The Anodic Behaviour of GoldPART I -- OXIDATION IN ACIDIC SOLUTIONS

Michael J. NicolNational Institute for Metallurgy, Randburg, South Africa

The anodic behaviour of gold is of fundamental importance not only in

processes involving the metal, such as its extraction, refining, elec-

troplating, electroetching and electropolishing, but also in the many uses

of gold in which its resistance to corrosion is critical. The first part of a

review of this subject is presented here. Part II, dealing with reactions in

alkaline media, will appear in th.e next issue of Gold Bulletin.

Gold is the most noble of metals. This property is,of course, the major factor contributing to itsdurability and to the fact that it normally occurs innature as the native metal. Despite its generally highresistance to corrosion, gold can fortunately be madeto corrode (or dissolve) freely in solutions containingappropriate complexing agents. This property of goldis put to use in the widely practised cyanide processfor its recovery from ores, in its electrolytic refiningin chloride solutions and in many applications in themetal finishing industry where it may be used as ananode in plating cells or is subject to electropolishingoperations.

In view of the diverse conditions under which goldis forced by direct or indirect methods to operate asan anode, an understanding of its anodic behaviour inthe presence of various complexing agents in aqueoussolutions is often of importance. This review has beencompiled with the object of providing the reader witha critical (albeit subjective) assessment of past andcurrent views on the factors determining thebehaviour of gold under oxidizing conditions, and ofthe mechanisms that have been proposed for thevarious reactions involved.

Thermodynamic AspectsAlthough not always reliable as a gauge of chemical

reactivity, thermodynamic data do provide a basis forthe study of electrochemical reactions and can be ofassistance in the interpretation of, and assignment ofreactions to, observed electrochemical effects. Forthis reason, and because later discussion will befacilitated by their availability, a summary of the mostrelevant data on gold will first be given.

In the absence of ligands which stabilize one orother of the predominant oxidation states of gold insolution, namely Au(I) and Au(III), its chemistry isrelatively simple and can be conveniently described

using the thermodynamic diagram of potentialagainst pH (Figure 1). The most important features ofthe diagram in relation to this review are as follows:(1)There is no significant region of stability for the

uncomplexed Au' or Aua ions(2)The most stable oxide of gold in the trivalent state,

Au 20 3, has a wide region of stability(3)There is no stability region for Au(I) or Au(II)

oxides. Aurous hydroxide, AuOH, has supposedlybeen isolated (1), but evidence for its existence issparse and no experimental thermodynamic datarelating to it are available

(4)The formation of gold peroxide, Au02, requiresrelatively high potentials, for instance thatnecessary at pH 1.0 is 2.6 V.

In the prescence of ligands that form complexeswith Au(I) and/or Au(III), the potential against pHdiagram is more complex than that shown in Figure 1and reflects greater domains of stability of the solublegold species. The chemistry of the various complexesof gold that can be formed with the common ligandsin aqueous solutions has been described in a previousarticle (2) in this journal. "T ie most significant aspectof this chemistry is the influence of the nature of theligand on the stability of the ions and, thereby, on thestandard reduction potentials of the correspondingcouples. Data for the ligands that will be discussed inthis review are recalled in Figure 2, in which therelative stabilities of the Au(III) and Au(I) complexesare compared. Thus, points below the diagonal linecorrespond to ligands (L) for which the Au(III) com-plexes AuL, are more stable than the correspondingAu(I) complexes AuL2, the latter being the preferredspecies for those ligands appearing above the line.The information in Figure 2 has interesting conse-quences which will be discussed later.

Included in Figure 2 is a comparison of the poten-tials corresponding to the reduction of the various

46

oxidized forms of the ligands (X 2) with those for theAu(l) complexes. On thermodynamic grounds, onewould expect anodic oxidation of the ligand inpreference to anodic dissolution of gold to occur onlyin those complexes positioned below the diagonalline. As will be seen, this prediction is generallyborne out by experimental observations.

Oxidation in Non-Complexing Media

The electrochemical oxidation of noble metals andthe reduction of oxide films on them have been

2.6Au Q,

2.2Au'

1.aAgo,

1.4^^' HAuo3-

Qi

0..s

02

-a2Au

-0,6

-to

-1.a

0 2 4 6 8 10 12 14P„

Fig 1. Potential against pH diagram for gold inaqueous solution at 25°C. The concentration of allthe soluble species is 10-4 M

actively investigated for many years. However, onlyrecently have studies revealed the complexity of theseprocesses on metals such as platinum and, to an evengreater extent, gold. The formation of oxide films canhave pronounced effects on the anodic dissolution ofthe metals and on the ease with which electrons canbe transferred to reactants in solution. The factorsgoverning the formation and properties of these filmsare therefore important for an understanding of theanodic behaviour of the metals. This topic has beenthe subject of several reviews (4, 5, 6), but these werenot written with the anodic dissolution behaviour ofgold specifically in mind.

Early work

Much of the early work (7 to 16) on the electro-chemical oxidation of gold was carried out withgalvanostatic methods that lack the sensitivity andresolution required for detailed studies. However,this work provided an overall picture of the oxidationof gold which has undergone considerable refinementin recent years. The major findings of the pre-1972researches were as follows:(I) Thick oxide layers can be grown on gold, but not

on platinum electrodes by anodization at highpotentials. Evidence for this is provided by theresults of Laitinen and Chao (9) shown in Figure 3.The amount of oxide (expressed in terms ofmillicoulombs/square centimetre) produced byanodization for 3 minutes in I M perchloric acidat various potentials was determined by cathodicstripping at constant current. After anodizationat low potentials, the stripping chargecorresponds approximately to that required for

2.6

2A

1.6

to

6.5

a

-05

- 2ie

Standard reduction. potentials of complexes ofAujlj and.Aujilli with:

cyanide .ihiosulpiiateiodidethlocyanatesulphitebromidechloride ions

and with thiourea and water.

`These are compared with those for the oxidationof the corresponding ligands to the diiniericspecies. After 12, 3, 4j

5H O

MORE STABLE

MÖIESTABLE

I Tu R SC( }^

-0.5 0 as 1.0 1.5 2.0 2.5

Ewm/w. V

47

ANODIZATION POTENTIAL, V

Fig. 3 Variation of the amount of oxide formed ex-pressed in terms of the electrical charge requiredfor subsequent stripping) by the anodization of agold electrode at various potentials for 3 minutes in1 M perchloric acid. After (9)

one-electron oxidation of the surface atoms of thegold (close to 0.4 mC/cm 2). At potentials above2 V, oxidation is rapid and the surface becomescoated with a dark orange film that has beenshown (8) to consist of hydrated Au z0 3 . Becausepotentials above about 1.8 V are of littlesignificance for practical applications, this regionhas been the object of few studies only and atten-tion will be devoted here mostly to the betterknown region of low potentials

(2) Anodic-charging curves at constant current inacid solutions showed a region of smoothpotential increases from about 1.3 to 1.75 V.This is followed by a plateau at about 1.8 V,which corresponds to the potential required forthe evolution of oxygen (9, 10, 14)

(3) Cathodic reduction (at constant current) ofanodized surfaces yielded chronopotentiograms(9, 10, 14) that appear to show a single arrest at apotential which was found to depend on theanodic potential used to oxidize the surface andon the time of anodization. These observationsled early workers to conclude that a singleoxidized species only was formed by anodizationof the gold

(4) At all potentials, an initial rapid oxidation isfollowed by slow growth of the oxide film (10,12, 13). Oxides formed over longer anodizationtimes are more difficult to reduce than thoseformed over short times. This `ageing' effect isgreatest at the lower potentials, despite thegreater thickness of the oxides formed at the highpotentials

(5) The rest potential of a gold electrode anodized tovarious potentials in 1 M perchloric (9) orsulphuric acid (16) was found to be relativelyconstant at 1.30 V, which is sufficiently close tothose (1.31 to 1.51 V) reported by variousauthors (4) for the Au 20 3/Au couple to supportthe conclusion that Au 20 3 is the product which isformed in the electrochemical oxidation of goldsurfaces

(6) The potentials for oxidation of the gold andreduction of the oxides shift approximately60 mV towards less positive potentials for eachten-fold decrease in acidity over a wide range inpH (9, 10, 14). This indicates control by reac-tions involving a proton/electron ratio of 1 and isconsistent with the formation and reduction ofAu 20 3 by the reaction:

2Au + 3H 20 + 6e - Au 203 + 6H'

(7) The ratio of the anodic charge passed in the for-mation of the oxide (Qa) to that required toreduce it (Q) is generally less than 1, even at lowpotentials where the evolution of oxygen does notoccur (8, 9, 10, 14). Furthermore, loss of oxide,as measured by Q, is observed when an anodizedelectrode is allowed to remain in acid solutionsunder open circuit conditions. Reactions whichhave been put forward to explain these observa-tions have included oxidation of water by Au 20 3

(9) (Such oxidation is possible from a thermo-dynamic point of view, since the H 2 O/0 2 line inFigure 1 lies below that for the Au/Au 20 3 couple)and chemical dissolution of the oxide to formAua ions (8, 10).

Based on these early observations, a fairly generalconsensus of opinion was reached on the grossfeatures of the reactions taking place during theanodic oxidation of gold in solutions of non-complexing acids. Thus, it was accepted thatchemisorption of oxygen begins at potentials aboveabout 1.4 V, monolayer coverage is approached atabout 2 V and, at higher potentials, the growth of alayer of bulk oxide that appears to correspond toAu20 3 in acid solutions and to Au(OH) 3 in alkalinesolutions follows. The nature of the chemisorbedoxygen layer was however not known with anycertainty.

48

Fig. 4 Effect of anodic potential sweep rate on theproportions of different intermediate species detec-ted by reduction at a constant cathodic sweep rate ofa gold electrode in 1 M.perehloric acid. After {17)

Recent AdvancesIn the 1970's, much of the experimental work on

the oxidation and reduction at gold surfaces under-went considerable refinement as a result of the use ofadvanced techniques such as cyclic volta:nmetry.These methods are eminently suited to the resolutionof `fine structure' in current-potential relationshipsand to the study of anodic film growth andrecrystallization kinetics. The use of modern opticaland spectroscopic techniques, such as ellipsometryand X-ray photoelectron spectroscopy, has also con-tributed to current knowledge of the nature of theoxide films.

One of the most significant conclusions drawn fromrecent work is that the process of anodic oxidation ofgold is not as simple as previously supposed. Forinstance, cyclic voltammograms (17) for a goldelectrode in acid solution (see Figure 4) indicate thatthe oxidation and reduction processes involve severalreactions, as evidenced by the multiplicity of thepeaks. Much effort has been devoted to the identifi-cation of the origins of these peaks. Ir

If single crystal gold electrodes with exposed planes L)of defined symmetry are used (18), only single peaksare observed for oxide formation and, at low potentialsweep rates and anodic limits, for oxide reduction.Three voltammograms obtained in this manner byDickertmann ei al. (18) are shown in Figure 5 forpolycrystalline and single crystal electrodes with the( 100) and (111) faces exposed. Repeated cycling of theelectrodes to potentials below 1.9 V produced nonoticeable change in the shape of the curves.However, anodization at potentials above 2 V resultedin roughening of the surface of single crystalelectrodes with apparent film recrystalIization to yieldvoltammograms of the polycrystalline type. Alsoevident for high anodic potential limits, was anadditional reduction peak that had previously beenobserved by Rand and Woods (19) when the anodiclimit was extended into the region where the charge-potential relationship (Figure 3) revealed a change inthe anodization characteristics.

More recent work (17, 20) has added considerablyto the knowledge of ageing and recrystalIizationprocesses in oxide films on gold. Using fast-scanvoltammetry, it is possible to distinguish severalcathodic peaks for the reduction of oxide filmsproduced even at low anodic potentials. Figure 4shows some typical curves obtained in perchloric acidsolutions at various anodic, but constant cathodic,

POTENTIAL

49

60

0

-50

-100

POTENTIAL, V

Fig. 5 Effect of the crystallographic orientation ofthe surface of single crystal gold electrodes on theformation and reduction of .oxide on then in .1 Mperchloric acid at a sweep rate of 10 mV/s. After 118)

ear ,O'

to-4

Cc0

1Q-5

M NaCI

M NaCI

t2 1.4 1.6 1.8 2.0POTENTIAL, V

Fig. 6 Steady-state current against potential curvesfor the oxidation of gold in solutions containing0,5 M1 sulpb.uric; acid and various amounts of chlo-ride. After (28)

sweep rates. Although the charge involved is less thanthat required for monolayer coverage, the resultsindicate relatively rapid transformations of the initialanodic product. Thus, the faster the anodic sweep,the greater the amount of the more reactive (asmeasured by the area under the least cathodicreduction peak) anodic product retained during thepotential scan. The behaviour of gold in sulphuricacid is somewhat different, the oxidation starting atslightly lower anodic potentials. Here, five reductionpeaks have been observed but, as pointed out byArvia's group (17) and others (18, 19), caution shouldbe exercised in the assignment of all these peaks tosurface oxides, because trace impurities could be atleast partially responsible for them. The mechanismproposed by Arvia's group (17) to explain theseeffects involves the initial formation of an adsorbedAuOH species that is susceptible either to furtheroxidation to a higher `oxide' or to chemical dispropor-tionation to a more stable surface oxide with astoichiometry comparable to that of Au 1O 3 . Thesespecies are envisaged as giving rise to different reduc-tion peaks, the relative magnitudes of which varywith the limit of anodic potential and the timeallowed for the chemical disproportionation reactionto take place. However, this interpretation is open todebate, since it ignores the polycrystalline nature ofthe surfaces employed in the experiments. Indeed, aspointed out by Dickertmann et al. (18), the formationof epitaxial surface oxides is possible (and even likely)and could give rise to multiple reduction peaks on apolycrystalline surface. Rapid transition of anepitaxial oxide to an oxide independent of thesubstrate could explain the ageing effects that areobserved. Work similar to that of Arvia's group (17),but using single crystal electrodes, might solve theseproblems of interpretation.

Studies (18) of the formation kinetics of oxide layerson gold as a function of their thickness have resultedin general agreement that formation of the first layeris relatively reversible (fast) and that its rate of forma-tion depends on the crystallographic orientation ofthe gold surface. For thicker layers (0.2 to 0.3 nm),however, the rate of growth is independent of thesubstrate orientation and is governed largely by themigration of ions through the oxide.

Several investigations (8, 9, 10, 21, 22) have shownthat dissolution of gold occurs during anodization andduring the reduction of oxide films in acidic sulphateor perchlorate solutions. In an elegant study, Cadleand Bruckenstein (21), using ring-disc electrodes,determined the rate of dissolution by collecting thedissolved gold on a platinum ring electrode. Dissolu-tion was observed at potentials greater than 1.38 Vwhere Au(III) was found to be the predominant solu-ble species. At potentials above 1.7 V, Au(I) was also

10-' 1

0.5 M NaCI

50

detected in significant amounts. Gold dissolution,primarily as Au(III), was also observed during thereduction of oxide films. Mechanisms were not pro-posed beyond the unlikely suggestion that an oxideAuO, is reduced to a soluble Au(III) species that canbe further reduced to metal or can diffuse into thesolution. The accurate measurement of the very lowdissolution rates involved is the major obstacle tomore detailed understanding of the factors influen-cing the dissolution of gold in non-complexing solu-tions. However, as pointed out by Duncan andFrankenthal (22), such dissolution and re-depositionof gold can have important practical consequences inthe thin film and integrated circuit technologies.

Ellipsometric (23) and optical reflectance techni-ques (24) have been applied to in situ observation ofthe formation of oxide films on gold electrodes.Although both have enabled detection of changes inthe optical properties of gold surfaces in the potentialregions of interest, the many assumptions requiredfor interpretation of the experimental data haveresulted in little new information being gathered onthe chemical nature of the anodic products. X-rayphotoelectron spectroscopy can, in theory, provideuseful information on the chemistry of surfaces, but itsuffers from the disadvantage that it cannot by used insitu. Nevertheless, gold 4f and oxygen is spectra (25)of a gold surface anodized in dilute sulphuric acid aresimilar to those of chemically prepared Au,0 3 .

Oxidation in Halide SolutionsAnodic oxidation of a gold electrode in acidic solu-

tions containing chloride (or bromide) ions results ina marked increase in the rate of dissolution of themetal by virtue of the increased stability of Au(I) andAu(III) chloro-complexes (Figure 2). This forms thebasis for the Wohlwill process for the electrolyticrefining of gold, the overall chemistry of which wasrecently reviewed in this journal (26). Several studieshave been made of the fundamental electrochemistryassociated with the dissolution of gold from chloridesolutions (26 to 33). While most authors agree on theconditions under which the passivation of goldanodes occurs and on the mechanism by which thispassivation is induced, the results for active dissolu-tion and the interpretations offered are not altogetherconsistent. This is particularly true with regard to theparticipation in the reactions of gold species of lowervalencies. This aspect will now be reviewed in somedetail, before discussing the passivation behaviour ofgold in acidic chloride solutions.

Dissolution in Chloride SolutionsHeumann and Panesar (28), in a study of the anodic

dissolution of gold in acidified sodium chloride solu-tions, found that both Au(I) and Au(III) were the pro-

ducts of dissolution. They were able to distinguishanalytically between the two species and obtainedcurrent-potential data that yielded Tafel plots forpotentials below about 1.2 V (Figure 6) for produc-tion of Au(I) and Au(III) under various conditions.Their results showed that the formation of Au(I)predominates at low current densities only and thatincreased chloride concentration (at constant current)enhances production of Au(I). Reaction orders of ap-proximately 2 were obtained for the dependence ofthe rates of dissolution on the concentration ofchloride ions for the formation of both Au(I) andAu(III). Corresponding reaction orders of 0.92 and0.67 were also derived for the dependence on the pro-ton concentration. At higher potentials in the range1.2 to 1.4 V, the current was found to be controlledby the diffusion of chloride ions to the surface of thegold electrode. At a potential of about 1.5 V, passiva-tion was observed, which was attributed to the forma-tion of an oxide layer. Heumann and Panesar (28) didnot attempt the derivation of a detailed reactionmechanism from their data.

Gaur and Schmid (29), in a study aimed primarilyat clarifying the mechanism involved in the passiva-tion of gold in dilute chloride solutions, obtainedsteady-state curves of current against potential for thepre-passive region and assumed that Au(III) was theproduct of dissolution. No mechanism was proposed.In a study of the deposition of gold from chloridesolutions, Harrison and Thompson (30) found thatthe reduction of Au(III) could be characterized by aTafel slope of 60 mV and suggested a fast one-electron first step followed by a slow chemical step.The observed Tafel slope of 84 mV for the anodicreaction was interpreted in terms of a slow step in-volving transfer of the first electron. Despite this, theproduction of Au(I) was not considered, althoughthe authors recognized that results of impedancemeasurements pointed to the possibility of asomewhat more complicated process.

An extensive study of gold dissolution in chloride-containing acidic media was recently published byArvia's group (31). The cathodic sweeps of cyclicvoltammograms indicated the production of Au(I) bythe anodic reaction, but the authors made no attemptto measure the relative yields of Au(I) and Au(III) orto analyze the data in terms of these yields. Theyfound a first-order dependence of the rate of activedissolution on the chloride concentration. Themechanism proposed for the anodic reaction involvesthe highly unlikely heterogeneous disproportionationof adsorbed Au(I) and does not allow for the directanodic oxidation of Au(I) to Au(III). Furthermore,this mechanism predicts that Au(I) should be themajor dissolution product at low concentrations ofchloride, in contradiction with the experimental

51

Fig. 7 Cyclic voltammograins for arotating gold disc electrode in solilions containing 1 M perchloric acidand various amounts of chloride at asweep rate of 20 mV/s. After (32)

observations of Heumann and Panesar (28) andSchalch and Nicol (26).

The latter workers (26) have made a detailed studyof the dissolution and passivation of gold in chloridesolutions as part of a general fundamental investiga-tion into its electrolytic refining. The relativeamounts of Au(I) and Au(III) formed by anodicdissolution under various conditions were obtainedfrom the charge required to strip a known amount ofgold deposited onto a rotating platinum disc elec-trode. It was found that the fraction of gold thatreported as Au(III) increased with the potential, butdecreased with increased chloride concentration anddecreased rotation speed at all potentials in the activeregion. These results are consistent with a mechanisminvolving the formation of Au(I) as an intermediatespecies that can either be further oxidized to Au(III)in a rate-determining two-electron step or diffuse intothe solution. The rate of dissolution as Au(I) wasfound to be proportional to the chloride concentra-tion, as was the oxidation of Au(I) to Au(III). Furtherevidence supporting this mechanism was obtainedfrom a study of the oxidation of Au(I) at a platinumelectrode. The kinetic parameters for this latterreaction were similar to those derived from ananalysis of the anodic dissolution of gold. Apreliminary study showed that the rate of dispropor-tionation of Au(I) in chloride solutions is extremely

slow (its half-life is of the order of hours) and this can-not reasonably be accommodated in the mechanismproposed by Arvia's group (31.)

Passivation in Chloride SolutionsAs shown in Figure 6, passivation of gold in

chloride-containing acidic media occurs at potentialsabove about 1.4 V and is accompanied by a noticeableinstability or oscillation of the current in the vicinityof the passivation potential. Cyclic voltammograms,obtained (32) using a rotating gold disc in perchloricacid solutions with and without added chloride, areshown for comparative purposes in Figure 7. Asdiscussed previously, the plateau observed in thecurve at potentials above about 1.45 V in the presenceof chloride is due to the formation of an oxide layer onthe surface of the gold. Thus, with added chlo-ride (10 -3 mol/litre), a curve results that indicatespassivation, but it is noticeable that no re-activation ofthe electrode takes place during the reverse sweep andthe cathodic peak due to reduction of the oxide is evi-dent. In the solution with a chloride concentration of10 -2 mol/litre, passivation is at somewhat higherpotentials, re-activation occurs during the reversesweep and there is no cathodic peak, indicating thatno oxide is present. These experiments confirm thetheory proposed by several authors (27, 28, 29, 31)that the formation of an oxide film on gold results in

52

passivation. It is also obvious that chemical dissolu-tion of the oxide film is possible in chloride solutions.It is this anodic formation of an oxide film and itschemical dissolution that cause the oscillation in thecurrent near the passivation potential. Thisphenomenon has been discussed in some detail in arecent article by Arvia's group (33).

Several authors (29, 31, 33) have attempted todevise quantitative models to account for the shape ofthe curves in Figure 5. These have been based on sim-ple and complex representations of the variation withpotential of gold electrode coverage by oxide.Although some success has been achieved, it isunlikely that a general model will be establishedunless account is taken of the relative amounts ofAu(I) and Au(III) produced, of the chemical dissolu-tion of the oxide in chloride solutions and of the ad-sorption of chloride ions on the surface of gold. Thislatter aspect was discussed at length by Cadle andBruckenstein (34) who used ingeniously the ring-discelectrode technique to confirm the adsorption ofchloride on gold at various potentials.

Bromide and Iodide Solutions

While it can be reasonably argued that the anodicbehaviour of gold in acidic chloride solutions is,despite areas of minor controversy, fairly wellunderstood, the same cannot be said for the elec-trochemistry of gold in the presence of the other com-plexing halides (bromide and iodide). The use ofbromide solutions has been advocated (35) for theanodic stripping analysis of gold, but the kinetics andmechanisms of the anodic process have apparentlynot been studied in any detail beyond that reportedby Gaur and Schmid (29). These authors inferredfrom a cyclic voltammetric experiment that gold pro-

UiUi

U

POTENTIAL, V

Fig. 8 Current against potential curves for the oxida-tion (iToT u.) of a rotating gold disc electrode insolutions. containing 0.1 M sulphuric acid and 0.001or 0.01 M bromide. Also shown are the correspon-ding partial currents for the production of bromine

as treasured al a platinum ring electrode

bably dissolves and passivates in a dilute bromidesolution much as it does in chloride solutions, withthe difference that bromine evolution (E° = 1.08 V) ispossible at potentials close to those required for theoxidation of gold to AuBr, (E° = 0,960 V) or AuBr¢

(E° = 0.854 V). No study of the gold-iodide systemhas been reported, but its characteristics are expectedto be similar to those of the gold-bromide system,with the additional possibility of passivation by in-soluble AuI.

Preliminary experiments carried out in the writer'slaboratory with a gold disc-platinum ring electrodehave shown that, depending on the potential and theconcentration of bromide ions, various proportions ofAu(I), Au(III) and bromine are produced at theanode. Figure 8 gives the results of some experimentswhere the ring was held at a potential (0.85 V) atwhich bromine can be detected by reduction tobromide, but at which Au(I) and Au(III) are elec-trochemically inactive. The most important conclu-sions to be drawn from these curves are that:(1)The shape of the curve for the anodic dissolution

of gold is similar in most respects to that obtainedin chloride solutions

(2) Passivation of the gold occurs at roughly the samepotential as that observed in chloride solutions andmay therefore also be attributed to the formationof an oxide film

(3)At low potentials, the oxidation of bromide ionsconsumes only a small fraction of the total current.At potentials approaching the passivation region,bromine is the major anodic product

(4)The proportion of the total current consumed inthe production of bromine decreases with in-creasing bromide concentration.

It is apparent that further work is required toestablish the mechanisms and details of the reactionsinvolved, even though these do seem to parallel thosein chloride solutions rather closely.

Oxidation in Thiourea SolutionsAu(I) forms the strong cationic complex

Au[SC(NH2)2J' with thiourea and this has been thebasis for a number of schemes proposed for therecovery of gold from ores and concentrates (36).However, despite the potential applications ofthiourea in the extractive metallurgy of gold, the rele-vant electrochemistry has not received much atten-tion. In fact, the only publication of any significancein this area is that of Groenewald (37), who in-vestigated the anodic characteristics of gold in acidicsolutions of thiourea complex. He used steady-statemeasurements of the current at varius potentials andcoulometric measurements of the amount of golddissolved from a rotating gold disc, and found thatgold dissolves as Au(I) with 100 per cent current effi-ciency at overpotentials below about 0.3 V (Figure 9).

53

--------^

Siah 5

- y

10

100

80

60

LLW

40 jU

1ü° S

t

20

2 0.4 0.6 0.8OVERPOTENTIAL, V

Comparison of the limiting current with thatcalculated for the reaction:

Au + 2SC(NH 2)2 ' Au[SC(NH2)2]2 + e

showed that dissolution of gold is largely diffusion-controlled in the plateau region. Analysis of the dataat low overpotentials yielded kinetic parameters

Fig. 9) ffect of overpotential on the anodic dissolu-tion of gold and the current efficiency of its dissolu-tion into a solution containing 0.1 M sulphuric acidand 0.1 M thiourea at 30°C. After (37)

POTENTIAL, V

Fig. 10 Cyclic voltammograms for the oxidation of arotating gold disc electrode in solutions containing0.1 M sulphuric acid and either 0.01 M chloride or0.01 M thiocyanate

which showed that the reaction given above isrelatively reversible. The decrease in current efficien-cy at high overpotentials was attributed to the loss ofthiourea by oxidation to formamidine disulphideand/or other products, with a simultaneous decreasein the rate of gold dissolution.

Oxidation in Thiocyanate SolutionsThe thiocyanate ion, SCN - , is isoelectronic with

the halide ions and displays pseudo-halide propertiesin many of its reactions. It may therefore be expectedto exert an influence on the anodic behaviour of goldsimilar to that of chloride or bromide ions. To someextent this is apparently true. For example, the com-plexes Au(SCN)2 and Au(SCN),- are well known, andthiocyanogen, (SCN)2, can be produced by oxidationof thiocyanate ions.

The only publication on the anodic reactions of goldwith thiocyanate ions is that of Arvia's group (38)who investigated these reactions in acetonitrile me-dium. It was found that, at potentials below 0.35 V,gold dissolved predominantly as the Au(I) complex,while the presence of Au(III) in anodic solutions pro-duced at higher potentials was inferred from the ultra-violet absorption spectra of the latter. Oxidation ofthiocyanate and/or the solvent occurred at potentialsabove 0.55 V. A mechanism involving the formationof Au(SCN)Z by oxidation of an adsorbed AuSCNspecies and the formation of Au(SCN), by dispropor-tionation of the Au(I) species was proposed, similar tothat previously suggested by the same group (31) forthe anodic dissolution of gold in chloride solutions.

Cyclic voltammograms obtained in the writer'slaboratory for a rotating gold disc electrode insulphuric acid solutions containing respectivelychloride and thiocyanate ions are shown for com-parative purposes in Figure 10. The similarity of thecurves in respect of shape, passivation potential andre-activation during the reverse sweep is apparent.The fact that the current is higher in the active regionin the presence of thiocyanate ions than in presence ofchloride ions can be explained in terms of oxidation ofthe thiocyanate by a multi-electron process. However,coulometric measurements have shown that thedissolution of gold consumes only a fraction of thetotal current in the active region and that this fractiondecreases from 20 per cent (assuming dissolution asAu(I)) at 0.95 V to 4 per cent at 1.15 V. This is notunexpected, since it is well known (39) that in thispotential region thiocyanate is oxidized at a platinumsurface to thiocyanogen (E° = 0.77 V) and, finally, tosulphate and cyanide ions.

SummaryTable I summarizes the salient features of the

anodic behaviour of gold in acidic solutions in the

54

Table

Summary of the Anodic Behaviour of Gold in Acidic Solutions

Anodic products Gold PassivationLigand yield*, by oxide

Au(I) Au(lll) dinier

Cl -- yes yes

Br- yes yesSC(NH2)2 yes no

SCN- not known

For current below the limiting diffusion current of the ligand.•" Decreases at high potentials.

no 100 yesyes 100** yesyes 100*" yesyes low yes

presence of various complexing agents that can beused under these conditions. Comparison of thesefeatures with the thermodynamic data of Figure 2shows that several predicted trends are borne out byobservation. For example, Au(I) is produced as thesole soluble gold species only in the presence ofthiourea. The available information for thiocyanatedoes not permit its inclusion in this comparison butAu(I) and Au(III) could be expected to form as pro-ducts in aqueous thiocyanate solutions. Also,simultaneous oxidation of the ligand is possible athigh potentials in all cases, but, as Figure 2 shows,this is most likely to compete with gold dissolution atlow potentials in the case of ligands such as thioureaand thiocyanate, whose positions lie close to thediagonal line.

For all practical purposes, only if chloride ions orthiourea are present can metal be dissolved with highcurrent efficiency from a soluble gold anode in acidicsolutions. This probably also holds good for theanodic dissolution of gold in electroetching processes.However, in electropolishing, where the metal isdissolved at high current densities under passive con-ditions, no such limitation applies and the use of anyof the ligands could be expected to promotepolishing, though with lower current efficiencies.

AcknowledgementsThis article is published by permission of the National Institute

for Metallurgy. The enthusiastic and excellent technical assistanceof Miss D. A. Velthuis in the preparation of this review is grateful-ly acknowledged.

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The second part of this article will be published in the July 1980 issue of Gold Bulletin.

55