Anafyhca Chwca Acta, 258 (1992) 167-170 Elaewer Science Pubbshers B V , Amsterdam

167

Potentiometric study of the reduction of gold(II1) with cobalt(U) in 1,100phenanthroline or 2,2’-bipyridine

medium: redox titration of gold(II1) and its application to alloys

BV Rao

&fence Metallurpcal Research Laboratory, P 0 Kianchanbagh, Hydembad-XXI 258 (induz)

(Received 8th July 1991)

Abstract

A stmple potentmmetrlc method was developed for the direct determmation of gold, based on the redox tltratlon of goId with cobalt(U) m the presence of l,lO-phenanthrolme or 2,2’-blpyrldme The reductton of gold(III) to the metal and the oxldatton of cobalt(U) to the respecttve cobalt(II1) complexes proceeds raptdly m the pH range 4-6 at WC A mmunum of a tenfold molar amount of either of the complexmg agents IS necessary for the quantltattve reactlon Many common metal Ions do not mterfere The mterference due to A&) and Pd(I1) can be overcome but the presence of H&l), Fe(II1) and Pt(IV) mvahdates the determmatlon The method was successfully appbed to gold-contammg alloys

Keywords Potentlometry, Tltnmetry, Alloys, Gold

In a premous mvestlgatlon 111, It was shown that gold(II1) can be reduced quanfitatlvely to the metal wth cobalt(I1) 111 the presence of l,lO- phenanthrolme or 2,2’-blpyrldme medmm ac- cordmg to the followmg reactions

Au3++ 3[Co(l,lO-phen),]‘+-,

Au + 3[Co(l,10-phen),]3+

Au3++ 3[Co(2,2’-blpy)$+ +

Au + 3[Co(2,2-blpy)3]3+

Gold (2-40 mg) was determmed mdu-ectly by omdatlon of the unreacted cobaId complex vvlth lron(II1) and visual or potentlometrlc titration of the resultmg ferrom or Fe@)-btpmdme complex wth standard cermm(IV) sulphate solution [l] In subsequent mvestigatlon [2], it was found that small amounts of gold (0 25-2 5 mg) can be de- termmed by tltratmg either of the unconsumed

cobalt(I1) complexes with cenum(IV) sulphate by visual, potentlometnc or bmmperometnc end- point detection In the above methods, fdtratlon of the preclpltated metal was essential to avoid mterference with the titrations

In the work described m this paper, the redox reactlon between gold(II1) and cobalt(I1) m l,lO- phenanthrohne or 2,2’-blpyr~dme medmm was studled m detail by dnzct potentlometnc tltra- tlons The proposed method pernuts the detemu- nation of gold0111 with cobaltU1) chloride as a stable tltrant

EXPERIMENTAL AND RESULTS

Apparatus A Leeds-Northrup Type k-3 potentiometer

with a platmum-SCE electrode pour was used for potentlometrlc tltratlons and a Metrohm pH me-

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BV RAO

ter with a combmatlon glass-calomel electrode for pH measurements

Reagents Analytical-reagent grade reagents were used to

prepare solutrons of various Ions meant for mter- ference studies

GoZd(III) chkwzde solutron, 005 M Prepared by dlssolvmg 4 925 g of gold (99 9%) m aqua regla, slowly evaporating the solutlon nearly to dryness, addmg 10 ml of hydrochloric aad (1 + 1) and dllutmg to 500 ml

Cobalt(ZZ) chlonde solution, 0 05 M Prepared by dlssolvmg 2 947 g of cobalt (99 99%) 111 hydro- chlorlc acid (1 + 2) and diluting to 1 1 The test solutions and the standard tltrant solutions were prepared from the 0 05 M solution by accurate dllutron

l,lO-Phenunthrolme, 0 1 M, and 2,2’-blpyndme, 0 1 M A 19 823-g amount of l,lO-phenanthrolme (Merck) and 15 62 g of 2,2’-blpyndme (Merck) were dissolved separately m water weakly acldl- fled with hydrochloric acid and dduted to 1 1 Solutrons of 0 05 and 0 02 M were obtained by appropriate dllutlon

Buffer solutions, pH 3 O-6 5 Prepared by mix- mg calculated molar amounts of sodium acetate and acetlc acid, the pH was checked with the combmed glass electrode

Prehminary utvestlgatwns Trtratlons were done to study the effects of

l,lO-phenanthrohne, 2,2’-blpyndme, pH and temperature To 3 0 ml of 0 01 M gold(II1) chlo- nde solutlon were added 15 ml of 0 05 M l,lO- phenanthrohne or 2,2’-blpyridme solutlon and 15 ml of buffer solution of pH 4 The solution was dduted to 40 ml and titrated vvlth 0 02 M cobalt(U) chlonde solution potentlometrlcally

Effect of phenanthrohne and blpyruime A number of potentlometrlc determmatlons of

a constant amount of gold(II1) were made with 0 02 M cobalt(U) chloride at pH 4 m the presence of various molar amounts of phenanthrolme or blpyrldme The results mdlcated that v&h ratios of Co to phenanthrolme and of Co to blpyndme of 1 1 and 1 2 the course of the reactlon 1s not

quantltatlve, whereas it 1s so when the ratio of Co to the complexmg agents 1s 1 3 From the redox reactlons, it was found that 1 mol of gold(II1) oxldrzes 3 mol of cobalt(U) and, therefore, the concentration of the complexmg agents required 1s nme tunes the molar concentration of gold(II1) It 1s therefore Inferred that the molar concentra- tion of either of the complexmg agents must be at least ten times higher than that of gold(II1)

Effect of pH Titrations were also done to study the effect of

pH on the course of the redox reactions From the tltratlons of gold(II1) m the presence of a constant molar excess, Au phen = 1 12 and Au blpy = 1 12, it 1s evident that with a gradual decrease m pH the potential decrease at the equivalence pomt increases from 120-150 mV at pH 5 5 to 220-250 mV at pH 4 At pH G 3 the gold(II1) forms compounds with the complexmg agents m a preclpltated condltlon However, at pH > 6 the potentral decrease IS comparatively smaller Hence the pH of the titrated solution 1s important but not critical as long as It lies v&hm the range 4-6

Effect of temperature An increase m temperature of the titrated

solution was found to enhance the rate of the reactions At temperatures between 40 and SO“C, the equlhbrmm IS established qmckly and the stablhzatlon of the potential of the solution 1s faster than at room temperature

Procedure Into a titration vessel, transfer a solution con-

taming 0 985-24 63 mg of gold(II1) together with a suitable volume of l,lO-phenanthrolme or 2,2’- blpyrrdme solution having a molar concentration at least ten tunes higher than that of gold(II1) Adjust the pH to 4-5 with a suitable sodium acetate-acetic acid buffer and heat to 50°C Titrate the solution m volumes of 40-75 ml with 0 01-O 025 M standard cobalt(U) chlonde solu- tion with stirring to a potentlometrlc end-pomt The potential break at the equivalence pomt 1s 180-250 mV for 0 04 ml of the standard solution The complete determination takes about 30 mm

POTENTIOMETRIC DETEIWINATION OF AdIII) 169

TABLJZ! 1

Potentmmetnc titration of gold(III) wth cobah chloride

Gold taken Complexmg Gold found (mg) agent ’ (rnp) b

0 965 Phen 0997*003 &W 1005*002

197 Phen 1% *003 394 Phen 397 *004

B~PY 3% *003 5 91 Phen 593 fOO4 788 Phen 791 *004

BW 791 *004 985 Phen 989 *006

14 78 Phen 1479 +006 B~PY 1482 rtOO5

19 70 Phen 1974 *006 2463 Phen 2465 *008

B~PY 2468 kOO7

’ Phen = l,lO-phenanthrolme, lhpy = 2,2’-blpyndme b Aver- age of five determmatlons, f standard dewatlon

Table 1 indicates the accuracy achieved for pure gold(II1) solutions

Effect of diverse wns Vanous ions were added to study their mflu-

ence on the determmatlon of gold To a solution contammg 7 88 mg of gold were added solutions contammg fivefold amounts of other ions sepa- rately A suitable amount of 0 1 M phenanthro- lme or blpyrldme was added, the solution was adjusted to about pH 4 with dilute sodmm hy- droxlde solution and 20 ml of sodium acetate- acetrc acid buffer (pH 4) were added The solu- tion was diluted to 75 ml, heated to 50°C and titrated potentlometrlcally wth 0 01 M cobalt011 chloride solution The results mdlcate that there is no interference from many common metal Ions, such as Ca(II), BatIT), Mg(II), M&I), Pb(II), Al(III), Cr(III), Se(IV>, Th(IV), W(VI) and U(VI> However, sllveril), palladmm(II), mercury01) and platmum(IV) exert an mhlbltory actlon on gold determmatlon even when present m low concen- tratlon, owmg to the formation of volurnmous precipitates The unfavourable effect of sliver(I) and palladmm(I1) can be overcome by quantlta- tlve preclpltatlon with sodmm chlonde and l,lO- phenanthrolme or 2,2’-blpyrldme, respectively, followed by filtration

MercutyfII) and platmum(IV) Interfere sen- ously The presence of even a small amount of iron(II1) causes a slgnrficant Interference as cobalt(I1) reduces iron(III) quantltatlvely m the complexmg medmm used, gwmg ferrom 131 or the lron(II)-blpyrldme complex [4] under the expen- mental comhtlons Ions such as CdII), Nl(II), Zn(II), Cd(I1) and MdVI) form stable complexes with l,lO-phenanthrolme or 2,2’-btpyrtdme m a 1 3 molar ratlo and thereby suppress the forma- tlon of the cobalt complexes, this mterference was prevented by the addltlon of a calculated excess of complexmg agents The presence of anions such as mtrate, sulphate and phosphate did not shown any undesirable effect

Anaiytxal apphcatwn of alloys The method has been employed successfully

for the accurate determmatlon of gold m gold- contammg alloys, accordmg to the followmg pro- cedure

Dissolve 0 1-O 25 g of alloy m 10 ml of aqua regla, evaporate the mixture carefully to a small volume but not to dryness and add 5 ml of hydrochloric acid (1 + 4) Cool the solution to room temperature and dilute to 100 ml m a volumetric flask Transfer an ahquot contammg 8-16 mg of gold mto a 150~ml beaker and add a

TABLE 2

Potentlometnc determmatlon of gold m alloys

Type of alloy

Gold found (%I

Hydroqumone Proposed method method [5] Complexmg Result

Au-Cu 75 36

agent

Phen 75 55 B~PY 75 23

Au-Cu a3 25 Phen 83 11 BIPY 83 44

Au-h+211 68 56 Phen 6865 B~PY 68 35

Au-Ag a 8050 Phen 80 15 B~PY 80 15

Au-Pd b 55 0 Phen 548 B~PY 55 3

’ Ag removed by preclpltatlon as AgCl wth Nail b Pd re- moved as complex wth IJO-phenanthrolme or 2,2’-lnpyn- dme

170 BV RAO

smtable volwne of 0 05 M l,lO-phenanthrolme or 2,2’-blpyndme solution such that rts concentra- tion 1s about twelve tnnes higher than that of gold011) Adjust the pH to about 4 with dilute sodium hydroxide and add 20 ml of sodmm ac- etate-au% acid buffer (pH 4 0) Mamtam the temperature at 50°C and titrate potentlometn- tally Hrlth 0 01-O 025 M cobalt(I1) chloride solu- tion In the vlcmlty of the equivalence pomt, wat 2 mm between the additions of the tltrant to allow the potential to stablllze Table 2 shows that the method gives results m agreement with those obtained by the hydroqumone method [5]

DISCUSSION

The normal oxldatlon-reduction potential of the Au(III)-Au couple 1s + 1 4 V Cobalt(I1) can- not be oxldlzed as the redox potential of the Co(III)-co(II) couple 1s + 179 V The formation of complexes with l,lO-phenanthrolme or 2,2’-bl- pyrtdme alters the oxldatlon potential of the sys- tem to 0 37 or 0 31 V respectively Under appro- priate expernnental conditions, gold(II1) oxldlzes cobah quantltatlvely to the respective cobalt- (III) complexes and Its reduction product, gold

metal, 1s precipitated In solutions with lo-12-fold molar amounts of complexmg agents, with a per- rmsslble pH range of 4 O-5 5 and at 5O”C, the reduction of gold(III) proceeds rapldly and direct potentlometrlc titration wth cobah chloride 1s possible The optimum experunental parameters are sunllar wrth the two complexmg agents except that the potential decrease at the equivalence pomt 1s greater m the presence of 2,2’-blpyndme This work has resulted m the development of a method using cobalt(H) chlorrde for the first time as a stable reductlmetrlc tltrant for the deten- nation of gold(II1) with apphcatlon to gold-con- tammg alloys The proposed method IS the reap- rocal of that for determmmg cobalt [6]

REFERENCES

1 B V Rao, S V Athavale and S LN Acharyulu, Chum Anal (Paris), 53 (19711 323

2 B V Rao, D V Rao and D P Lahm, Mlkrochlm Acta, II (1977) 201

3 F Vydra and R Prlbll, Talanta, 5 (1960) 44, Collect Czech Chem Commun , 26 (1961) 2169

4 F Vydra and R Prlbd, Talanta, 8 (1961) 824 5 R Belcher and A J Nutten, J Chem Sot, (1951) 546 6 B V Rao, S V Athavale, T H Rao, S L N Acharyulu and

R V Tamhankar, Anal Chun Acta, 70 (1974) 169