Direct and Indirect Atomic Absorption Methods of Determining Various Forms of Iodine in Waters and in Aqueous Solutions
V. V. Sukhan, O. M. Trokhimenko, and V. N. Zaitsev
Shevchenko National University, Kiev, Ukraine
Received November 11, 2008
Abstract—The article provides data on direct and indirect (along the analytical lines Hg, Ag, Cu, Cd, Cr, Fe, Se, As) atomic absorption methods of determining various forms of iodine (iodide, elementary iodine, iodate, periodate, alkyl iodide) in waters, aqueous solutions of various objects of analysis after correspond�ing sample preparations. Indirect methods of determining have been classified into extraction and non�extraction ones with the use of reactions of precipitation and complexation.
DOI: 10.3103/S1063455X10020037
Key words: iodine, atomic absorption analysis, direct and indirect methods of determining various forms of iodine, water, aqueous solutions.
INTRODUCTIONS
A large ecological and biological significance of iodine stimulates perfection of the methods of its determi�nation. A special interest is determination of various forms of iodine in natural waters [1] and aqueous solu�tions of other objects of analysis after respective sample preparation [2]. In natural waters iodine is, mainly, in the form of iodide and, to a lesser degree, is present as iodate [3]. In a neutral solution these two forms of iodine coexist. In surface layers of waters, sea, and oceans in places of aggregation of algae in noticeable amounts there form halogen–derivative of hydrocarbons including those containing iodine (CH3I, CH2I2, CH2ClI, CH2BrI, CH3–CH2–CH2–CH2I, etc.), which enter the atmosphere and subsequently into weather precip�itations. As part of organic of organic compounds iodine is also contained in vegetable and animal organisms. A feedstock for commercial production of iodine is along with sea algae natural mineral waters and brines accompanying oil and gas deposits and fields [3]. Thus, in underground geothermal waters of the Crimean peninsular the iodine concentration constitutes 10–120 mg/dm3 and in the form of iodide—more than 90–95% [4]. When determining the total content of iodine in solid samples of analysis (ecological objects, food products, agricultural feedstock) depending on the methods of their preliminary sample preparation aqueous solutions are obtained containing various ionic forms of iodine: iodide after dry alkaline mineralization and iodate after humid acidic and alkaline mineralization.
Iodide is the most often used form for determination of iodine by direct [5] and indirect [6] methods of analysis. Determination of iodine in the form of its oxoforms, elementary iodine, or organic compounds con�taining iodine, is performed more rarely. However, using corresponding sample preparation for transferring one form of iodine into the other or various forms into one (total iodine) one obtains analytical results by any method of analysis or other forms of iodine.
The atomic absorption method (AA) is one of the most promising modern analytical methods of determin�ing metals [7–19]. Thanks to high accuracy of fast reaction and selectivity of determining many elements in dissolved solutions it became very popular. Among non�metals the indirect AA method is useful in determin�ing B, Si, As, Se, and Te. Most of commercial atomic absorption devices are suitable for registration of absorp�tion only in the visual and UV regions (up to ~ 190 nm) of the spectrum, while absorption of radiation (wave�lengths < 200 nm) by oxygen creates difficulties in analytical interpretation and reliability of experimental data [11]. Therefore inert gases, halogens, C, H, N, O, S, P. and other non�metals, whose resonance lines have the wavelength < 200 nm (the vacuum UV part of the spectrum) are not included into a number of elements being determined. Nevertheless, the direct AA method is used for determination of about 70 elements [12] including iodine.
78
DIRECT AND INDIRECT ATOMIC ABSORPTION 79
DIRECT AA METHODS OF IODINE DETERMINATION
Main spectral lines of iodine atoms (178.30; 183.00; 184.40; 187.60, and 206.24 nm) except line 206.24 nm are in the vacuum UV portion of the spectrum [13–25]. The most intensive among them is the emission–absorption resonance line at 183.00 nm, which corresponds to electron transition 4P5/2 → 2P3/2 between elec�tron states respectively 6s and 5p. Since the main resonance line of iodine is in the vacuum UV region of the spectrum, its use as an analytical line for the direct AA method is experimentally difficult.
Direct AA methods of determination of various forms of iodine are given in Table 1. For improving the ana�lytical signal : noise ratio one minimizes the optical route and wash it with nitrogen or argon. Iodine atomiza�tion is carried out by a flame or electrothermal method. In this case commercial discharge lamps are used just as lamps with a hollow cathode [13] and electrodeless ball lamps [14] made in laboratory conditions [14–18, 23–25]. The use of self�adjusting lasers is limited by their high costs [22].
Table 1. Direct atomic absorption methods of iodine determination in model solutions*
Iodine form being determined
λ, nm; atomizerFeatures of sample
preparation and atomization
Metrological characteristics
of the technique
Impact of accompanying components Refs.
I– 183.00; lamp with a hol�low cathode; graphite oven
Harmful influence of Fe(III), Co(II), V(V) is eliminated by passing the analyte solution through a cation–exchange resin No harm of: Ni, Mn, Cr, Zn, Mg, Na, K, Ca, Al, SO4
2–, PO4
3–. Harm: SCN–, S2– Hg(II)
[15]
I–, IO3– 183.00; electronless dis�
charge tube; flame N2O–C2H2
Laiperth multiplication
reaction: I–
IO3– I2. Iodine is
extracted by methylisobu�tylketone and the extract is reduced to flame
– Harm: Cu(II), Fe(III), Y(III)
[16]
″ 183.00; through a plati�num loop, set before the entrance slot of the monochromator, elec�tric current is passed for evaporation of water and atomization of the ana�lyte. Light source: the electronless lamp cooled by the N2 flow
Atomic steam is produced at 1227°C by evaporation of the film being formed on the Pt�loop submerged in the analyte solution. Oxygen is removed by blowing the Ar or N2 opti�cal system
For removing the harmful impact of a number of anions and cations ion–exchange separation and extraction are carried out. F–, Cr(III), Al(III) are harmful
[17]
″ The resonance (183.00) and less sensitive nonres�onance (206.24 nm) lines are used. The last line does not require the cre�ation of the inert atmo�sphere on the way of the rays being absorbed. Light source is a elec�tronless discharge tube. Graphite oven
Drying: 60 s at 125°C and 30 s at 400°C. Atomiza�tion—3 s at 2100°C
C0 = 0.4 μg/cm3; RSD 7% at the concentration of iodide 10 ng/cm3 (183.00 nm); RSD 7%— at 100 ng/cm3 (206.1 nm)
Impact of foreign matter is substantially decreased with correction of the background. Harmful are: Br–, PO4
3–, Ca2+
[18]
→bromine
→iodide
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80 SUKHAN et al.
Note: C0 is the lower boundary of the concentrations being determined; RSD is relative standard deviation; GG is graduation graph.
I2 206.24; Lamp with the hollow cathode; flame air—C2H2
Not indicated Linearity—GG 2–40 μg/cm3
– [19]
I2 183.00; dismountable lamp with the hollow cathode for determina�tion of volatile elements; flame N2O–C2H2, enriched with N2
An advantage of the dis�mountable lamp com�pared with a sealed off one—an increase of radi�ation intensity when passed through an argon lamp
C0 =14 μg/cm3; linearity—GG up to 22 μg/cm3
– [20]
I2 183.00; 206.24; electron�less discharge tube; flame air—C2H2 or N2O–C2H2, enriched with Ar
When using the line 183.00 nm the optical sys�tem is washed with Ar
Optical system is washed with Ar at the pressure 533.3 Pa
C0 = 3 μg/cm3 I– or I2 RSD 4% at iodide concentra�tion 10 mg/cm3; GG linearity—up to 15 μg/cm3; sample volume 5 cm3
– [22]
I–, IO3– 178.30; 183.00; electron�
less discharge tube; graphite oven
Radiation is focused on the graphite tube by means of fluorite lenses. The graphite tube is blown through with nitrogen. Radiation is recorded by means of a photoelec�tronic multiplier and an amplifier with a photosen�sitive detector
C0 = 0.04 μg/cm3; GG linearity—up to 6 μg/cm3
(183.00 nm). C0 = 0.02 μg/cm3; GG linearity—up to 6 μg/cm3 (178.30 nm). Measure�ment accuracy is lower at 178.30 nm, injection vol�ume for both wavelengths—10 μl
Na+, K+, Cl–, NO3–,
PO43– cause nonselective
absorption, which may be taken into account by mea�suring absorption of the nonresonance iodine line 184.40 nm. At 1000�fold excesses of salts of interfer�ing cations (except nitrates) such an account is impossible. It is recom�mended to transform inter�fering salts into nitrates
[23]
I–, IO3–,
IO4–,
total iodine
183.0; electronless HF tube (quartz tube filled with 5 mg of I2) with mechanical modulation; flame N2O–C2H2, enriched with nitrogen
Mass 50�fold excesses of Cu, Co, Cr, K, Na, Ni, Mg, Mo, Zn, Al, Hg and 100�fold excesses of Cl–, Br–, NO3
–, PO43–, SO4
2–
do not interfere
[24, 25]
Table 1. Direct atomic absorption methods of iodine determination in model solutions*Table 1. (Contd.)
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DIRECT AND INDIRECT ATOMIC ABSORPTION 81
INDIRECT AA METHODS OF IODINE DETERMINATION
In indirect AA methods of iodine determination, jus as other indirect methods of analysis, the analytical signal of the element being determined (iodine) is not measured, but the AA signal of other element (metal) chemically bound with iodine, is measured [26].
Publications on indirect AA methods of iodine determination have started to be published since 1984 [27–55]. More than half of these papers refer to iodine determination in sea objects—water, salts, algae and also in weather precipitations and underground mineral or geothermal waters. The most often used analytical form for determination of iodine in indirect AA methods is iodide ion. Iodine in the form of iodate [28, 32, 33, 39, 49], periodate [32] or as part of organic compounds containing iodine (iodoform, alkyl iodides, organic com�pounds of other classes) [28, 32, 49] are determined more rarely.
The basis of indirect AA determination of iodine is the running of one or several chemical reactions between the analyte and the auxiliary substance (substances), which afterward are found directly by absorp�tion. Theoretical determination of such an analyte does not present any difficulties if stoichiometry of the aux�iliary reaction is known. However, in practice the running of chemical processes depends on a number of fac�tors, which increases labor input of the techniques and complicates the possibility of their automation [38, 49].
It should be noted that often indirect AA methods of determination are brought about by means of auxiliary reactions of sedimentation of iodine [30, 31, 33, 53], or complexation without extraction [27, 29, 35], or extraction of preliminary obtained complexes of the analyte [28, 32, 34, 38]. The most important details of analytical techniques of indirect AA determination of various forms of iodine are given in Table 2.
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparation
Iodine form being deter�mined
Element by which
iodine con�tent is deter�
mined; wavelength,
nm
Atomization method Sample preparation
Metrological characteristics
of the technique
Determina�tion impedi�
mentsRef.
I– Hg; 253.7 Graphite laboratory dish, drying: 30 s at 150°C
Samples of mineral waters or atmo�spheric precipitations are acidified with nitric acid and at pH 3.4–4.8 the analyte is transformed into the HgI4
2– complex. At atomization two peaks are formed owing to the pres�ence in the solution of two forms of mercury: an excess of Hg2+ and the HgI4
2– complex. Iodide is deter�mined by the intensity of the mercury peak as part of the HgI4
2– complex
C0 = 0.019 μg/cm3
in the interval of the GG lin�earity—0.13–6.4 μg/cm3
CN–, S2–, S2O3
2–, Ag+[27]
I–, IO3–,
alkyl iodide
Ditto Graphite lab. dish, drying: 15 s at 100°C and 15 s at 110°C; atomization: 7 s at 1000°C
Iodine as part of sea products or air samples over the ocean (volatile alkyl iodides) by means of the method of alkaline (NaOH) high temperature mineralization is transformed into iodide and extract dissolved compo�nents by water. The analyte forms the HgI4
2– complex, which is bound into an ionic pair with 2,2’�dipyridyl and extract its complex at 6.8–7.6 with methylisobutylketone. Iodide is determined by mercury concentra�tion in the organic layer
C0 = 8.84 × 10–4 μg/dm3; RSD = 7.8% within GG linearity up to 75 μg/dm3
– [28]
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82 SUKHAN et al.
I– ″ Method of cold steam
Samples of rain waters and brines are acidified with sulfuric acid and trans�formed the analyte into the complex of the HgI4
2– composition. The resultant solution is passed through the KU�2 for sorption of the Hg2+ excess. Iodine is determined by the mercury concentration bound into the complex with iodide
C0 ≤ 0.07 μg/cm3: RSD = 2.4% within the inter�val of the GG linearity 0.07–80 μg/cm3
Br–, Cl– [29]
I– ″ Graphite laboratory dish
From the dry residue obtained after sample preparation of milk products by alkaline (NaOH) high�tempera�ture mineralization soluble compo�nents are extracted by water, iodide is precipitated in the form of HgI2 by an excess of Hg(NO3)2 and mercury concentration is determined in the solution after centrifugation of the mixture
– CN–, S2– [30]
I– ″ Cold steam method, 250°C
Drinking or underground waters are treated with a mixture of nitric and sulfuric acids, bring to pH 7–8, by an excess of Hg2+ bind I– into HgI2
and by the mercury concentration in the solution over the sediment calculate the iodide concentration
C0 = ∼ 0.003 μg/cm3; RSD = 3.0% within the GG linearity interval
Halogenides [31]
I–, IO3–,
I4– total
iodine
″ Electrother�mal atomiza�tion at 1000°�
Drinking water or milk are mineral�ized at 250°C, the remainder is dis�solved at 7.2–7.4 an ionic pair is formed between 1,10�phenanthro�line, Hg(II) and iodide, which is extracted by methylisobutylketone and the organic layer is atomized
C0 = 8.5 μg/cm3
Accuracy—98.1%; RSD = 10%;
– [32]
I–, IO3–,
total iodine
– Cold steam method
From dry residue after sample prepa�ration of sea algae or bottom sedi�mentations by alkaline (NaOH) or high�temperature mineralization method soluble components are extracted by water. The resultant solution is neutralized until 7–8 and iodide is precipitated by Hg(II). After the HgI2 sedimentation is separated iodide is determined by the concen�tration in the solution of the Hg(II) excess
RSD = 2–4% within the GG linearity.
– [33]
I– ″ Ditto Samples of mineral or underground geothermal waters and weather pre�cipitations are acidified and iodide is bound into a complex with mercury then an ionic pair is formed between 2,2’�dipyridyl and HgI4
2–, which at pH 7.2–7.5 is extracted by ethylace�tate. Mercury (4 mol/dm3) is re�extracted by nitric acid and by the mercury concentration in the aque�ous phase calculate the iodine con�centration
C0 = 1.48 μg/ dm3; Detection limit 0.68 μg/dm3; RSD = 2.25% at the iodide con�centration in the sample15 ng
Cl– in the presence of Fe3+, S2O3
2–, Cu2+, Ag+, Ba2+
[34]
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparationTable 2. (Contd.)
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DIRECT AND INDIRECT ATOMIC ABSORPTION 83
I– ″ ″ The solution of the sample of bioliquids (blood plasma, urine) pre�liminary treated with zinc acetate is passed through an ion–exchange col�umn. I– contained in the eluate is bound into a complex with Hg2+. An excess of Hg2+ is selectively reduced to Hg0, whose content is measured by the cold steam method. Mercury as part of the complex is not reduced
Correctness: 92–100 %; GG linearity 2.5–25 μg/cm3
CN–, S2–, S2O3
2–, Ag+[35]
I– ″ Graphite oven. Drying: 20 s at 50°C, atomization: from 50 to 900°C 30 s and 15 s at 900°C
Iodide contained in the samples of mineral waters is bound in a nitroacid medium by means of Hg2+ into the HgI4
2– complex. In the solution that formed one registers two peaks one of which is determined by the Hg2+ excess, the other one—by mercury as part of the complex. By registering the intensity of the last signal the I– concentration is calculated
C0 = 3 μg/dm3; RSD = 6.7% within the GG linearity range (3–20 μg/dm3)
CN–, S2–, S2O3
2–, Pb2+, Zn2+
[36]
I–, total iodine
″ Cold steam method, cell temperature for atomiza�tion— 180°C
The solution obtained by alkaline mineralization of the samples of sea silt (NaOH) and neutralization of the resultant aqueous extract by sulfuric acid to pH 7–8 is treated by the Hg2+ excess for binding iodide into HgI2. The sediment is separated, while in the solution we determine unreacted Hg2+ by the content of which we cal�culate the iodide concentration
RSD = 4.9–8.8%; correct�ness 94–103%
Ag+, Pt(IV) [37]
IO3– – Cold steam
methodIO3
– contained in the solution of cooking salt is bound into a ionic pair with 2,2�dipyridyl, which is extracted at pH 6.95–7.05 by methyl�isobutylketone and re�extracted by nitric acid (4 mol/dm3). The aqueous phase is analyzed for the mercury content
RSD = 2.4% at the sample con�centration of 1.0 μg of iodide; correctness 92–112%: GG lin�earity—to 1.18 μg
S2–, S2O32–,
Ag+, EDTA[33, 38]
IO3– Ag; 328,1 Flame H2–
airIn the aqueous solution IO3
–, is reduced by saccharose to I–, iodide is precipitated in an acid medium by sil�ver nitrate. The precipitate is sepa�rated and dissolved in the excess of KI. In the resultant solution we deter�mine silver by whose content we cal�culate the iodide concentration
C0 = 0.1 μg/cm3 Not indicated [39, 49]
I– Ditto Flame air—C2H2
Iodide of mineral waters is precipi�tated in the acid medium by silver nitrate and by the content in the solu�tion of silver excess we determine the iodide concentration
RSD = 3.61% within the GG linearity interval (1–6 μg/cm3)
Br–, CN–, SCN–
[40]
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparationTable 2. (Contd.)
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84 SUKHAN et al.
I–, total iodine
″ Flame air– C2H2–oxi�dizer
After the high�temperature (K2CO3) ashing of samples (food products with protein, fat, or carbohydrate basis) and extraction from the dry residue by water iodide and is precipitated in the form of Ag I; after separation the precipitate is dissolved with cyanide. By the content of silver in the result�ant solution we calculate the iodide concentration
After the high�temperature (K2CO3) ashing of the samples of biobjects, iodide from the dry residue is extracted by water and precipitated in the form of Ag I. The precipitate is separated and dissolved by cyanide. By the content of silver in the result�ant solution we calculate the iodide concentration
C0 = 0.13 μg/cm3; RSD = 1.1% at the iodide con�centration 2.54 μg/cm3
Cl–, SCN–, CrO4
2–[42]
I– Ag; 193.7 Hydrogen generator
Iodide as part of mineral waters in the HClO4 medium is oxidized by Ce(IV) to elementary iodine, which is distilled off and absorbed by the solution. The absorbing solution (pH 8) in the phosphate buffer is treated with an excess of As(III) for reducing I2 to I–. Then I– is precipitated in the form of AgI. The solution obtained after separation of the precipitate by centrifugation, is analyzed for the content of silver (GG is obtained with a negative inclination)
RSD = 2.9% at the concentra�tion 12 ng of iodide in the sample; GG lin�earity 3.2–25.4 μg/cm3
– [43]
I–, total iodine
Ag; 328.1 Flame air—C2H2
After the high�temperature alkaline (K2CO3) ashing of the samples (food products with the protein, fat, or car�bohydrate basis) the dry residue is dissolved in HNO3. The solution is dissolved with water to pH 1.3–6.2 and use for determination of the Cl– and I– concentration. For this goal both anions are precipitated in the acid medium in the form of AgCl and AgI. The resultant precipitate is rinsed by a solution of nitric acid and partially dissolved in ammonia. AgCl is dissolved, while AgI remains in a sediment). The difference in the con�centrations of silver at the stages of precipitation and dissolution is pro�portional to the content of iodide
Detection limit—6 μg/cm3; RSD = 2.3% within the limits of GG linearity and 4.9% at the iodide concen�tration 100 μg/dm3; Correctness—97.2–104.7%; GG linearity 10–20 μg/cm3
CrO42–, ClO–,
CN–, [Fe(CN)6]3–
[44]
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparationTable 2. (Contd.)
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DIRECT AND INDIRECT ATOMIC ABSORPTION 85
Ditto ″ Graphite oven
The dry residue after alkaline high�temperature mineralization (Na2CO3 + Mg(NO3)2) of the sam�ple (food products with the protein, fat, or carbohydrate basis) is dissolved in nitric acid. In the resultant solution iodide is precipitated in the form of AgI, the precipitate is separated by centrifugation and by the content of silver in the solution of the iodide concentration is calculated
Correctness measure 97.5–102.3%
[45]
I– ″ Flame air—C2H2
Iodide as part of biological liquids is precipitated in the form of AgI and by the content of silver in the solution after centrifugation of the sediment the iodide concentration is calculated
I– ″ Flame Iodide in the solution of the sample of sea salt is precipitated in the form of AgI, which then is extracted by meth�ylisobutylketone in the presence of the Ag+ excess in 0.8 mol/dm3 of NH3
RSD = 1.52–4.17% within the limits of the GG linearity
[45, 47]
I– Cu; 324.7 Flame air—C2Н2— oxidizer
From the dry sample of sea algae I– is extracted by acetone nitrile, is re�extracted by water and form a chelate complex [Cu(2�benzoylpyridine thi�osemicarbazone)] I, which at pH 6.5 is extracted by butylacetate. In the organic phase we determine the Cu concentration
C0 = 1.5 μg/cm3; RSD = 3.2% at the iodide con�centration 5 μg/cm3; Cor�rectness—97.3–101.4%; GG linearity—0.4–10.4 μg/cm3
Br–, Cl– [48]
I– Ditto Flame air—C2H2
To the solution of the sample the solutions of CuSO4 and thiocyanate are added and in the medium of the buffer solution K2HPO4 –KH2PO4 the ionic pair [CuSO4(SCH)+][I] formed is extracted by methyl�isobutylketone. The Cu concentra�tion is determined in the organic layer
C0 = 0.104 μg/dm3; RSD=1.29% at the iodide con�centration 5 μg/cm3; Cor�rectness—98–106%; GG lin�earity—0.79–9.52 μg/cm3
Halogenides 49
I–, total iodine
″ Flame A sample of sea algae is incinerated by a high�temperature alkaline method (K2CO3), macerated with H3PO4 and bring to pH 4.5. The aliquot of the resultant solution is added with KH2PO4, thiourea and CuSO4 for the formation of neocuproine–cum�prume(I)–iodide complex, which is extracted by phenyl chloride. The copper concentration is determined in the organic layer
RSD ≤ 2.9% at the iodide con�centration 5 μg/cm3; Cor�rectness—93.3–106.7%; GGlinearity—0–5 μg/cm3
[50]
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparationTable 2. (Contd.)
JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY Vol. 32 No. 2 2010
86 SUKHAN et al.
I–, total iodine
″ Flame air—C2H2
A sample of urine and blood plasma is incinerated by the high�temperature alkaline method (K2CO3) and form a complex of neocuproine—CuI, which is extracted into phenyl chlo�ride and in the organic layer deter�mine the copper concentration
C0 = 0.04 μg/cm3; GG linearity—0.12–4.8 μg/cm3
[32, 49]
I– ″ Ditto Iodide contained in sea water is pre�cipitated in the form of CuI and dis�solved in Na2S2O3. The copper con�centration is determined in the resultant solution and calculated the I– content
GG linearity—0–800 μg/cm3; correctness—99.9–103%
Halogenides [49, 51]
IO3– Cd; 228.8 ″ In the solution of the food product
samples obtained after acid sample preparation, iodate is reduced to iodide and form an ionic pair between the Cd(II) complex with 1.10�phena�throline. The ionic pair is extracted by nitrobenzene at pH 5.2 and in the organic phase determine the Cd con�centration
Formation and extraction from model solutions of iodide by nitrobenzene at pH ~5 by the ionic pair (1.10�phenanthroline) CdI2 and determination of Cd(II) in the organic phase
RSD = 0.6% at the iodide con�centration 5.1 μg/cm3; GG linearity—0.5–5.1 μg/cm3
Br–, ClO3–,
IO4–, NO3
–[49, 52]
I– ″ Flame air–C2H2
Formation and extraction from the model solutions of iodide by nitrobenzene at pH 3.5–5.5 of the ionic pair (1.10–phenanthroline) CdI2
and determination of the con�tent of Cd in the organic phase
GG linearity—0.6–5.1 μg/cm3
– [53]
I– Cr; 357.9 Ditto Reduction by iodide as part of min�eral water in the medium 3 mol/dm3 of hydrochloric acid Cr(VI) to Cr(III); extraction of the Cr(VI) excess by methylisobutylketone. Determination of the Cr(III) con�centration in aqueous, and Cr(VI)—in organic layers
– – [32, 49]
IO3–,
total iodine
Fe; 248.2 – Determination by iodate as part of the solutions of food products obtained after acid sample prepara�tion, in the medium 9 mol/dm3 of hydrochloric acid, Fe(II) to Fe(III); extraction of HFe(III) by diethyl ether and determination of Fe(III) concentration in the organic phase
– – [40, 49]
I– Se; 204.0 Ditto Reduction by iodide as part of min�eral water in the acid medium Se(IV) to Se0; separation of the elemental Se by filtration through a milliporous fil�ter and determination in the filtrate of the Se(IV) excess
– – [49]
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparationTable 2. (Contd.)
JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY Vol. 32 No. 2 2010
DIRECT AND INDIRECT ATOMIC ABSORPTION 87
Methods with the Use of Precipitation Reactions
In methods using precipitation reactions iodine is determined by the excess of ions of silver or mercury introduced into the solution after separation of hardly soluble iodides by centrifugation [30, 31, 33, 37, 40, 43, 45, 46]. Iodide is also found by the content of silver in the iodide [39] or cyanide [41, 42] solution of the sep�arated sediment of silver iodide and also by the content of copper in the thiosulfate solution CuI [51].
Methods Using Complexation Reactions
Without the extraction stage the indirect methods of determining iodide through the complexation reac�tions include the addition of the Hg(II) aliquot to the iodide solution and subsequent registration of the absorbed signal of mercury atoms bound into the HgI4
2– complex [27, 36] including after ion–exchange sep�
aration of Hg2+ [29].The other technique is based on extraction of iodine containing chelates of any metals (Hg, Ag, Cu, Cd).
Thus, more than half of the papers on indirect AA determination of one or other forms of iodine include pre�liminary extraction of ionic pairs of iodine�containing complexes with organic reagents (2,2’�dipyridyl [28, 34, 38], 2�benzoylpyrydinethiosemicarbazone [48], necuproine [49, 50], 1,10�phenanthroline [32, 40, 49, 52, 53]).
Methyl isobutyl ketone [28, 32, 38,49], ethyl acetate [34], butyl acetate [48], phenyl chloride [32, 50], nitrobenzene or diethyl ether [40, 49] are used as solvents. Extracts of the complexes containing iodine are directly introduced into an atomizer. One or other forms of iodine are determined also by the content of the corresponding metals in aqueous reextracts of ethyl acetate [34] and methyl isobutyl ketone [38] extracts.
In addition to the elements (Hg and Ag) mentioned above iodine is determined by analytical lines Cu [32, 48–51], Cd [40, 49, 52, 53], Cr [32, 49], Fe [40, 49], Se [49], As [55] with the use of one or other preliminary reactions and techniques of separating the excess of metal ions from the stoichiometrically bound with iodine.
The AA determination is brought about by various methods of atomization depending on the composition of the solution being analyzed. For example, most often for final determination of iodine by the analytical lines Hg spectroscopy of cold steam is used [29, 31, 33–35, 37, 38], rarely—a graphite stove or a flame [27, 28, 30, 32, 36].
A shortcoming of the indirect methods is mainly low specificity of preliminary reactions being run. Indirect methods are recommended for the use only in those cases when the concentration of all other accompanying components in solution from analysis to analysis may be maintained constant.
METROLOGICAL CHARACTERISTICS OF THE METHODS
Metrology of the direct AA methods of determining iodine is characterized only on model aqueous solu�tions containing iodine (see Table 1) rather in the analysis of standard samples or real objects [49].
Metrological characteristics of the indirect methods of analysis in great measure depend on the selectivity of preliminary performed chemical reactions. The comparison of metrological characteristics of various indi�rect methods of determination is a complex task. Thus, published papers give one and the same metrological parameters of the techniques some of them varied within the last 30 years. In addition, experimental measure�ments were performed on different equipment. This is especially evident in the cases of using such metrolog�
I– As; 193.7 Hydrogen generator
Iodide as part of the sea water is oxi�dized by Ce(IV) in the medium HClO4. Iodine is sublimated and passed through the absorption solu�tion, which is treated of I2 to As(III) excess for the reduction by iodide at pH 8 (phosphate buffer). Iodide is precipitated in the form of AgI. The precipitate after centrifugation is thrown away, while the solution above the precipitate is analyzed for the content of As (GG is obtained with a negative slope)
RSD 2.9% at the concentra�tion of iodide 12.8 ng; GG linearity—3.2–25.4 μg/cm3
Br–, Mn(II) [49, 54]
Table 2. Indirect atomic absorption methods of iodine determination in waters and aqueous solutions of the samples after corresponding sample preparationTable 2. (Contd.)
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88 SUKHAN et al.
ical characteristics as “the limit of detection”—LOD, characteristic mass (m0), characteristic concentration (C0), sensitivity (see Table 2). Observation of the IUPAC recommendations by determination of metrological characteristics of the method will help overcome the existing issue.
CONCLUSIONS
The direct AA method of iodine determination due to a complex hardware execution presents a scientific rather than practical interest.
The indirect AA iodine determination methods are used in the practice activity chemists–analysts when solving their own specific tasks. In terms of sensitivity these methods are not inferior to spectrophotometric ones, while in terms of selectivity catalytic ones are superior, however, they are inferior to the mentioned ones by labor input and the possibility of their automation.
The effective development and the employment of the indirect AA iodine determination methods is deter�mined by the successful search and the use of sensitive and selective preliminary chemical reactions of the ana�lyte with auxiliary substances.
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