Hydrogen cyanide [74-90-8], hydrocyanic acid,prussic acid, formonitrile, HCN, Mr 27.03, is acolorless liquid with the characteristic odor ofbitter almonds. Hydrogen cyanide in aqueous
solution was first prepared by SCHEELE in 1782[22]. The acid occurs naturally in combinationwith some glucosides, such as amygdalin.
Hydrogen cyanide is currently produced bydirect reaction of alkanes with ammonia, andindirectly as a byproduct of the manufacture of
Article with Color Figures
DOI: 10.1002/14356007.a08_159.pub3
acrylonitrile (by ammoxidation of propene !Acrylonitrile). Major end uses include the pro-duction of adiponitrile, methyl methacrylate,cyanuric chloride, chelating agents, sodium cya-nide, and methionine and its hydroxy analogues.
1.1. Properties
Physical Properties [1, 23–26]:
Melting point �13.24 �CBoiling point 25.70 �CVapor pressure
at 0 �C 35 kPa
at 20 �C 83 kPa
at 50 �C 250 kPa
Enthalpy of formation (HCN liquid), DH 3910 kJ/kg
Critical temperature 183.5 �CCritical density 0.20 g/cm3
Critical pressure 5 MPa (50 bar)
Density (20 �C) 0.687 g/cm3
Specific heat, liquid (20 �C) 2630 J kg�1 K�1
Specific heat, gas (25 �C) 1630 J kg�1 K�1
Heat of fusion 310 kJ/kg
Heat of vaporization 935 kJ/kg
Heat of polymerization �1580 kJ/kg
Explosive range in air 5.5 – 46.5 vol%
Flash point �17.8 �CIgnition temperature 535 �CDynamic viscosity, h (20 �C) 0.192 mPa � sSurface tension (20 �C) 18.33 mN/m
Dielectric constant
at 0 �C 158.1
at 20 �C 114.9
Dissociation constant
aqueous solution, pK (20 �C) 9.36
As a result of its high dielectric constant, HCNhas acquired some importance in preparativechemistry as a nonaqueous, ionizing solvent [27].
Chemical Properties. Some comprehen-sive reviews on hydrogen cyanide and cyanogencompounds have been published [1–3]. The acidoccurs only in the nitrile form.Although isomericisonitrile HNC has been detected in interstellarspace, all efforts to isolate this compound havefailed. As the nitrile of formic acid, HCN under-goes many typical nitrile reactions. For example,hydrogen cyanide can be hydrolyzed to formicacid by aqueous sulfuric acid or hydrogenatedto methylamine. Hydrogen cyanide adds tocarbon – carbon double bonds and forms cyano-hydrins with carbonyl groups of aldehydes orketones. The most important uses of this type are
in the manufacture of acetone cyanohydrin (anintermediate in the production of methyl meth-acrylate) and in the production of adiponitrilefrom butadiene and hydrogen cyanide. Anotherexample is themultistep synthesis of amino acidsvia hydantoins. Hydrogen cyanide can beoxidized by air over silver or gold catalysts at300 – 650 �C to yield cyanic acid (HOCN) andcyanogen (CN)2 in an approximate 2 / 1 ratio.
The reaction of hydrogen cyanide with chlo-rine gives cyanogen chloride (see Section 4.2).For industrial purposes, the latter compound isusually directly trimerized to cyanuric chloride,the starting material for the chemistry ofs-triazines.
In the presence of oxygen or air, hydrogencyanide burns with a very hot flame. For thereaction
2 HCNþ1:5 O2!N2þ2 COþH2O
the heat of formation is calculated to be �723.8kJ/mol and the adiabatic flame temperature is2780 �C. Pure liquid or gaseous HCN is inertto most metals and alloys such as aluminum,copper, silver, zinc, and brass. At higher tem-peratures (> 600 �C), the acid reacts with metalsthat can form carbides and nitrides (titanium,zirconium, molybdenum, and tungsten).
Pure liquid hydrogen cyanide has a tendencyto polymerize to brown-black, amorphous poly-mers, commonly called azulmic acid [26746-21-4]. The reaction is accelerated by basicconditions, higher temperature, UV light, andthe presence of radicals. Since the decomposi-tion is exothermic, the polymerization reactionis autocatalytic and can proceed with explosiveviolence to form the HCN dimer iminoacetoni-trile [1726-32-5] and the HCN tetramer diami-nomaleonitrile [1187-42-4] as intermediates[28, 29]. Both compounds are presumed to beimportant in the evolution of life, and cyanocompounds may play a role in prebiotic synthe-ses [30, 31]. In the liquid phase, hydrogencyanide is stabilized by the presence of smallamounts of acids (0.1 wt% H3PO4, 1 – 5%HCOOH or CH3COOH) or 0.2 wt% SO2 in thegas phase.
TheHCN – H2OSystem. At 25 �C, hydro-gen cyanide is miscible with water in all ratios.The solution is a weak acid, with a dissociationconstant of the same order ofmagnitude as amino
674 Cyano Compounds, Inorganic Vol. 10
acids. The relation between total cyanide con-centration (cHCN þ cCN�) and dissociated cya-nide (cCN�) in a dilute aqueous solution, as afunction of pH, is illustrated in Figure 1.
Figure 2 shows the liquid – vapor equili-brium diagram at atmospheric pressure. Becauseof the high HCN partial vapor pressure it is
difficult to separate the acid from a gas mixtureby absorption in water, which is an importantconsideration for industrial practice [32].
The stability of hydrogen cyanide solutionsdepends on the degree of dilution: at concentra-tions below 0.1 mol/L HCN, the acid is stable;addition of traces of acid prevents decomposi-tion. Like pure HCN, the exothermic polymeri-zation of an aqueous solution is accelerated bythe presence of alkali. In addition to azulmicderivatives, small amounts of amino acids andpurine bases are formed. This fact is of someimportance to biological chemistry. The C�Ntriple bond is hydrolyzed by strong alkali or acidto give formic acid and ammonia. Higher tem-peratures or hydrothermal conditions favor thesereactions.
1.2. Production [1, 33, 34]
Hydrogen cyanide can be produced when suffi-cient energy is supplied to any system contain-ing the elements hydrogen, nitrogen, andcarbon. Generally, only processes starting fromhydrocarbons and ammonia are of economicimportance today; however, the production ofhydrogen cyanide from formamide [35] iscarried out in a plant at Ludwigshafen, Germanyand since 1997 in a plant in Niigata, Japan.Alternative raw materials such as methanol,carbon, or carbon monoxide [36–45] are ofinterest since the 1980s. The ammoxidation ofmethanol to HCN is performed in Japan in a10 000 – 15 000 t/a plant.
These routes can compete with conventionaltechnology given specific site conditions. Thereaction of hydrocarbons with ammonia
CxH2xþ2þxNH3! xHCNþ2�ð2xþ1ÞH2
is highly endothermic and needs a continuousheat supply. The means of providing this energyrequirement are manifold and characteristic ofthe different processes [47]. Direct microwaveheating of the catalyst has been developed forsmall-scale local synthesis of HCN [48]. Themain processes currently used to make hydrogencyanide: the Andrussow ammoxidation process,which involves the reaction of ammonia, meth-ane, and air over a catalyst gauze; and thetwo ammonia dehydration routes, the methane –ammonia (BMA) and the Shawinigan processes,
Figure 1. HCN/CN� equilibrium as a function of pH
Figure 2. Liquid – vapor diagram of the system H2O –HCN at atmospheric pressure
Vol. 10 Cyano Compounds, Inorganic 675
developed by Degussa and Gulf Oil Company,respectively. The latter are performed in exter-nally heated ceramic tubes or in an electricallyheated fluidized coke bed. Twenty-five percentof the hydrogen cyanide in the United Statesand 20% in Western Europe is obtained as abyproduct in the manufacture of acrylonitrile bythe oxidation of propene in the presence ofammonia (Sohio technology).
1.2.1. Andrussow Process
The Andrussow process was developed around1930 by L. ANDRUSSOW of I.G. Farben [49–51]and is the most widely used method for directsynthesis of hydrogen cyanide. The average ca-pacity of single commercial installations is (5 –30) � 103 t/a; large plants are operated by Du-Pont, Rohm and Haas, Novartis and Cyanco inthe United States; ICI, Butachemie, R€ohm andElf-Atochem inWestern Europe; and MitsubishiGas in Japan.
Figure 3 is a flow diagram of the Andrussowprocess. Natural gas, essentially sulfur-freemethane, is mixed with ammonia. Compressed
air is added in a volume ratio that correspondsclosely to the theoretical reaction [52]:
CH4þNH3þ1:5 O2!HCNþ3 H2O DH ¼ �474 kJ=mol
The mixture is passed over a platinum – rho-dium or platinum – iridium gauze [53] catalyst;temperature and upper flammability limit shouldbe monitored carefully [54, 55]. The reactiontakes place at > 1000 �C, at around atmosphericpressure, and with a gas velocity through thecatalyst zone of about 3 m/s. To avoid decompo-sition of HCN, the effluent gas from the reactor isquickly cooled in a waste-heat boiler, whichproduces the steam used in the process.
After the waste-heat boiler, the gas is washedwith dilute sulfuric acid to remove unreactedammonia; this is necessary to prevent polymeri-zation of HCN. Because disposal of the resultingammonium sulfate solution is expensive, othersystems have been patented [56–58]. Alternative-ly, the off-gas from the reactor is passed through amonoammonium phosphate solution [59, 60],which converts the ammonia to diammoniumphosphate. To effect thermal reversal of thephosphate equilibrium, the absorption solution is
Figure 3. Simplified diagram of the Andrussow processa) Reactor and ammonia scrubber; b) HCN absorption tower; c) HCN rectifier; d) Condenser
676 Cyano Compounds, Inorganic Vol. 10
boiled in a stripper by injection of steam. Thereleased ammonia is condensed and recycled tothe reactor, while the regenerated monoammo-nium phosphate solution is pumped back to theabsorber.
After the ammonia scrubber, the gas is passedthrough a countercurrent column in which thehydrogen cyanide is absorbed in cold water andthe resulting solution is stabilized by adding acid(ca. 0.1%). The hydrogen cyanide is strippedfrom the aqueous solution in a rectifier and con-densed. The end product is highly pure and has awater content of less than 0.5%. The aqueousabsorber solution, containing traces of HCN, iscooled and fed back to the absorption tower. Theresidual gases, H2, CO, and N2, can be used forheating or methanated in a separate unit andrecycled as feedstock for HCNmanufacture [61].
The advantages of the Andrussow processinclude (1) long catalyst life, up to 10 000 h;(2) well-tested technology, with a simple andsafe reaction system; and (3) high-purity HCN.A disadvantage is that the process is dependenton pure methane as a raw material to avoidcarburization of the platinum catalyst; forexample, a small percentage of higher hydro-carbon impurities rapidly causes problems withthe catalyst system and reduces conversionrates. Other important poisons for platinum aresulfur and phosphorus compounds. A furtherproblem is the relatively low yield based onmethane (60 – 70%) and ammonia (70%), aswell as the low hydrogen cyanide concentrationin the product gas, so that recovery equipmentmust handle large volumes of gas.
The compositions of reaction and residualgases in the Andrussow and BMA processes arecompared in Table 1.
1.2.2. Methane – Ammonia (BMA)Process
The basis of the Degussa BMA process is theformation of hydrogen cyanide in the absence ofoxygen [62–66]. The reaction
CH4þNH3!HCNþ3 H2 DH ¼ þ252 kJ=mol
is endothermic, requires temperatures above1200 �C, and is performed in externally heated,alumina tube bundles, which are coated with athin layer of a special platinum catalyst [67, 68].
Several of these bundles are fixed in a reactionfurnace unit. Amixture of ammonia andmethane(natural or refined gas with a content of 50 –100 vol% methane) is passed through the tubesand quickly heated to 1300 �C at normal pres-sure. To avoid the formation of any disturbingdeposits of carbon black, the NH3/CH4 ratio iskept between 1.01 and 1.08. After leaving thereaction tubes, the product gas is cooled to300 �C by passage through a water-cooledaluminum chamber. A kinetic study has shownthat a particular temperature profile is essentialfor this process [69].
The subsequent reaction steps, ammoniaabsorption and hydrogen cyanide isolation, aresimilar to those of the Andrussow process. Adistinct advantage is the higher HCN content(Table 1) of the product gas, so that the numberof steps and the size and cost of recovery equip-ment, are greatly reduced. The tail gas consistsmainly of pure hydrogen. If this is not needed forother syntheses, it can be used as fuel for heatingthe furnace. About 80 – 87% of the ammoniaand 90 – 94% of the methane are converted tohydrogen cyanide. The specific energy consump-tion of ca. 4 � 106 kJ/100 kg HCN reported thusfar has been considerably decreased by morerecent developments [70, 71]. A large part ofthe heating energy is recovered and used in the airpreheater or steam generator.
If the methane supply is limited, the processcan be carried out directly with liquefied hydro-carbons or ethanol, or in a three-step reactionstarting from methanol [72–74]. Both Degussaand Lonza utilize the BMA route to producehydrogen cyanide.
Table 1. Composition of off-gases and residual gases (volume frac-
tions, %) on the basis of pure methane and ammonia in the BMA and
Andrussow processes
Compound BMA Andrussow*
After Residual After Residual
reaction reaction
HCN 22.9 <10�2 7.6 <10�2
NH3 2.5 <10�2 2.3 > 0
H2 71.8 96.2 13.3 14.7
N2 1.1 1.5 49.2 54.6
CH4 1.7 2.3 0.3 0.4
CO 3.8 4.2
H2O 23.1 25.7
CO2 0.4 0.4
* Calculated because no consumption figures are available.
Vol. 10 Cyano Compounds, Inorganic 677
1.2.3. Shawinigan Process
In the Shawinigan process, hydrocarbon gasesare reacted with ammonia in an electricallyheated, fluidized bed of coke. The process,sometimes called the Fluohmic process, wasdeveloped in 1960 by Shawinigan Chemicals[75–78], now a division of Gulf Oil Canada.
In a circular reaction cavity constructed fromalumina and silicon carbide, the mixture of am-monia and hydrocarbon (N/C ratio slightly > 1)passes through a fluidized bed of coke, heated byelectrodes immersed in the bed. The chemicalreaction is similar to the methane – ammoniaprocess, but no catalyst is required and tempera-tures are kept above 1500 �C. Other carboncompounds, such as naphtha or lighter hydro-carbons, can also be converted. Propane is usu-ally the main feedstock. The reaction can bedescribed as
3 NH3þC3H8! 3 HCNþ7 H2 DH ¼ þ634 kJ=mol
Unreacted feed gas is almost completelydecomposed to the elements. This reduces thequantity of ammonia to be removed from theproduct gas and leads to the formation ofcoke particles. The control of coke particle sizein the bed is an important operating parameter.
The reactor effluent contains up to 25 vol%HCN, 72 vol% H2, 3 vol% N2, and only0.005 vol% NH3. Coke is removed in a water-cooled, cyclone-entrained bed. The gas is fur-ther cooled and enters the absorption equipmentwhere HCN is removed. The residual gas, nearlypure hydrogen, can be used for other chemicalprocesses. Some of the hydrogen is recycledto the reaction unit to inhibit the formation ofsoot.
Coke from the cyclone is screened, and threefractions are separated, stored, and then fed backto the reactor system in the desired proportions tocontrol the particle size distribution. By regulat-ing the rate of coke recycling, the level of thefluidized bed and the reaction temperature can becontrolled.
In practice, at least 85% of the ammonia andup to 94% of the hydrocarbon are converted tohydrogen cyanide. Because of the high electricpower consumption (6.5 kWh per kilogram ofHCN) the Shawinigan process would probablyonly be attractive where low-cost electricity isavailable.
This process is employed by Polifin in SouthAfrica, by Arogenesas in Spain, and by Ticor inAustralia.
1.3. Storage and Transportation [79,
80]
Handling, storage, and transportation of hydro-gen cyanide are determined by its low boilingpoint, high toxicity, and instability in the pres-ence of moisture, bases, or other impurities. Theliquid acid is relatively uncorrosive. Materialscompatible with HCN at normal temperaturesare stainless steel, Hastelloy, and Monel. Toprevent polymerization, stabilizing agents, suchas sulfuric acid, phosphoric acid, oxalic oracetic acid, and sulfur dioxide are used. Thetype and quantity of stabilizer (usually< 0.5%)depend on storage capacity, temperature, andresidence time in a container. A combination ofH2SO4 and SO2 prevents the decomposition ofHCN in the liquid and vapor phases. Largerquantities of hydrogen cyanide are stored at amaximum temperature of 5 �C and must bepermanently recirculated.
Additionally, the color of the liquid is moni-tored and should not exceed APHA 20. To keepthe concentration of gas below the danger level,good ventilation of buildings in which HCNis stored and handled is of primary importance.Hydrogen cyanide is usually classified by gov-ernmental authorities as very poisonous, requir-ing special packaging and transportation regula-tions. Similar strict procedures exist for solutionswith an HCN content of 5% or more. Smallerquantities of the stabilized acid are transported inmetal cylinders of up to 56 kg nominal watercapacity in the United States and up to 60 kg inGermany.Cylinders cannot be chargedwithmorethan 0.55 – 0.60 kg of liquid HCN per 1 L bottle,and resistance to deformationmust be tested up to10 MPa before first filling. The water contentshould not exceed 3%, and storage time shouldbe less than one year. According to nationalregulations, transportation of quantities> 100 kgrequires special permission. Procedures coveringdetails of tank car size (up to 50 t), shipping,loading, and handling must be obeyed.
Hydrogen cyanide is mainly shipped by railunder the provisions of the applicable transportregulations. Transport in railcars is permitted
678 Cyano Compounds, Inorganic Vol. 10
only by approval of the competent authorities.The transport classifications of themost commontransport regulations are as follows:
RID/ADR: Class 6.1, 1�; (a).CFR 49: Class 6.1, UN 1051, PG I, ‘‘Poison
Inhalation Hazard, Hazard Zone A’’.
1.4. Economic Aspects and Uses
In 1998 the production capacity of synthetichydrogen cyanide was about 650 � 103 t/a inthe United States, 400 � 103 t/a in Europe, and35 � 103 t/a in Japan. Without detailed knowl-edge of the type and age of catalyst used invarious acrylonitrile plants, the additionalamount of byproduct HCN can only be estimat-ed. With an estimated HCN formation in theacrylonitrile processes of 10%, an additional180 � 103 t/a (United States), 130 � 103 t/a(Europe), and 60 � 103 t/a (Japan) is available.Currently, between 60 and 80% of the existingproduction capacity is being utilized. Thevariety of uses has changed enormously in thepast 15 years. Acrylonitrile production startingfrom acetylene was formerly the major consumerof hydrogen cyanide; in contrast, the Sohioprocess now in use yields HCN as a byproduct.
Today, the main outlet for HCN is the manu-facture of methyl methacrylate. Acetone is trea-ted with hydrogen cyanide to produce acetonecyanohydrin, which is treated with sulfuric acidandmethanol to formmethyl methacrylate. HCNis used to a similar extent in adiponitrile produc-tion, in which 2 mol HCN are added to 1 molbutadiene in a two-stage process. Virtually all theadiponitrile is then converted to hexamethylene-diamine, a nylon precursor. Hydrogen cyanidereacts with chlorine to form cyanogen chloride(see Section 4.2) which is usually directly tri-merized to cyanuric chloride. Herbicides basedon cyanuric chloride have been successfullyemployed in recent years.
D,L-Methionine, one of the largest volumeamino acids produced commercially, is manu-factured in a complex, multistep synthesis whichemploys hydrogen cyanide as one of the rawmaterials. Other uses of HCN are in the produc-tion of chelating agents such as ethylenediami-netetraacetic acid (EDTA) or nitrilotriacetic acid(NTA), starting from aldehydes, amines, andhydrogen cyanide. A wide variety of organic
intermediates can also be made from HCN andhave considerable potential utility. The mostimportant derivatives of hydrogen cyanide ininorganic chemistry are alkali metal cyanidesand cyanide complexes of iron.
2. Metal Cyanides
Metal cyanides are compounds of metals inwhich one or more cyanide ions act as mono-dentate ligands with carbon as the preferreddonor atom. Simple metal cyanides are repre-sented by the formula M (CN)x. Depending onthe metal, simple cyanides dissolve more or lessreadily in water, forming metal ions and cyanideions:
MðCNÞx�MxþþxCN�
The solubility is influenced by pH andtemperature, especially because hydrogencyanide is formed by the hydrolysis of cyanide:
CN�þH2O�HCNþOH�
The cyanide complexes can generally bedescribed by the formula Ay [M (CN)x], whereA is an alkali, alkaline earth, or heavy metal, andM is normally a transition metal. Most cyanidecomplexes in which A is an alkali or alkalineearth metal are highly soluble in water and formalkali or alkaline earth metal ions and complextransition metal cyanide anions, e.g.,
Ay½MðCNÞx�� yAþþ½MðCNÞx�y�
The complex cyanide anionmay then undergofurther dissociation and release cyanide ions:
½MðCNÞx�y�� zCN�þ½MðCNÞx�z�ðy�zÞ�
The dissociation of the cyanide complexdepends strongly on the type and valency of thecomplexed metal ion, as well as on the pH andconcentration of the solution. Insoluble or slight-ly soluble cyanide complexes are formed whenthe alkali or alkaline earth metal anion A isreplaced by a heavy metal ion. In this case, morecomplicated cyanide complexes are oftenformed, which cannot be described by theabove-mentioned formula. Furthermore, mostheavy metal ions allow the formation of mixedcyanide complexes in which one or more of thecyanide groups can be replaced by other ligands
Vol. 10 Cyano Compounds, Inorganic 679
such as halides, pseudohalides, nitrogen oxides,sulfur compounds, water, or ammonia [4]. Theproperties and toxicities of these compoundsdepend on the structures of the metal cyanides.
2.1. Alkali Metal Cyanides
Since the introduction of the Castner process,sodium cyanide [143-33-9] has been the metalcyanide with the greatest commercial impor-tance. Until about 1900, potassium cyanide[151-50-8] was the more common compound,because it could be produced more easily bymelting potassium carbonate with potassiumhexacyanoferrate(II), one of the oldest knowncyano compounds. The properties and reactionsof both alkali cyanides are very similar.
2.1.1. Properties
Sodium and potassium cyanide are colorless,hygroscopic salts with a slight odor of hydrogencyanide and ammonia in moist air. They arefairly soluble in water (Fig. 4), and the sodiumsalt forms two hydrates: NaCN � 2 H2O below35 �C and NaCN � 1/2 H2O at higher tempera-
tures. Further physical properties are listed inTable 2. In the absence of air, carbon dioxide,and moisture, the alkali metal cyanides are
Figure 4. Solubility of NaCN and KCN in water* Solid phase: ice–NaCN � 2 H2O or ice–NaCN** Solid phase: ice–KCN
Table 2. Physical properties of NaCN [143-33-9] and KCN [151-50-8]
Ethanol, 100% 1.235 g (25 �C) 0.57 g (19.5 �C)Ethanol, 95% 2.445 g (25 �C)Methanol 6.44 g (15 �C) 4.91 g (19.5 �C)
4.58 g (25 �C)4.10 g (67.4 �C)
NH3, liquid 58 g (�31 �C) 4.55 g (�33.9 �C)
680 Cyano Compounds, Inorganic Vol. 10
stable, even at fairly high temperature, and canbe stored indefinitely [2].
Dry CO2 does not react with dry alkali metalcyanides; however, in moist air slow decompo-sition takes place, even at normal temperature,releasing HCN:
2 NaCNþCO2þH2O!Na2CO3þ2 HCNWhen this occurs, the salt sometimes becomes
brownish because of the formation of polymeri-zation products of HCN (azulmic acid). Alkalimetal cyanides are totally decomposed to HCNby the action of strong acids, e.g.,
2 NaCNþH2SO4! 2 HCNþNa2SO4
At elevated temperatures, oxygen reacts withalkali metal cyanides, to form the metal cyanateand carbonate, nitrogen, and carbon dioxide[83]:
2 NaCNþO2! 2 NaOCN
2 NaOCNþ3=2O2!Na2CO3þN2þCO2
The oxidation of alkali metal cyanides tocyanates by bubbling an air stream through analkaline melt is used for the bath nitriding ofsteel [84, 85]. Oxides of lead, tin, copper, nickel,and iron react with alkali metal cyanides above560 �C to form the corresponding cyanate andcarbonate, nitrogen, carbon dioxide, and thecorresponding metal [86, 87]. Violent oxidationof alkali metal cyanides can result from theaddition of strong oxidants such as nitrate,nitrite, or chlorate to a melt:
NO�3 þCN� !N2þCO2�3
5 NO�2 þ3 CN� ! 4 N2þO2�þ3 CO2�3
These reactions have been utilized for thedestruction of waste salts from hardening shops[88, 89].
When alkali metal cyanides are dissolved inwater, a pH-dependent, reversible equilibriumis established between hydrocyanic acid andalkali-metal hydroxide [90, 91]:
NaCNþH2O�HCNþNaOHFor example, in an aqueous solution ofNaCN
at pH 9.4, half of the total cyanide is present asHCN [92] (cf. Fig. 1). As a result of this hydro-lysis, solutions of alkali-metal cyanides in waterare always strongly alkaline. Furthermore, com-
mercial products always contain small amountsof alkali metal hydroxide to enhance stability;hence, the actual pH values of solutions areusually higher than those calculated on the basisof the cyanide concentration and the hydrolysisreaction (Table 3).
Nevertheless, when alkali metal cyanidesare dissolved in tap water, discoloration andprecipitation of brownish-black amorphouspolymerization products of HCN (azulmic acid)may occur when the water contains too muchCO2 or other acidic components. When solu-tions of alkali-metal cyanides are stored for along time or heated, slow hydrolysis of the C�Nbond takes place to yield the alkali metalformate and ammonia [93, 94]:
NaCNþ2 H2O!HCOONaþNH3
The hydrolytic decomposition of concentrat-ed NaCN solutions can be suppressed, even at60 – 80 �C, by the addition of 0.5 – 2% NaOH[95]. However, the hydrolysis is fast and com-plete when the reaction is carried out at 170 �Cunder pressure [96, 97]. This process has beeninvestigated for the destruction of cyanidewastesfrom metal hardening plants [98]. Atmosphericoxygen achieves only partial oxidation of cya-nide in aqueous solution at elevated temperature[99]; however, the reaction can be enhanced bythe use of catalysts such as activated carbon [100,101]. A fast conversion of cyanide to cyanate isbrought about by ozone at pH 10 – 12; this isfollowed by a slower oxidation to nitrogen andcarbonate [102, 103].
Cyanide is also oxidized to cyanate by hydro-gen peroxide [104] and by peroxomonosulfate,peroxodisulfate [105], and permanganate ions[106]. The anodic oxidation of cyanide leads,via cyanogen and the cyanate, to the carbonate,nitrogen, ammonia, and urea [107]. Halogensreact with cyanide in aqueous solution to givecyanogen halides, XCN (X ¼ Cl, Br, I), whichare hydrolyzed more or less quickly, depending
Table 3. pH values of aqueous solutions with varying concentrations
of NaCN and KCN [76]
Concentration, mol/L
1 10�1 10�2 10�3 10�4 10�5
NaCN 11.64 11.15 10.67 10.15 9.6 8.9
KCN 11.37 10.91 10.35
Vol. 10 Cyano Compounds, Inorganic 681
on the pH and temperature of the solution [108](see Section 4.1).
Reaction with chlorine or hypochlorite abovepH 11.5 is the most widely used process fordetoxification of solutions and wastewaters con-taining cyanide at concentrations below 1 g/L.
The reaction of alkali cyanides with sulfur orpolysulfides in aqueous solution at elevatedtemperature is used for the production of alkalimetal thiocyanates [109]. The reaction withthiosulfate to give thiocyanate is used therapeu-tically in the treatment of cyanide poisoning[110]. Heavy metal ions react in aqueous solu-tion with cyanide to yield insoluble cyanides, allof which dissolve in the presence of excesscyanide to give stable complex salts [111].These compounds play an important role inmetallurgy and electroplating [112]. Some basemetals (e.g., zinc or nickel) and, in the presenceof oxygen or oxidizing agents, even preciousmetals (e.g., gold or silver) are dissolved byaqueous solutions of alkali metal cyanides[113]. This property of alkali metal cyanideshas been utilized for about 100 years in theleaching of precious metals ores:
4 NaCNþ2 Auþ1=2 O2þH2O! 2 Na½AuðCNÞ2�þ2 NaOH
Tantalum, titanium, and tungsten are notattacked by alkali metal cyanide solutions atroom temperature.
Alkali metal cyanides react with carbonylgroups of organic compounds in aqueous solu-tion to form cyanohydrins [115, 116]:
Organic compounds with labile halogenatoms can be converted by reaction with alkalimetal cyanides in aqueous solution to nitriles,which can be further processed to give carboxylicacids, amines, etc.:
RClþNaCN! RCNþNaCl
2.1.2. Production
Sodium and potassium cyanide are produced byreaction of: gaseous or liquid HCN reacts with a
solution of sodium or potassium hydroxide. Theformation of cyanides is chemically a simpleneutralization reaction between a weak acid(HCN) and a strong base (OH�). The neutraliza-tion reaction has an energy release of 460 kJ/mol,which leads to additional heating of the reactionsolution and saves energy.
The efficiency of the neutralization reaction isvery high, and the yield based on HCN is close to100 %. Thus all of the HCN used is transferredinto the end products NaCN or KCN, and onlyminimal losses of HCN occur.
The reaction results in the formation of adissolved salt. The raw solution can either beconcentrated or diluted with water, dependingon what kind of intermediate product form isproduced. Normally, the aim is to form aconcentrated solution with ca. 30 % cyanide.Commercial solid sodium and potassium cya-nide can be crystallized by evaporation ofexcess water.
There are patented cyanide production pro-cesses available which directly form solidmaterial in a fluidized-bed reactor using analkali solution and HCN gas as raw materials.Also processes are patented that use a cyanidesolution as the cyanide source, and solid cyanideis produced by a drying process in a fluidized-bed reactor.
2.1.2.1. Classical Production Processes
The following processes are termed classical asthey are not applied to any significant extent inEurope presently but were used in earlier cyanideproduction.
In theCastner – Keller process sodiummetalreacts with charcoal and ammonia at around600 �C in a melt [117]:
NaþCþNH3!NaCNþ3=2 H2
This process was mainly developed byDegussa and applied there until 1971.
The calcium cyanamide process is very simi-lar to the Castner process, but uses calciumcarbonate as the carbon source for the cyanideion.
In the ‘‘blood process’’ potassium or sodiumcarbonate was fused with potassium hexacyano-ferrate(II), for which mainly animal blood andwastes were used as raw materials.
682 Cyano Compounds, Inorganic Vol. 10
2.1.2.2. Current Production Processes
Cyanides are made by melt processes or byprocesses in aqueous solution using HCN as thecyanide source. Most cyanide is produced now-adays using solution processes. Only where(electrical) energy costs are sufficiently low canmelt processes be competitive.
Melt processes do not apply any cyanidesources as raw materials, but produce the cya-nides from non-cyanide components. The resultof the processes is a solid cyanide with differentkinds of impurities, e.g., carbon black fromusing carbon as raw material.
Cyanides can be produced in batch and incontinuous operation. Liquid cyanide solutionsare easily pumped as the viscosity is moderateeven at the highest achievable concentration.
Figure 5 shows a flow sheet for NaCNproduction.
Production of Cyanide Solution. Theproduction of the cyanide solution (NaCN,KCN)is the basic first production step. Further proces-sing of the resulting raw cyanide solution usuallyconsists of concentration of the solution by
evaporation of excess water, cooling, filtering,and storage. The exact ratios of cyanide in waterare strongly dependent on the water balance.When a concentrated NaOH solution (50 %) andpure HCN (99 %) are used a cyanide solutionwith ca. 46 % sodium cyanide is formed whichneeds to be further diluted with water to give acommercial product.
Solutions are often directly used in caseswhere the site of industrial application is closeenough to the cyanide production site to keep thetransport of solution competitive (energy forevaporation of the excess water, formulation ofthe solid material). This is, therefore, usuallyonly applied if the application site can directlyuse a concentrated cyanide solution (30 wt%).This is, for example, the case for gold miningoperations in Nevada, South Africa, and Austra-lia. Cyanide solution is normally shipped in tanktrucks to the gold mine, subsequently stored inliquid tanks at the site, and diluted for further use,if necessary.
Many gold mines are too far away fromthe raw material production site to be suppliedwith liquid material and, therefore, need solidNaCN.
Cyanide solutions normally contain approx.24 – 30 %NaCN.Cyanide solutions transportedat cold times of the year or in cold regions maycontain lower cyanide concentrations to preventcrystallization during transport and storage inunheated devices.
In cases where an application requires HCNas a liquid or a gas and no HCN productionis available on site, cyanide solutions or solidscan be used as the source. They need only beacidified, and the HCN is distilled and takento production. Normally, it is not storedintermediately.
Production of Solid Cyanides. Solidcyanide is made from solution by crystallizationof solid cyanides from the liquid, separation ofthe wet solid with further drying and mechanicalhandling to form stable solid product forms.Figure 6 shows a plant for the production ofsolid cyanides. The preferred solid forms arebricks, granules, tablets, and powder. Bricks aremade by compacting of the wet filter cake withsubsequent drying and also by first drying thecrystals followed by compacting. Bricks are alsomade from granules. Powder can be made by
color
fig
Figure 5. Example of a solid cyanide production plant
Vol. 10 Cyano Compounds, Inorganic 683
grinding and subsequent sieving of finesmaterial.
The cyanide content in solids is usually veryhigh and specified for normal applications in arange of higher than 98 % and even higher than99 %.
2.1.2.3. Energy Consumption
The energy costs for cyanide production aremainly due to electricity and steam and corre-spond to approx. 1.3 – 1.5 t of steam per ton ofsolid 100 % NaCN or KCN.
2.1.3. Packaging and Transport [118, 119]
Production includes packaging of cyanides forthe safe storage and transport. Normally cyanidesare filled into metal drums holding between 50and 100 kg or 1 t big bags which are transportedin wooden boxes. All packaging must have UNregistration for the packaging of hazardousmaterials. Returnable packaging for multiple useis an important alternative to the use of one-way
packaging. Both one-way packaging and return-able packaging for solid material are used world-wide. Another alternative for transport is theso-called solid to liquid system (SLS). Here, solidcyanides are filled into a unit containing approx.20 t of material. This unit is transported to thecustomer’s site and the cyanide is dissolveddirectly at the user’s facility and pumped to astorage tank. The transportation unit can bereused and refilled with solid material manytimes. The preferred type of packaging oftendepends on the specific transport conditions,which are due to technical, economical, andenvironmental considerations.
Figure 7 shows storage of solid cyanide indrums.
Transport is regulated by national and inter-national regulations and laws:RID/ADR and ADNR Class 6.1UN no 1680 (KCN)
1689 (NaCN)
Road transport of liquid cyanides is allowedin various European countries. However liquidcyanide is preferably shipped by rail cars withinEurope. Only certified companies with a specialpermit for the transportation of dangerousgoods are approved. Some European countriesrequire special permits for the transport ofcyanides on the road, e.g., Italy. Each transportin Europe is also required to have a transportemergency card with relevant information oncyanide on board.
In Germany, it is allowed to ship cyanides andacids together in the same shipment (trucks). Butfor safety reasons cyanides and acids should notbe transported and stored together.
Figure 6. Flowchart of themain production steps for cyanideproduction (NaCN as example)
color
fig
Figure 7. Storage facility for solid cyanide in drums
684 Cyano Compounds, Inorganic Vol. 10
Transportation of cyanide solution is mostlyfavored in cases where transport routes to theend-users are considered to be safe and thedistance short enough that transport of liquidproducts is economic compared to the alternativeof solid cyanide. Also the customermust be in theposition to directly use the solution.
Production of solid material from cyanidesolutions is mostly the best alternative when theproduct is transported over long distances,when the transport of liquid is not allowedaccording to national regulations, and when thecustomers are prepared to store and use solidmaterial.
2.1.4. Responsible Care, ProductStewardship, Sustainable Development,Certification [120–132]
Production facilities can be certified according tointernational standards referring to product andmanagement system quality including the rele-vant aspects of (workers’) health, safety, andenvironmental management or any other systemsthat lead to similar results. This includes certifi-cation according to ISO 9001, 14001, or anyother management system that is applicable tomanage company and stakeholder needssufficiently.
Cyanide producerswho supply cyanide to goldmines are increasingly being asked to fulfill therequirements of the cyanide producers verifica-tion audit protocol according to the InternationalCyanide Management Code (ICMC) provisions.The overall basis for this kind of certification isthe International Cyanide Management Code forthe gold mining industry.
All cyanide producers supply material safetydata sheets (MSDS) to their customers coveringall specific information needed for the safehandling and use of their products. Informationand training programmes are in place fortraders, end-users, and transport companies.
Transport route analysis from the productionfacility to the sites of the end-users (mainly forthemining industry) are carried out by producers.
Major European cyanide producers exchangeinformation on safety, health, and environmentalaspects of their products an a regular basis withinthe CEFIC Cyanide Sector Group. This group
has, for example, developed a Mutual AidScheme (MAS) for transportation incidentswithcyanides and published the ‘‘Guidelines forStorage, Handling, and Distribution of AlkaliCyanides’’.
2.1.5. Emissions and Consumptions[133–139]
Hazardous Emissions from cyanide pro-duction can be emissions of HCN and NH3 gasesand of cyanides, the last-named mainly in waste-water or as particles in the workplace.
NH3 and HCN can be detected by humans dueto their specific odor at low detection limits.These limits are far below any workplace limits.Therefore, people may be warned even withoutany technical monitoring devices. However, asignificant number of persons are not able todetect HCN, and a much smaller number cannot detect NH3 by its smell. Hence, in accordancewith standard workplace regulations, monitoringdevices must be installed, mainly for the detec-tion of HCN and also for NH3.
The production of cyanide solutions is oper-ated under closed conditions where no HCN canleak from the units. As a result the overallemission ofHCN from cyanide production plantsin Europe and elsewhere are below the Europeanand national regulation limits (e.g.,TA Luft inGermany) and below 10 kg HCN per year for a20 000 t/a production facility.
NH3 emissions may result from heatingcyanides and cyanide solutions. NH3 formationfrom cyanide production can result from thehydrolysis of cyanides at higher temperature,which can result in the formation of cyanate.The formation and the subsequent emissionof NH3 to the environment is very low andbelow reportable limits, i.e., less than 15 kgNH3 per year.
Detoxification of Cyanides. The detoxifi-cation of cyanides can be carried out by usingdifferent kinds of oxidizing agents, such ashypochlorite or peroxides. Pure oxygen is notsuitable. Using hypochlorite creates a signifi-cant salt load of the effluent and can createadditional contributions to AOX load. UsingH2O2 based technology does not create any of
Vol. 10 Cyano Compounds, Inorganic 685
these byproducts and can be seen as the bestchoice for cyanide detoxification at a cyanideproduction plant.
NaCNþOCl�!OCN�þNaCl
CN�þH2O2!OCN�þH2O
Cyanide which is dissolved in process waste-water can be treated by the above-mentionedmethods, but preferably peroxides are used.
Wastewater. The process solution candirectly be fed to a biological treatment plantunder predefined conditions. Also dilutedcyanide effluent streams may directly be addedto a biological wastewater treatment, where bac-teria break down the cyanide to nontoxic sub-stances and use the chemical as feed.
As a result of detoxification the free cyanidein the treated water is far below 0.01 ppm.The total amount of cyanide transferred to theenvironment from a 20 000 t/a NaCN plantnormally is below 50 kg/a. These small cyanideemissions which enter, e.g., rivers at a very lowconcentration do not pose harm to fish and theenvironment and are much likely also detoxifiedfurther by microorganisms.
Excess water from concentrating cyanidesolutions is evaporated and either recycled tothe process water after condensation or detoxi-fied as mentioned above. No contaminated wateris emitted to the air.
While most of the evaporated water is sub-sequently condensed, a small amount of thevapors and noncondensables must be treatedbefore release to the atmosphere. This is accom-plished through the use of wet scrubbers, incin-eration, or a combination of both. Wet scrubbersystems utilize liquid scrubbing solutionsdesigned to contain or neutralize the noncon-densable gases. Destruction efficiencies forthese systems exceed 98 %. Incineration orthermal oxidation systems are also utilizedfor detoxification of the gasses. Destructionefficiencies for these systems exceed 99 %.
2.1.6. Quality Specifications, Impurities
The main impurities arising during the produc-tion of cyanides are cyanate (OCN�), formate
(HCOO�), and carbonate (CO2�3 ). However,
generally speaking, none of these are critical inany way for the applications in gold mining,the chemical industry, and electroplating. Veryoften these impurities are formed in applicationprocesses anyway.
Carbonates originate mainly from contact ofexcess alkali in the cyanide solution with atmo-spheric carbon dioxide. Hydrolysis of cyanide athigher temperatures (e.g., in drying sections, inhot solutions) leads to an increased content offormate, and ammonia is also formed. Cyanatecan be formed by oxidation.
Ammonia can also occur as a possible impu-rity, especially in solutions after longer storage atelevated temperatures. If slow hydrolysis in solid(wet) material takes place, the ammonia formedcan be released to the gas phase.
Due to reactions that lead to the formation ofHCN and/or NH3 commercial cyanide solutionscan – mainly after prolonged storage under non-ideal conditions – have a smell of HCN and/orNH3. Absolutely dry cyanides normally do notsmell of these species, as the presence of water isnecessary for any of the decomposition reactionsthat lead to the release of HCN and/or NH3.
For high-quality applications in the electro-plating industry the content of a number of heavymetals is critical. Therefore, these must be belowspecified limits, which are, for example, definedin the DIN-ISO 50971. To achieve the DINstandards the respective alkali metal source(NaOH, KOH) must meet these requirements,as only these raw materials can act as sources ofthese metals. Due to its production processes,HCN is free of these metals.
For applications in high-quality electroplatingand for most chemical synthesis applications,especially for the pharmaceutical industry, thespecification for cyanides according to DIN-ISO50971 can be regarded as a typical minimumquality requirement (Table 4).
2.1.7. Uses
2.1.7.1. Gold Mining [140–145]
Themajority ofworld sodium cyanide productionis used in goldmining to extract gold from its ores(! Gold, Gold Alloys and Gold Compounds,Section 4.2). Elemental gold is very stable and
686 Cyano Compounds, Inorganic Vol. 10
does not react easily, even with very aggressivechemicals. Usually only in the presence of astrong oxidant together with a complexing agentcan gold become water-soluble (formation ofsoluble gold salts). A classical example of howgold can be dissolved is the use of aqua regia, amixture of the strong oxidant HNO3 and thecomplexing agent HCl, where the chloride ionis the complexant for the gold. More weaklyoxidizing conditions can be applied if a verystrong complexing anion is used. In gold mining,cyanide is the complexing agent for the gold.Even by oxidation with oxygen (air, peroxides)under normal reaction conditions (dilute, slowreaction speed, moderate to low temperature)elemental gold is dissolved by oxidation and thensubsequently complexed with cyanide:
4 NaCNþ2 Auþ1=2 O2þH2O!2 Na½AuðCNÞ2�þ2 NaOHWhile the gold chloride complex is unstable in
dilute aqueous solution, even very low concen-trations of the gold cyanide complex are stable inwater over a wide pH range.
Cyanides are used in gold mining, althoughthey are acutely toxic, mainly because of theirhigh efficiency and selectivity, and they cannot easily be substituted by other chemicals.In gold mining, not using cyanides would ba-sically mean the termination of primary goldproduction. Only approx. 10 % of gold produc-tion today runs without the application of cya-nides. There have been numerous attempts todevelop gold mining processes using alterna-tive lixiviants for gold extraction. None of thealternative lixiviants or processes have everbeen applied successfully in any large-scaleoperation on a worldwide basis. In this sense,only cyanide can fulfil the requirements of thegold mining industry to extract gold from thevariety of ores in an economical and environ-mentally sound manner.
2.1.7.2. Chemical Synthesis
In a number of reactions, cyanides are used asstarting materials or intermediates. In organicchemistry the cyano group basically acts as abuilding block which can easily be transformedinto a variety of other chemical functionalgroups, such as carboxylic acids, esters, hydro-xylamines, cyanohydrins, and many more.Important examples in chemical synthesis are
the cyanidation of acyl chlorides forming acylcyanides as intermediate products:
RCð¼ OÞClþNaCN!RCð¼ OÞCNþNaCland the nucleophilic substitution of halogenatedalkanes by alkali metal cyanides for the elonga-tion of the molecular chain by one carbon atom:
RBrþNaCN!RCNþNaBrAnother basic reaction is the formation of
cyanogen chloride from sodium cyanide andchlorine:
NaCNþCl2!ClCNþNaCl
Trimerization of cyanogen chloride results inthe formation of cyanuric chloride (! CyanuricAcid and Cyanuric Chloride, Chapter 3), animportant raw material for the production ofherbicides, pesticides, fungicides, and insecti-cides. Cyanides are therefore important rawmaterials for crop-protection agents in theworldwide agricultural market.
2.1.7.3. Electroplating of Metals(! Electrochemical and Chemical Deposition)
The electroplating industry uses silver and goldcyanides for electroplating processes:
KCNþAgNO3!AgCNþKNO3
KCNþAuCN!K½AgðCNÞ2�Beside the similar gold cyanide and potassium
dicyanoaurate(I) the trivalent gold cyanide com-poundK[Au(CN)4] has the advantage ofworkingin lower pH ranges and is able to deposit golddirectly onto stainless steel base materials.
Electroplating of platinummetals is performedin cyanide baths and, for special applications,in cyanide melts. The high-temperature electro-plating (HTE process) runs in the temperaturerange of 500 – 600 �C in an eutectic sodium –potassium cyanide mixture under argon gas.
2.1.7.4. Hardening of Metals
Strongly treated steel, such as that camshafts formotors, must be hardened by boron, nitrogen, orcarbon. One of the most favorably appliedmethods is the carbonization of the metals inmelts with cyanide. A superficial layer of iron
Vol. 10 Cyano Compounds, Inorganic 687
carbide, Fe3C (martensite) with a depth ofseveral micrometers is formed at a temperatureof about 900 �C. By the addition of bariumchloride as activator the amount of cyanidecould be minimized to some extent. The mainchemical reactions of the hardening process are:
2 NaCNþ2 O2!Na2CO3þN2þCOand
3 Feþ2CO!Fe3CþCO2
and hence the overall reaction:
4 NaCNþ4 O2þ3 Fe!Fe3Cþ2 Na2CO3þ2 N2þCO2
The regeneration of such melts is possiblewith nonhazardous polymeric cyanic acid, azul-minic acid [CNH]x, cyanamide NH2CN, dicya-namide, ormelamine under nitrogen atmosphere.
2.1.8. Economic Aspects
The most important alkali metal cyanides interms of production volume are sodium cyanideand potassium cyanide. Potassium cyanide pro-duction worldwide amounts to less than 10 % ofthe sodium cyanide production. The same pro-duction facilities can be used for the manufactureof NaCN and KCN.
In Europe, NaCN and KCN are produced atless than ten sites for various purposes. A signifi-cant amount is produced for captive use, but themajority is produced for applications withinEurope and for export outside Europe.
Cyanide production efficiency is mainly driv-en by cost-effective production conditions. Theproduction costs are mainly influenced by thecosts for the raw materials HCN and NaOH/KOH, which can differ significantly.
In the 1990s the cyanide producer market inEurope consolidated to a significant extent, and anumber of production sites have been closed.Existing production sites in Europe as of 2005follow (asterisk denotes captive use):
Wesseling, Germany NaCN solution,
NaCN solid, KCN solid
Geleen, the Netherlands NaCN solution
Billingham, United Kingdom NaCN solution
Ludwigshafen, Germany NaCN solution
Antwerp, Belgium NaCN solution
Kolin, Czech Republic NaCN solid
Pitesti, Rumania NaCN solution
Seal Sands, United Kingdom* NaCN solution
Saint-Avold, France* NaCN solution
Roussillon, France* NaCN solution
The production capacity of cyanides inEurope is in the range of 90 000 t/a (based on100 % solid material). Most of this is solidsodium cyanide; a smaller part is solid potas-sium cyanide. Worldwide sodium cyanide pro-duction is estimated at approx. 500 000 t/a.
About 70 % of world cyanide production isused in the gold mining industry. The secondbiggest application is in industrial chemicalsynthesis, and third is electroplating togetherwith metal hardening.
In Europe the majority of cyanide productionis used in chemical synthesis. The secondlargest application is electroplating togetherwithmetal hardening, and the third is theminingindustry.
The differences in prices between NaCN andKCN are mainly reflected in the price differ-ences of the raw materials NaOH and KOH. AsKOH is significantly higher in price, KCN isalso more expensive than NaCN. Selling pricesin Europe for NaCN/KCN are also subject tofluctuation on the world market. Prices weremainly less than 1650 $/t forNaCN (100 %) andless than 3000 $/t for KCN (100 %).
2.2. Alkaline Earth Metal Cyanides
Properties and Uses. The properties of thealkaline earth metal cyanides differ from those ofthe alkalimetal cyanides. They are less stable anddecompose at elevated temperatures to releaseHCN. Even when alkaline earth metal cyanidesare in contact withmoist air or dissolved inwater,hydrolysis takes place, e.g.,
CaðCNÞ2þ2 H2O!CaðOHÞ2þ2 HCN
As a consequence, pure solid calcium cyanideis unstable, and the commercial product can onlybe manufactured in amorphous form with acontent of about 45 – 50% NaCN equivalent,known as black cyanide. As a solution, calciumcyanide is only sufficiently stable for commercialuse at concentrations below 17%.
Liquid and solid calcium cyanide are onlyproduced in South Africa, where they are used
688 Cyano Compounds, Inorganic Vol. 10
for the leaching of precious metals from ores[148].
Apart from this compound, only barium cya-nide has any commercial importance as a resultof its use in electroplating to separate carbonatefrom cyanides containing electrolytes [146].Some of the physical properties of these com-pounds are listed in Table 5.
Production. The alkaline earth metal cya-nides can be obtained by reaction of the corre-sponding metal hydroxides with hydrogen cya-nide [5]. However, only liquid calcium cyanide isproduced by this method. The resulting producthas a brownish color due to polymerization ofHCN, liberated by hydrolysis.
The production process of the commerciallyavailable solid calcium cyanide is different.Black cyanide is produced by treating calciumcarbide with nitrogen gas to give calciumcyanamide. The crude cyanamide is then mixedwith sodium chloride and heated in electricfurnaces above 1000 �C [149]. The cyanamideis converted into calcium cyanide, which isquenched to produce amorphous flakes. Thereaction product is gray-black because of thepresence of 1 – 2% carbon. The cyanide con-tent of the amorphous black cyanide is ca. 45 –50% NaCN equivalent. Other constituentsare 32% NaCN, 12% CaO, 2 – 3% CaCl2,and 2 – 3% CaNCN.
2.3. Heavy-Metal Cyanides
Heavy metals usually form stable, insoluble orsparingly soluble simple cyanides M (CN)x,which dissolve in the presence of excess alkalimetal cyanide to form stable complex salts [111]:
MðCNÞxþyCN�� ½MðCNÞxþy�y�
where x ¼ 1 � 3; y ¼ 1 � 4When dissolved in water, the complex
cyanide anions tend to dissociate, and variousdissociation equilibria are established. Thedissociation properties of the most importantheavy-metal cyanides are shown in Table 6.
2.3.1. Iron Cyanides
2.3.1.1. Properties
The most important iron cyanides are the hex-acyanoferrate(II), ferrocyanide, [Fe (CN)6]
4�,and the hexacyanoferrate(III), ferricyanide, [Fe(CN)6]
3�, complexes, which have an octahedralconfiguration. They are among the most stable
Table 5. Physical properties of the alkaline-earth-metal cyanides
Property Mg (CN)2 Ca (CN)2 Sr (CN)2 Ba (CN)2[4100-56-5] [592-01-8] [52870-08-3] [542-62-1]
Mr 76.37 92.12 139.67 189.40
mp, �C > 350 � 600
Decomp., �C > 300 � 640 > 500 > 600
Other crystal forms Mg (CN)2 � 2 NH3 Ca (CN)2 � 2 NH3 Sr (CN)2 � 4 H2O Ba (CN)2 � 2 H2O
Table 4. Specifications and average analysis of NaCN and KCN (data
given in wt% or ppm ¼ mg/kg; M ¼ Na or K)
NaCN KCN
DIN 50 971 Average
analysis
DIN 50 971 Average
analysis
MCN, % > 98.0 98 – 99
CN�, % 51.5 > 52.0 39.0 > 39.0
MOH, % � 0.5 � 0.1
M2CO3, % � 0.5 � 1.0
Cl�, % � 0.02 � 0.0015
SO2�4 , % � 0.05 < 0.01
SO2�3 , % < 0.05 < 0.01
S2�, ppm < 1 < 1
MOCN, % � 0.1 � 0.1
SCN�, % < 0.01 < 0.01
HCOOM, % � 0.5 � 0.3
H2O, % � 0.2 � 0.2
Sb, ppm < 5 < 5 < 5 < 5
As, ppm < 5 < 5 < 5 < 5
Pb, ppm < 50 < 50 < 50 < 50
Cd, ppm < 10 < 10 < 10 < 10
Fe, ppm < 50 < 50 < 50 < 50
Cu, ppm < 10 < 10 < 10 < 10
Ni, ppm < 50 < 50 < 50 < 50
Zn, ppm < 20 < 20 < 20 < 20
Sn, ppm < 80 < 80 < 30 < 30
Na, % < 48 < 0.5 < 0.3
Insolubles, % < 0.005 < 0.001 < 0.005 < 0.001
Vol. 10 Cyano Compounds, Inorganic 689
inorganic complexes and are practically nontoxicbecause of the strong bonding between iron andcyanide (for LD50 values, see 6). Practically noneof the known cyanide reactions can be observedwhen these compounds are dissolved in water.Only when irradiated with UV light does thehexacyanoferrate(II) ion decompose slowly atnormal temperature to form cyanide ions [151]:
In the absence of UV light, only very strongoxidants like ozone are able to oxidize the cya-nide and destroy the hexacyanoferrate(II) com-plex slowly at elevated temperature [152]:
Permanganate, peroxodisulfate, hypochlorite,chlorine, iodine, cesium(IV), and other oxidantsoxidize hexacyanoferrate(II) to hexacyanofer-rate(III) at moderate temperatures [153], e.g.,
5½FeðCNÞ6�4�þMnO�4 þ8Hþ!5½FeðCNÞ6�3�þMn2þþ4H2O
This reaction is used for the determinationof hexacyanoferrate (II) in water. Hydrogen per-oxide oxidizes hexacyanoferrate(II) to hexacya-
noferrate(III) below pH 3 and reduces hexacya-noferrate(III) to hexacyanoferrate(II) abovepH 7:
2½FeðCNÞ6�4�þH2O2þ2 Hþ ! 2½FeðCNÞ6�3�þ2 H2O
2½FeðCNÞ6�3�þH2O2! 2½FeðCNÞ6�4�þ2 HþþO2
When acidified with mineral acids, solutionsof hexacyanoferrates decompose slowly at roomtemperature and more rapidly when heated, re-leasing HCN and forming precipitates of iron(II)hexacyanoferrate(II) [14460-02-7] [153]:
Insoluble or sparingly soluble metal hexacya-noferrates(II) are formed by the addition ofaqueous solutions to neutral or weakly acidicsolutions of metal salts, particularly heavy metalsalts [153]:
4Mþþ½FeðCNÞ6�4� !M4½FeðCNÞ6�ðsparingly solubleÞ
2M2þþ½FeðCNÞ6�4� !M2½FeðCNÞ6�ðsparingly solubleÞThese reactions are used to separate heavy
metals from solutions for the purpose of refiningand for the production of colored pigments, e.g.,Prussian blue pigments, which contain the hex-acyanoferrate(III) anion:
MþþFe2þþ½FeðCNÞ6�3� !MFe½FeðCNÞ6�
M ¼ K, Na, NH4, CuMultinuclear complexes are formed by the
thermolysis of hexacyanoferrate(II) complexeswith complex cations [5]. Many anionic andneutral ligands can be introduced into thehexacyanoferrate anion by substitution ofone CN group, to form pentacyano complexes[4, 5].
½FeðCNÞ6�3�=4�þX! ½FeðCNÞ5X�2�=3�þCN�
where X ¼ H2O, NH3, NO, NO2, NO5, CO
½FeðCNÞ6�3�=4�þX� ! ½FeðCNÞ5X�3�=4�þCN�
where X ¼ SCN, NCS.Some of these compounds are used in analyti-
cal chemistry. Hexacyanoferrate(II) reacts with avariety of organic amino compounds to forminsoluble or slightly soluble salts of general
Table 6. Stability of heavy-metal cyanide complexes [4, 111]
Dissociation equilibria pKdissociation
[Pb (CN)4]2� a Pb2þ þ 4 CN� 10.3
[Cd (CN)4]2� a [Cd (CN)3]
� þ CN� 2.5
[Cd (CN)3]� a Cd2þ þ 3 CN� 14.7P
17.2
[Zn (CN)4]2� a [Zn (CN)3 ]
� þ CN� 1
[Zn (CN)3]� a Zn2þ þ 3 CN� 17.9P
18.9
[Ag (CN)2]� a Agþ þ 2 CN� 20.9
[Ni (CN)4]2� a Ni2þ þ 4 CN� � 22
[Cu (CN)4]3� a [Cu (CN)3 ]
2� þ CN� 1.5
[Cu (CN)3]2� a [Cu (CN)2]
� þ CN� 5.3
[Cu (CN)2]� a Cuþ þ 2 CN� 23.9P
30.7
[Fe (CN)6]3� a Fe3þ þ 6 CN� � 36
[Au (CN)2]� a Auþ þ 2 CN� � 37
[Hg (CN)4]2� a Hg2þ þ 4 CN� 40.5
[Fe (CN)6 ]4� a Fe2þ þ 6 CN� � 42
[Co (CN)6]4� a Co2þ þ 6 CN� � 64
[Pt (CN)4]2� a Pt2þ þ 4 CN� � 40
[Pd (CN)4]2� a Pd2þ þ 4 CN� � 42
690 Cyano Compounds, Inorganic Vol. 10
composition BA, B3A2, B2A, B3A, and B4A,where B is the cationic nitrogen compound andA the anionic hexacyanoferrate(II) [4]. Thesereactions are used in analytical and pharmaceu-tical chemistry. Hexacyanoferrate(III) is a strongoxidant which can dissolve metals; it is used inphotographic processes [154] and etching tech-niques [155]:
½FeðCNÞ6�3�þM! ½FeðCNÞ6�4�þMþ
The hexacyanoferrates(II) of sodium, potas-sium, and calcium crystallize with differentamounts of water of crystallization, whereaspotassium hexacyanoferrate(III) is obtained inanhydrous form. The hexacyanoferrates(II) startto lose water of crystallization above 30 �C andbecome anhydrous white powders above 80 �C.All hexacyanoferrates decompose above 400 �Cto form alkali metal or calcium cyanide, elemen-tal iron, carbon, and nitrogen, e.g.,
Na4½FeðCNÞ6� ! 4 NaCNþFeþ2 CþN2
The alkali metal hexacyanoferrates are readilysoluble in water; the solubility of the calciumsalt is more than twice that of the sodium salt(Table 7). Double salts of low solubility may beformed in the presence of Ca2þ and Kþ or NHþ4ions.
2.3.1.2. Production
Historically, the preparation of hexacyanoferrates(II) was based on the fusion of potash with ironcompounds and animal residues such as hide,horns, or dried blood, which led to the Germanname gelbes Blutlaugensalz (yellow salt of bloodlye). Later, the absorption of hydrogen cyanidefrom coal gas on iron hydroxide was used toproduce hexacyanoferrates. Today, synthetic hy-drogen cyanide, iron(II) chloride, and alkalimetal
or calcium hydroxide are the rawmaterials for thelarge-scale production of hexacyanoferrates(II)on a large scale.
Calcium hexacyanoferrate(II) is usually theprimary product, which is subsequently con-verted to the potassium and sodium salts. Liquidhydrogen cyanide and an aqueous solution of iron(II) chloride are mixed with calcium hydroxidesolution in stoichiometric amounts in a stirredreactor:
3 CaðOHÞ2þFeCl2þ6 HCN! Ca2½FeðCNÞ6�þCaCl2þ6 H2O
After filtration, the solution is concentrated byevaporation ofwater under reduced pressure, andthe calcium hexacyanoferrate(II) crystallizeswith elevenmolecules of water of crystallization.The relatively coarse-grained salt is then sepa-rated by filtration and generally used withoutdrying. For the conversion of calcium hexacya-noferrate(II) to the potassium or sodium salt, twomethods are used.
In the first, a stoichiometric amount of potas-sium chloride is added to the filtered calciumhexacyanoferrate(II) solution and the hexacya-noferrate(II) precipitates as a sparingly solublepotassium calcium double salt [156]
Ca2½FeðCNÞ6�þ2 KCl!K2Ca½FeðCNÞ6�þCaCl2The double salt is separated by filtration,
redispersed in water, and then converted to thesoluble potassium hexacyanoferrate(II) by theaddition of potassium carbonate:
K2Ca½FeðCNÞ6�þK2CO3!K4½FeðCNÞ6�þCaCO3
After separation of the insoluble calcium car-bonate, the potassium hexacyanoferrate (II) so-lution is concentrated by evaporation, and thepotassium salt crystallizes with three moleculesof water of crystallization. After filtration, thesalt is carefully dried and packaged.
A similar method can be used for the produc-tion of sodium hexacyanoferrate(II): sodiumcarbonate is added to the calcium hexacyanofer-rate(II) solution and the precipitated calciumcarbonate is separated:
Ca2½FeðCNÞ6�þ2 Na2CO3!Na4½FeðCNÞ6�þ2 CaCO3
Usually, the sodium hexacyanoferrate(II) issynthesized directly from solutions of sodiumcyanide and iron(II) chloride [157]:
6 NaCNþFeCl2!Na4½FeðCNÞ6�þ2 NaCl
Table 7. Solubilities of hexacyanoferrates in water (wt% of anhy-
drous salt)
Compound Temperature, �C
20 50 80
Na4[Fe (CN)6 ] 16 26 38
K4[Fe (CN)6 ] 22 32 40
Ca2[Fe (CN)6 ] 36 42 44
K3[Fe (CN)6 ] 31 39 45
Vol. 10 Cyano Compounds, Inorganic 691
The crystalline Na4 [Fe (CN)6] � 10 H2Ocan be obtained when the solution is concentrat-ed; it must be dried very carefully to avoid loss ofwater of crystallization, which would influenceits solubility properties.
The disadvantages of the double-salt processare the necessity of recycling the various motherliquors and the formation of large quantities ofsolid waste products.
In the second conversion method, the hexa-cyanoferrate(II) is precipitated from a solution ofthe calcium salt as Berlin white by the addition ofiron(II) chloride solution:
Ca2½FeðCNÞ6�þ2 FeCl2! Fe2½FeðCNÞ6�þ2 CaCl2This iron(II) hexacyanoferrate(II) is separated
by filtration, redispersed in water, and convertedto sodium or potassium hexacyanoferrate(II) bythe addition of stoichiometric amounts of sodiumor potassium cyanide:
Fe2½FeðCNÞ6�þ12 KCN! 3 K4½FeðCNÞ6�Potassium hexacyanoferrate(III) can be
obtained by the oxidation of potassium hexacya-noferrate(II). Usually, anodic oxidation isapplied by means of nickel electrodes [158]:
2 K4½FeðCNÞ6�þ2 H2O! 2 K3½FeðCNÞ6�þ2 KOHþH2
Alternatively, hydrogen peroxide can be usedfor the oxidation of hexacyanoferrate(II) whenthe reaction is carried out below pH 3 [159]:
3 K4½FeðCNÞ6�þH4½FeðCNÞ6�þ2 H2O2
! 4 K3½FeðCNÞ6�þ4 H2O
The dry, ruby-red potassium hexacyanofer-rate(III) is obtained by concentration of thesolution at reduced pressure and crystallizationat ca. pH 7. The fact that H2O2 reduces hexa-cyanoferrate(III) to hexacyanoferrate(II) athigher pH must also be considered.
490.296, r 1.68 g/cm3, yellow monoclinic crys-tals, highly soluble in water (148.4 g in 100 gH2O at 20 �C).
PotassiumHexacyanoferrate(III) [13746-66-2], potassium ferricyanide, K3 [Fe (CN)6],Mr 329.25, r 1.858 g/cm3, orange to ruby red,monoclinic crystals, readily soluble in water(46.4 g in 100 g H2O at 20 �C).
The commercial products are fine, crystalline,free-flowing powders. Normally, they are pack-aged in polyethylene bags of 25-kg net weight,andmust be stored under cool, dry conditions andkept away from acids. Hexacyanoferrates are notdangerous goods according to appendix C ofthe EVO, GGVS, RID, ADR, and IMCO Code.The specifications and average analyses for thecommercially used hexacyanoferrates are listedin Table 8.
Uses and Economic Aspects. Sodium,potassium, and calcium hexacyanoferrates(II)are equally suitable for use in most applications,but sometimes one product may be preferred.For the production of blue pigments, the mostimportant application of hexacyanoferrates, thepotassium salt is used in Europe, whereas thesodium salt is preferred in the United States.Generally, blue hexacyanoferrate(II) com-pounds are formed by the reaction of iron(III)salts with hexacyanoferrates(II):
FeCl3þK4½FeðCNÞ6� ! FeCl2þK3½FeðCNÞ6�þKCl
!KFeðIIIÞ½FeðIIÞðCNÞ6�þ3 KCl
The production of blue pigments is morecomplicated, however, and is based primarily onthe oxidation of iron(II) hexacyanoferrate(II)precipitates in the presence of alkali metal or
692 Cyano Compounds, Inorganic Vol. 10
ammonium salts [2]. The properties of bluepigments are not dependent only on the compo-sition; the precipitation method and the finish-ing of the product are also very important. Theuse of hexacyanoferrates(II) for the productionof classified documents is also based on thisreaction. Large amounts of all three types ofhexacyanoferrates (II) are used as additives forrock salt to prevent caking [160]; hexacyano-ferrates(II) act as crystal growth inhibitors inthis application. The ability of hexacyanofer-rates(II) to form insoluble or almost insolubleheavy metal compounds is used, e.g., in theproduction of citric acid by fermentation[161], in the refining of wine [162], in theelectroplating of tin [163], and in analyticalchemistry. Potassium hexacyanoferrate(III)and alkali hexacyanoferrate(II) combined withperoxodisulfates play an important role asbleaching agents in color photography [154].Both hexacyanoferrates are also importantdepressants for the separation of molybdenumfrom copper by flotation [164].
2.3.2. Cyanides of Copper, Zinc, andCadmium
Properties.Copper(I) Cyanide [544-92-3], CuCN, Mr
89.56, mp 473 �C, forms white monoclinic pris-matic crystals, r 2.92 g/cm3,which are insoluble
in water, dilute mineral acids, and organic sol-vents; CuCN is soluble in ammonia and in alkalimetal cyanide solution because equilibria areestablished with highly soluble cyano com-plexes:
CuCNþ2 NaCN�Na2½CuðCNÞ3�þNaCN
ðslightly solubleÞ ðslightly solubleÞ
Na½CuðCNÞ2�þNaCN�Na2½CuðCNÞ3�þNaCN
�Na3½CuðCNÞ4�
ðslightly solubleÞ
These complexes can also be obtained assolids. In the absence of air and moisture, pureCuCN can be stored indefinitely withoutdeterioration.
Zinc Cyanide [557-21-1], Zn (CN)2, Mr
117.42, mp 800 �C (decomp.), forms a whitecrystalline powder with a slight HCN odor,r 1.852 g/cm3 (25 �C), which is insoluble inwater but soluble in aqueous solutions of alkalimetal hydroxides or cyanides by formation ofhydroxy ([Zn (OH)4]
2�) or cyano complexes([Zn (CN)4]
2�). In the absence of air, carbondioxide, andmoisture, zinc cyanide can be storedindefinitely without deterioration.
Cadmium Cyanide [542-83-6], Cd (CN)2,Mr 164.45, mp 200 �C (decomp.), forms white
(25 �C), which are soluble in water (1.7 g in100 g H2O at 15 �C). Cyano complexes areformed when cadmium cyanide is dissolved inaqueous solutions of alkali metal cyanides: [Cd(CN)3]
�, [Cd (CN)4]2�. In the absence of air and
moisture,pureCd (CN)2 isstableandcanbestoredindefinitely; in moist air, it decomposes slowly.
Production. The preferred method for theproduction of copper cyanide is based on thereaction of copper(II) salts, metallic copper, andhydrogen cyanide in hydrochloric acid solution[165, 166]. The reaction solution can be recycled,which avoids environmental problems. The Cu(CN)2 formedwhen copper(II) salts react directlywith alkali cyanide or HCN is unstable anddecomposes to CuCN and cyanogen, (CN)2 (seeSection 5.2).
Zinc cyanide can be obtained from the reactionof zinc salts with alkali cyanide in aqueoussolution.However, the reaction of zinc oxidewithhydrogen cyanide in acetic acid has the advantagethat no additional salt is produced [167].
Cadmium cyanide can be produced by evap-oration of a solution of cadmium hydroxide inaqueous hydrogen cyanide [168] or by precipita-tion from a cadmium salt solution with alkalicyanide [169].
Commercial Forms, Specifications, andPackaging. Commercially, CuCN and Zn
(CN)2 are supplied as white, fine crystallinepowders, packaged in steel drums of 10 kg,25 kg, and 50 kg net weight. Also 1 t packages,as described for the alkali metal cyanides areavailable. The specifications and the averageanalyses are shown in Table 9.
Water-soluble, complex copper and zinccyanides, as well as mixtures of CuCN and Zn(CN)2 with alkali metal cyanides (containing ca.23 or 29% Cu or 23% Zn), are available for usein electroplating processes.
For transportation, storage, and handling, thesame precautions must be observed as for alkalimetal cyanides.
Uses and Economic Aspects. The simplecyanides of copper and zinc are used primarily incombination with alkali metal cyanides in bathformulations for the electroplating industry. Themain applications are copper, zinc, and brassplating.
Copper(I) cyanide is an important reagentfor the introduction of cyano groups into aro-matic rings to produce benzonitrile derivatives(Sandmeyer reaction). Furthermore, it is usedin the synthesis of insecticides and fungicides[170], in the preparation of special paintsfor ships, as a catalyst for the polymerizationof organic compounds, and for the synthesisof anticancer drugs. Copper and zinc cyanideare used in the production of phthalocyaninedyes.
Table 9. Specifications (DIN 50971) and average analysis of the cyanides of copper, zinc, and cadmium
CuCN Zn (CN)2 Cd (CN)2
DIN 50 971 Average analysis DIN 50 971 Average analysis DIN 50 971 Average analysis
2.3.3. Cyanides of Mercury, Lead, Cobalt,and Nickel
The cyanides of mercury, lead, cobalt, and nickeldo not have any major industrial importance.Water-soluble complex cyanides can be obtainedby the reaction of the simple cyanides with alkalimetal cyanides.
Properties, Preparation, and Uses.Mercury(II) Cyanide [3021-39-4], Hg (CN)2,Mr 252.65, r 4.0 g/cm3, forms white crystals,which are very soluble in water; it is highlytoxic. The substance can be prepared by thereaction of HgO with aqueous HCN. Whenheated, it decomposes to Hg and (CN)2,and may explode if detonated [168, 171].The complex salts Na[Hg (CN)3], Mr 301.66,and K2 [Hg (CN)4] [591-89-9], Mr 382.87, arewhite crystalline powders which are verysoluble in water. Mercury (II) oxide cyanide[2040-54-2], Hg (CN)2 � HgO, Mr 469.26, isless stable and more sensitive to shock thanpicric acid [172].
Lead Cyanide [13453-58-2], Pb(CN)2, Mr
259.25, can be obtained by the reaction of aque-ous solutions of lead salts and alkali metalcyanides in the presence of HCN [2]. In theabsence of HCN, oxide cyanides are formed,e.g., Pb(CN)2 � 2 PbO.
Cobalt(II) Cyanide [542-84-7], Co (CN)22 H2O, Mr 110.95, mp 280 �C (decomp.), r1.872 g/cm3, forms red-brown needles which areinsoluble in water. It can be obtained by thereaction of aqueous solutions of cobalt salts andalkali metal cyanides, and can be converted tosoluble complex compounds with ammonia orexcess alkali metal cyanide.
Complex cobalt(III) cyanides are formedwhen solutions of complex cobalt(II) cyanidesare heated in the presence of oxygen:
2 K4½CoðCNÞ6�þH2Oþ1=2 O2! 2 K3½CoðCNÞ6�þ2 KOH
Cobalt cyanides are among the most stablecomplexes; therefore, cobalt compounds havebeen used as antidotes for cyanide poisoning[173].
Nickel Cyanide [557-19-7], Ni (CN)2, Mr
110.74,mp> 200 �C (decomp.), r 2.393 g/cm3,
forms brownish-yellow crystals which are insol-uble in water. It can be obtained by the dehydra-tion of Ni (CN)2 � 4 H2O, which is precipitatedwhen alkali metal cyanide is added to a solutionof a nickel salt.
258.97, forms orange-red crystals which arevery soluble in water. The complex salt isformed when excess KCN is employed in thepreparation; K2 [Ni (CN)4] is used as an addi-tive to electrolytes in the plating of gold,silver, and zinc, to improve the quality of thedeposits.
2.3.4. Cyanides of Precious Metals
The precious metal cyanides can be regarded asthe historical starting point of inorganic cyanidechemistry. Their importance for the recovery,refining, and electroplating of precious metals isas great today as it was 100 years ago.
Gold(I) Cyanide [506-65-0], AuCN, Mr
222.98, r 7.12 g/cm3, forms yellow crystalswhich are insoluble in water and hot diluteacids, but soluble in ether, alkali metal hydro-xide, and thiosulfate solutions. Water-soluble[Au(CN)2]
� complexes are formed when AuCNis dissolved in alkali metal cyanide solution orwhen gold metal is attacked by alkali metalcyanide in the presence of oxygen or oxidizingagents.
Silver(I) Cyanide [506-64-9], AgCN, Mr
133.89, mp 320 �C (decomp.), r 3.95 g/cm3,forms white hexagonal crystals which are insol-uble in water but soluble in aqueous alkali metalcyanide solution, forming complex [Ag (CN)2]
�
compounds. Like AgCl, AgCN becomes brownif exposed to light. The production of AgCN isbased on the reaction of AgNO3with alkali metalcyanide in aqueous solution. Its main applicationis in electroplating.
Platinum(II) Cyanide [592-06-3], Pt(CN)2, Mr 247.27, is insoluble in water and
Vol. 10 Cyano Compounds, Inorganic 695
dilute acids. Both palladium(II) cyanide andplatinum(II) cyanide can be prepared from thecorresponding chlorides by precipitation withHg (CN)2. Tetracyano complexes are formedwith excess alkali metal cyanide: K2 [Pd (CN)4] � 3 H2O forms white rhombohedral fluo-rescent crystals, and K2 [Pt(CN)4] � 3 H2Oforms rhombic yellow prisms with blue fluores-cence. The barium salt Ba [Pt CN)4] � 4 H2O isused to prepare the fluorescent coating of X-rayscreens; the alkali metal tetracyanoplatinatesare used in electroplating. The unstable Pd(CN)4 is also known.
2.4. Cyanide Analysis
The cyanide content of solid alkali metalcyanides, alkaline earth metal cyanides, anddissociating heavy metal cyano complexes suchas [Zn(CN)4]
2� may be determined by theargentometric titration of a dilute aqueoussolution of the cyanide, according to the methodof Liebig – Denig�es, in which potassium iodideis used as an indicator [174]:
2 CN�þAgþ ! ½AgðCNÞ2��
A very accurate indication of the end pointof this reaction can be obtained potentiometri-cally by using a silver – calomel or silver –Thalamide (Schott Glaswerke,Mainz) electrodepair. This method of determination is called thedetermination of free cyanide and is also animportant parameter in the leaching of gold andsilver ores.
For more stable cyano complexes or insolublecyanide compounds [e.g., iron(II) hexacyanofer-rate(II)] or in the presence of substances thatdisturb the argentometric titration, liberation andseparation of the cyanide must be carried outbefore determination. This can be done byacidifying and boiling the cyanide solution orsuspension, and absorbing the released hydro-gen cyanide in sodiumhydroxide solution [175].Depending on the concentration, the cyanidecontent of the absorption solution may be ana-lyzed argentometrically or colorimetrically,e.g., by using pyridine-barbituric acid reagents[176]. This method of determination is calledthe determination of total cyanide, because allexcept extremely stable cyanide complexes,
such as that of cobalt, liberate cyanidefor determination. There is an ISO standardfor total cyanide determination (ISO/DIS6703/1) [177]. The ASTM determination meth-od for total cyanide is also used internationally[178].
To evaluate the toxicity of wastewaters andeffluents that contain cyanide, readily liberablecyanide (ISO/DIS 6703/1) is a reliable methodfor determining less stable complexes of cya-nide. Hydrogen cyanide, alkali and alkalineearth metal cyanides, and the cyanides of zinc,cadmium, silver, mercury, copper, and nickelare determined by this method. The complexcyanides of iron, cobalt, gold, and nitriles [177]are not amenable to this method. In this method,hydrogen cyanide is liberated at pH 4 in thepresence of zinc metal and ethylenediaminete-traacetic acid. The HCN is transferred to anabsorption vessel and analyzed argentometri-cally or colorimetrically. The ASTM methodfor the determination of less stable complexesof cyanide is called the weak acid-dissociablecyanide method [178]. Another fast determina-tion method of cyanide analysis, similar to theabove two distillation methods, is the colori-metric picric acid method [179]. This methodhas gained in popularity since the 1970s, ityields very similar results to the above twostandard methods but is much quicker. This isimportant, where effluents are discharged con-tinuously (e.g., gold mine effluent).
Other cyanide determination methods suchas the colorimetric method with pyridine –barbituric acid reagents [176] and ion-selectiveelectrode [180] methods for free cyanide areprone to interference by various substances. Theion-chromatographic method has been a subjectof research since the 1980s, and techniques havebeen developed to determine several cyanidespecies selectively, especially at low concentra-tions. However, these techniques and instru-mentation have not yet achieved widespreadapplication [180].
Quick tests to check the concentration ofcyanides in wastewater are based on variouscolorimetric methods [176, 179]. Generally, thereactions permit detection of very small amountsof free cyanide, but they can be disturbed byvarious substances, e.g., reducing agents or thio-cyanate, sulfide, and nitrite, which all producesimilar colors.
696 Cyano Compounds, Inorganic Vol. 10
3. Detoxification of Cyanide-Containing Wastes
Whenever cyanides are manufactured or used,effluents and wastes containing various amountsof cyanide are produced. Because of the hightoxicity of cyanide to all forms of life (seeChap. 6), the effluents andwastesmust be treatedto reduce the cyanide content to concentrationsthat are acceptable with the regard to the partic-ular environmental conditions [181–185].Depending on the quantity and type of cyanidicwaste, various detoxification methods are used[186–190]. In addition to effectiveness andcost of treatment, the formation of undesirablebyproducts and additional salting of the waste-water are factors of growing importance inchoosing an effluent-treatment method.
Heavy metals may be precipitated and sepa-rated as hydroxides, carbonates, or sulfides, whencomplex heavy metal cyanides are oxidized.Wastewater treatment can be carried out batch-wise or continuously, and the process may bemonitored and controlled automatically.
3.1. Wastewater Treatment
Chlorination. Alkaline chlorination is themost frequently used process for the treatment ofeffluents containing< 1 gCN� per liter [191]. Inprinciple, this method allows the destruction ofall commercially used simple and complex cya-nides, with the exception of complex iron cya-nides, which are only attacked above 80 �C. Thetreatment can be carried out with chlorine andalkali [NaOH, Ca(OH)2] or with readymade hy-pochlorite solutions that contain about 12%NaOCl. At first, toxic cyanogen chloride isformed, which is hydrolyzed quickly to cyanateand chloride at pH > 11 [192]:
CN�þOCl�þH2O! ClCNþ2 OH�
ClCNþ2 OH� !OCN�þCl�þH2O
Additional salting of the water is the maindisadvantage of alkaline chlorination: at least5 kg ofNaCl is produced per kilogramof cyanidefor the oxidation of cyanate, and an additional6.5 kg of NaCl per kilogram of cyanide resultsfrom total oxidation [193]. Alkaline chlorination
is frequently used for the treatment of smallerquantities of effluent and less for large continu-ous operations. It is frequently used, for example,in the detoxification of electroplating effluents,when permissible. Lower chloride limits in thetreated effluent make an alternative process nec-essary. Hypochlorite solutions tend to lose theiractive oxygen content relatively fast.
Hydrogen Peroxide Oxidation. The use ofhydrogen peroxide for cyanide detoxification hasincreased because, in this case, the oxidation ofcyanide leads directly to cyanate without theformation of toxic intermediates and byproducts,and the oxidant does not cause additional salting[193, 194]:
CN�þH2O2!OCN�þH2O
The H2O2 process is used mainly for thebatchwise treatment of effluents from organicnitrile synthesis and metal hardening plants, andcan be monitored and controlled automatically[195, 196]. One process allows delivery of accu-rate amounts of H2O2 to continuous effluentstreams, which is controlled by on-line measure-ment of the total oxygen demand [197]; thisprocess is used, for example, in the treatmentof tailings from gold mines [180, 198]. Morerecently, the H2O2 process has increasinglyfound applications in the seasonal detoxificationof effluents from gold mines, because of the easeof operation and relatively low investment costs.The H2O2 process has been optimized by theaddition of compounds that reduce the quantitiyof H2O2 required [199].
Other Methods of Treatment. A methodfrom 1982 is based on the treatment of effluentswith amixture of sulfur dioxide (2.5%) and air inthe presence of small amounts of copper salts(> 50 mg Cu per liter) as catalyst [205]:
CN�þO2þSO2þH2O!OCN�þH2SO4
This is called the Inco process. The detoxifi-cation of cyanide-containing pulps is becomingmore and more important. In addition to the Incoprocess method above, the Caro’s acid process isalso used to create cyanide-containing pulpsaccording to the reaction:
CN�þH2SO5!OCN�þH2SO4
Vol. 10 Cyano Compounds, Inorganic 697
A further development uses the synergiesbetween the Inco process and the Caro’s acid orhydrogen peroxide process to detoxify cyanide-containing slurries more efficiently. This processis called the CombiOx process [206].
In the case of highly concentrated cyanidesolutions, the volatilization and catalytic oxi-dation of HCN after acidification of the solu-tions can be used [189]. Sometimes, the hydrogencyanide can be reused by absorption in alkalimetal hydroxide solution and conversion toalkali metal cyanide. Atmospheric oxygen isoccasionally used for the destruction of cyanidein weak solutions, in combination with activatedcarbon [101, 201] or microorganisms [202].Removal of cyanides by ion exchange and reverseosmosis does not solve the problem entirelybecause the concentrates require further treatment[203, 204].
The hydrolysis of cyanide at 180 – 230 �Cunder pressure can be used for the destructionof simple and complex cyanides, even in highconcentrations [98]. The process does not requirechemicals, but the investment for the plant ishigh.
The conversion of cyanide to thiocyanate bythe addition of sulfur is a process that used to beemployed but is now regarded more critically[186]. This process has the advantage of lowreagent costs, but the investment for plant canbe high. It is less suitable for seasonal detoxifi-cation, since start up and decommissioning cantake time.
Very effective oxidation of cyanide to cya-nate, carbonate, and nitrogen takes place withozone [98, 200]; however, this requires highcapital investment and causes problems with theadaptation to varying oxygen demand.
The conversion of cyanide to hexacyanofer-rate(II) by reaction with iron(II) salts in alkalinesolution followed by precipitation of hexacya-noferrate(II) as iron(II) hexacyanoferrate(II)at pH 3 – 4 is one of the oldest methods forcyanide removal [207]. However, this should beused only for the removal of complex ironcyanides, if at all, because the sludge must bedisposed of in such a manner that waterresources are protected. Furthermore, thefiltrates must be treated again to destroy theremaining traces of cyanide.
In many cases, low concentrations of com-plex iron cyanides (e.g., < 20 mg of cyanide
per liter) in effluents discharged to sewagesystems are tolerated because they are precipi-tated in contact with metal salts (e.g., the ironsalts present in domestic wastewater) and areseparated with the sludge of municipal sewagetreatment plants.
3.2. Solid Wastes
For a long time, disposal of the solid wastesfrom metal hardening plants, which containalkali metal cyanides, cyanates, nitrites, andnitrates in addition to other inorganic salts, wasa special problem [88, 89]. New processes havebeen developed that, in many cases, allowregeneration of waste salts. In any case, solidwastes should not be treated or destroyed by awet chemical process. The safest and mostpractical solution to the disposal problem atthe present time is to deposit them in disusedsalt mines.
4. Cyanogen Halides
Halogen cyanides, usually known as cyanogenhalides, have been the subject of considerableinterest, and some aspects of their chemistry havebeen reviewed in detail [2, 6–11]. These com-pounds, except for cyanogen fluoride, have beenknown for a long time [208, 209]. Cyanogenfluoride was prepared in 1981 [210, 211].Depending upon the electronegativity of thehalogens relative to the cyanide group, the cyan-ogen halides can act as halogenating or cyanatingagents. The cyanogen halides can be regarded aspseudohalogen or interhalogen compoundswith adefinite nitrile structure (X�C�N) [212]. Themost important cyanogen halide is cyanogenchloride, which is used for the commercial pro-duction of cyanuric chloride, a starting materialfor pesticides, dyes, and drugs (! Cyanuric Acidand Cyanuric Chloride).
4.1. Properties
Physical Properties. The pure cyanogenhalides are colorless, highly volatile, and poison-ous compounds. Some physical properties aregiven in Table 10.
698 Cyano Compounds, Inorganic Vol. 10
Chemical Properties. Pure cyanogenhalides are stable at normal temperature. Impuri-ties catalyze the exothermic trimerization to thecorresponding cyanuric halides. Catalysts areproton acids, Lewis acids (e.g., heavymetal salts),and bases. Relatively little is known about thepreparative use of FCN [211], which explodes at� 41 �C on ignition.Mixtures of FCN and air aremore explosive thanmixtures of acetylene and air[213]. In a catalytic reaction,ClCN formsnot onlythe trimer (cyanuric chloride [108-77-0]) but alsoa stable tetramer, 2,4-dichloro-6-isocyanodi-chloro-s-triazine [877-83-8] [214].
Cyanogen bromide polymerizes more readilyto cyanuric bromide [14921-00-7] than doesClCN to cyanuric chloride [215].
When heated to 130 �C in a sealed tube,cyanogen iodide does not polymerize [216].Above 150 �C, ICN eliminates iodine:
2 ICN!ðCNÞ2þI2The reaction is not reversible [217].Cyanogen chloride is an intermediate in the
detoxification of cyanide-containing wastewaterwith sodium hypochlorite solutions; therefore,the cyanogen halide must be removed immedi-ately by appropriate hydrolysis [218]. Alkaline
hydrolysis of cyanogen halides proceeds in dif-ferent ways:
XCNþ2 NaOH!NaOCNþNaXþH2OðX ¼ Cl ;BrÞ
ICNþ2 NaOH! ½NaOI�þNaCNþH2O
Thus, ClCN and BrCN are hydrolyzed to thecyanate and halide salts; ICN, however, formshypoiodite, which disproportionates to iodideand iodate. This is due to themore electropositivecharacter of iodine relative to chlorine [11].Therefore, ClCN can be used for the electrophilicintroduction of a cyano group into a substrate; useof ICN results in electrophilic iodination. Thereaction pathway of BrCN often depends onthe reaction conditions and on the nature of thesubstrate [219]. Cyanogen halides add to olefinsand acetylenes in the presence of acid catalysts[220, 221]. Aromatic ring systems can be cya-nated by ClCN or BrCN under Friedel – Craftsconditions [212, 223]. The reactions of cyanogenhalides, particularly ClCN and BrCN, withnitrogen, oxygen, and sulfur nucleophiles leadto cyanamides [224–226] cyanates [227, 228],and thiocyanates [229].
In some cases, the newly formed compoundundergoes further reactions at the C�N triplebond to form guanidines [224, 225], ureas, orheterocyclic ring systems [230]. Cyanogenbromide reacts with tertiary alkylamines to formammonium bromides, which decompose to givedialkyl cyanamides and alkyl bromides (vonBraun reaction) [231, 232]. Reaction of sulfurtrioxide with ClCN leads to the very reactive
Table 10. Physical properties of the cyanogen halides [10, 11, 212]
Enthalpy of vaporization DHv, kJ/mol 22.39 26.75 33.82 40.02
Enthalpy of fusion DHf, kJ/mol 11.39 19.88
Enthalpy of sublimation DHs, kJ/mol 45.21 59.90
Dipole moment m, D 2.17 2.80 2.94 3.71
*Triple point, p ¼ 132.1 kPa; subl. ¼ sublimation.
Vol. 10 Cyano Compounds, Inorganic 699
chlorosulfonyl isocyanate [233, 234]. Cyanogenhalides undergo radical reactions at elevatedtemperatures on irradiation with UV light or inthe presence of peroxide promoters. Thus, cyan-ogen chloride and acetonitrile are convertedto malonodinitrile [235–238]. In the gas phase,aromatic methyl compounds and cyanogenchloride react to give arylacetonitriles [239].
4.2. Production
Cyanogen fluoride is prepared by the reaction oftetracyanomethane with cesium fluoride [240]:
CðCNÞ4þCsF! FCNþCsCðCNÞ3The pyrolysis of cyanuric fluoride at 1300 �C
under reduced pressure gives FCN in ca. 50%yield [210, 211, 213].
The preparation of the industrially importantClCN has been developed extensively; the syn-thetic pathways are as follows:
1. Electrolysis of an aqueous solution of HCNand NH4Cl [241]
2. Reaction of complex cyanide salts (e.g., Na2[Zn (CN)4]) with chlorine below 20 �C [242]
3. Formation of ClCN from cyanide salts (most-ly NaCN) and chlorine in an exothermicreaction; in a continuous process, sprayedaqueous NaCN solution is contacted withchlorine, and the reaction heat evaporates theClCN [243].
4. Processes involving hydrogen cyanide andchlorine as the most convenient starting ma-terials; the reaction is carried out in aqueoussolution [244–249] (for a detailed procedure,see also ! Cyanuric Acid and CyanuricChloride) in organic [250] and inorganic sol-vents [251], and in the gas phase [252, 253].To avoid the byproduct HCl, attempts havebeen made to reoxidize HCl to chlorine withoxygen [248] and hydrogen peroxide [254,255] in catalyzed reactions.
5. Chlorinolysis of cyanogen in the gas phase at300 – 600 �C in the presence of a catalyst[256, 257]
6. Pyrolysis of cyanuric chloride at 600 –900 �C in the presence of a charcoal catalyst[258]
7. High-temperature syntheses based on elemen-tal chlorine, nitrogen, and carbon [259]
Usually, these processes lead to impureClCN;therefore, in many cases the product must bepurified before further use. Water can be re-moved by treatment with calcium salts or molec-ular sieves and by fractional distillation [260].Chlorine is strippedwithwater (mixedwithHCNand HCl) [261] or with solutions containing asoluble iron(II) salt and an insoluble carbonate[262]; chlorine may also be eliminated by treat-ment with g-aluminum oxide [263]. Cyanogenchloride, free of HCl, is obtained by treatmentwith water [261] or with organic solvents [264,265]. Hydrogen cyanide is removed by distilla-tion with chlorine [266] or by washing withFeSO4 solutions [267].
Cyanogen bromide can be prepared frombromine and cyanide salts or hydrogen cyanide[268, 269]. Cyanogen iodide is synthesized anal-ogously [270, 271].
4.3. Storage and Transportation
In most cases, ClCN is used in the gas phaseimmediately after preparation. Relatively smallamounts are condensed and stored in gas contain-ers as liquids. Steel cylinders, which must meetspecific requirements in each country, can beused for shipment. The condensed and bottledClCN must be very pure; moreover, it must bemixed with a stabilizing agent (generally sodiumpyrophosphate) to inhibit exothermic polymeri-zation caused by impurities [272–274]. ImpureBrCN can polymerize during storage [275]; thedanger of explosion exists with closed BrCNbottles [276]. Cyanogen halides should behandled carefully because they are very toxic(similar to HCN) and have a strong lacrimatoryeffect.
4.4. Uses
Most of the cyanogen chloride is used for theproduction of cyanuric chloride. The reactionwith amines leads to diphenylguanidines (vulca-nization accelerators) [277]. Cyanogen chlorideand bisphenols react to give cyanate esters, whichcan polymerize to polytriazine resins [227, 228].
Halogen cyanides and cyanamide react toform dicyanamide [504-66-5] [278, 279], whichis used for the preparation of pharmaceutical
700 Cyano Compounds, Inorganic Vol. 10
bisbiguanides. The industrial production ofmalonodinitrile [109-77-3] [235–238], andchlorosulfonyl isocyanate [1189-71-5] [233,234] based on cyanogen chloride has becomeincreasingly important. Cyanogen chloride is themost economically significant of all cyanogenhalides. Its worldwide production capacity ismore than 230 000 t/a (mainly in Europe andthe United States).
Cyanogen bromide and cyanogen iodideare commonly used in the laboratory, e.g., forthe dealkylation of tertiary amines [231, 232],the selective cleavage of peptides [280], and thepreparation of heterocyclic compounds.
5. Cyanogen
The first synthesis of cyanogen [460-19-5],dicyanogen, oxalonitrile, (CN)2, Mr 52.05, wascarried out by GAY-LUSSAC in 1815 by pyrolysisof silver cyanide [267]. The chemistry of thisreactive compound is discussed in [1, 12–17]. Inmany respects, cyanogen can be compared to thehalogens. In addition, many of its properties are aresult of the reactivity of the cyano groups.Although various procedures for the synthesisof cyanogen are known, commercial interest incyanogen is limited.
5.1. Properties [1, 17]
Physical Properties. At room temperature,cyanogen is a colorless, flammable, and verypoisonous gas with a pungent odor. Some impor-tant physical properties are listed below:
Melting point �27.98 �CBoiling point �21.15 �CCritical pressure, pcrit. 60.79 bar
Critical temperature, tcrit. 128.3 �CDensity 2.321 g/L at 0 �C
1.25 g/cm3 at �95 �C0.954 g/cm3 at �21.17 �C
Vapor pressure
t, �C �21.15 �4.88 20.88 44.43 72.40
p, bar 1 2 5 10 20
Solubility at 20 �C: water dissolves 4.5 timesits own volume of cyanogen; diethyl ether, 5.0times; and ethanol, 23.0 times.
Bond length:
C�C 138 pm
C�N 113 pm
Bond angle: C�C�N 179� 380
Enthalpy of fusion, DHf 8.112 kJ/mol
Enthalpy of vaporization 23.341 kJ/mol
MAK 10 ppm
The values reported for the ignition limits ofcyanogen – air mixtures vary widely, dependingon experimental conditions, e.g., with humid air(1.7% H2O); the reported values are 6.4 –7.25 vol% for the lower limit, and 26.2 –30.6 vol% for the upper limit.
Chemical Properties. Cyanogen has a highthermal stability; nevertheless at high tempera-tures (and when irradiated with UV light), poly-merization to paracyanogen occurs [17, 282,283]. With stoichiometric amounts of air oroxygen, cyanogen burns with the hottest flame(ca. 5000 K) ever observed in chemical reactions[284, 285].
Cyanogen undergoes reactionswhich dependeither on its properties as a pseudohalogen or onthe reactivity of the C�N triple bond [1, 15, 17]:
1. Hydrogenation of cyanogen at 550 – 675 �Cgives hydrogen cyanide [286].
2. At 300 – 500 �C, chlorine and cyanogen re-act to give cyanogen chloride and, subse-quently, cyanuric chloride in the presence ofan active charcoal catalyst [287, 288].
3. The direct introduction of a cyano groupinto an aryl or alkyl compound proceeds viaa radical- or heat-initiated reaction[289–292].
4. Two moles of hydrogen cyanide add to cyan-ogen to form diiminosuccinonitrile [293].
5. Cycloadditions (Diels – Alder) with organicdienes give cyanopyridines [294].
6. Alkaline hydrolysis leads to cyanide andcyanate [295].
7. The acid-catalyzed hydrolysis of cyanogenfirst gives cyanoformamide [296, 297] andthen oxamide and oxalic acid. Oxamide [471-46-5] is a nitrogen fertilizer with good depotactivity. Its industrial preparation has been
Vol. 10 Cyano Compounds, Inorganic 701
published in several papers; most of thesynthetic paths use the isolated (CN)2 as anintermediate [298–303], but in situ proce-dures are also known [304].
Heterocyclic compounds are formed by thereaction of cyanogen and disulfur dichloride[305] or sulfur [306]:
5.2. Production
Numerous synthetic pathways to cyanogen havebeen devised [1, 13, 15, 17]. In the laboratory, thecompound is normally prepared via cyanidesalts; in this case, the reaction of alkali metalcyanides with copper(II) salts is more commonthan the thermal decomposition of Hg(CN)2 orAgCN [17].
CuSO4þ2 KCN!CuðCNÞ2þK2SO42 CuðCuÞ2!ðCNÞ2þ2 CuCN
However, the oxidation of hydrogen cyanidein the presence of a catalyst has greater indus-trial interest. These reactions are based on theprinciple of JACQUEMIN [307], that is the con-version of cyanides, by treatment with coppersalts, to cyanogen in aqueous solution. In acontinuous process, the copper(I) salt formedmust be reoxidized rapidly to recycle the cata-lyst. Reoxidation of copper(I) to copper(II) canbe carried out directly with air or oxygen [308,309] or with NO2 [310] the NO formed in thesecond case can then be reoxidized with air oroxygen [311, 312]. The oxidation of hydrogencyanide to cyanogen in nonaqueous solutionshas also been achieved [313, 314]. Hydrogenperoxide is another reoxidation agent[315–318].
The continuous production of (CN)2 fromHCN and O2 or H2O2 is utilized industrially ona small scale.
In addition to the liquid-phase oxidation ofHCN, various gas-phase oxidations have beeninvestigated [319–324].
Cyanogen iodide decomposes to cyanogenand iodine (Chap. 4). In 1984 the reaction ofcyanogen halides with trimethylsilyl cyanide inthe presence of Lewis acids has been described[325].
Cyanogen prepared by the above procedurescontains impurities such as O2, N2, NO, CO2, orClCN. Its purification can be achieved by frac-tional vaporization of the crude material [326] orby scrubbing the gas with aqueous H2O2 [327].
5.3. Storage, Transportation, and Uses
Storage and Transportation. The flamma-bility and toxicity of cyanogen necessitate specialstoring and shipping conditions. Cyanogenshould be stored in cool, well-ventilated loca-tions, not near to flammable goods or openflames.Dry and pure (CN)2 can be stored in steel cylin-ders under pressure and at normal temperatures[328]. Comprehensive studies have beenmade onthe stability of pure cyanogen to heat, pressure,chemical additions, and severe mechanical shock[329]. Shipment must be in accordance withgovernmental regulations in each country.
Uses and Economic Value. Cyanogen isused mainly in the laboratory. In small quantities,cyanogen is used industrially for drug synthesis.The preparation of oxamide (a slow-releasefertilizer) has been carried out on a small scale[288]. Large-scale industrial applications are notyet known.
6. Toxicology and OccupationalHealth
Toxicity and Cyano Compounds. Hydro-gen cyanide and other cyano compounds that canformHCNor free cyanide ions are highly toxic tomost forms of life. The lethal dose depends on anumber of factors, and differs from species tospecies. A useful survey on this subject can befound in [184]. As shown in this report, thetoxicity of metal cyanide complexes is a functionof the bond strength between themetal atoms andthe cyanide ligands. Toxicity decreases with
702 Cyano Compounds, Inorganic Vol. 10
increasing bond strength, and complexes such ashexacyanocobalt(II) are nontoxic. Hexacyano-ferrates(II) and (III) also have very lowtoxicities [330]. These differences in toxicityare particularly important when the acceptableconcentrations of cyanides in wastewater arebeing considered. Some of the lethal concentra-tions are shown in Table 11. The LD50 values ofcomplex hexacyanoferrates (rat, oral) are asfollows:
Ca2 [Fe (CN)6 ] � 11 H2O > 5110 mg/kg
Na4 [Fe (CN)6 ] � 10 H2O > 5110 mg/kg
K4 [Fe (CN)6 ] � 3 H2O 3613 mg/kg
K3 [Fe (CN)6 ] 3503 mg/kg
Poisoning byHydrogenCyanide andAlkaliMetal Cyanides. Hydrogen cyanide is easilyabsorbed, even through intact skin and mucousmembranes. Not even full respiratory protectioncan prevent hydrogen cyanide from penetratinginto the organism. The speed of absorption oforally ingested cyanide depends on the contentsof the stomach and gastric pH; empty stomachand low pH lead to rapid absorption of cyanide.Symptoms [340] of the poisoning may varysomewhat, depending on the quantity of hydro-gen cyanide absorbed or the quantity of cyanideingested. Low concentrations of hydrogen cya-nide irritate the nasopharyngeal cavity; occa-sional complaints involve headache, anxiety,and nausea. Rapid inhalation of the lethal dose(ca. 50 – 100 mg) of hydrogen cyanide resultsin collapse of the individual. This apoplectiformsyndrome, characterized by a short spasmodicstage with subsequent respiratory paralysis,
results in immediate death. Poisoning symp-toms following oral ingestion of cyanides, onthe other hand, exhibit a slow progress, evenwith large doses (150 – 250 mg), because of theslow release of hydrogen cyanide. Case histo-ries described in the literature involve poisoningdurations up to several hours. Cyanide affectsmany enzymes. The inhibition of cytochrome coxidase is responsible for the acute effects andthe marked metabolic changes (lactic acidosis).As in other intoxication, treatment includesthe support of vital signs (supportive therapy),prevention of further poison absorption, andenhancement of poison elimination. The admin-istration of antidotes in severe cyanide poison-ing is mandatory, although this opinion has beencontradicted.
The following concentrations of hydrogencyanide and other cyanides in air are consideredacceptable at the workplace:
TLV–TWA (hydrogen cyanide) 10 mg/m3 [341]
MAK (hydrogen cyanide) 11 mg/m3 [342]
TLV–TWA (cyanide dust, as CN) 5 mg/m3 [341]
MAK (cyanide dust, as CN) 5 mg/m3 [342]
Therapy for Poisoning by HydrogenCyanide and Other Cyanides. Different anti-dotes or combination of antidotes are preferred indifferent countries. The mechanisms of antidotalaction follow different principles:
1. Direct complexation of the cyanide andexcretion as cyanide – antidote complex(hydroxocobalamin, dicobalt edetate)
2. Methemoglobin-forming compounds whichshift the cyanide from the intracellular to the
Table 11. Lethal concentrations of some cyanides (in mg/L)
3. Sulfate donors, which enhance the naturalpathway of detoxification (thiosulfate).
The first aid given to victims of hydrogencyanide poisoning is not fundamentally differ-ent from the measures taken in other acuteemergencies. The success of the treatment, how-ever, depends on the immediate start of antidotetherapy. Seconds may determine the success orfailure of the treatment of cyanide poisoning.The therapeutic measures required depend onthe particular poisoning syndrome encountered.The initial treatment is of supreme importanceand must be aimed at safeguarding vital func-tions such as respiration and circulation.
Cyanogen Chloride, Cyanogen Bromide[340]. Because of their high volatility, cyanogenchloride and cyanogen bromide are strongirritants to mucous membranes (eyes, lungs).Their effects are essentially identical to thoseproduced by hydrogen cyanide; however, theyare rarely encountered because the irritatingeffects serve as a warning of potential danger.When handling liquid cyanogen chloride, atten-tion must be paid to the potential for skinabsorption. The exposure limit of cyanogenchloride (TLV – TWA) is 0.6 mg/m3 [341].The therapy is analogous to that for hydrogencyanide poisoning.
Cyanogen [340]. Cyanogen irritates mu-cous membranes and is probably hydrolyzed inthe organism to hydrogen cyanide and cyanicacid. The symptoms are similar to those producedby hydrogen cyanide poisoning.
TLV – TWA 20 mg/m3 [341]
MAK 22 mg/m3 [342]
The therapy is analogous to that for hydrogencyanide poisoning.
References
General Reference1 Gmelin, System no. 14, D 1, Kohlenstoff (1971) .
2 H. E. Williams: Cyanogen Compounds, 2nd ed., Arnold
Co., London 1948.
3 M. H. Ford-Smith: The Chemistry of Complex Cyanides,
Dep. of Scientific and Industrial Research, Her Majesty’s
Stationery Office, London 1964.
4 A. G. Sharpe: The Chemistry of Cyano Complexes
of the Transition Metals, Academic Press, London
1976.
5 A. M. Golub et al.: Chemie der Pseudohalogenide,
H€uthig Verlag, Heidelberg 1979.
6 V. Migrdichian: The Chemistry of Organic Cyanogen
Compounds, Reinhold Publ. Co., New York 1947, pp.
97 – 124.
7 Houben-Weyl, 8, 90 – 93.
8 G. Brauer: Handbuch der pr€aparativen anorganischen
