Analytica Chimica Acta, 140 (1982) 335-338

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Short Communication

POTENTIOMETRIC REDOX DETERMINATION OF GOLD(II1) AND ITS APPLICATION TO ALLOYS

B. V. RAO

Defence Metallu&cal Research Laboratory, P-0. Kanchanbagh, Hyderabad 500 258

(India)

(Received 14th September 1981)

Summary. The simple potentiometric method proposed for the indirect determination of l-10 mg of gold(II1) is based on reduction to the metal with excess of cobalt(I1) in the

presence of l,lO-phenanthroline or 2,2’-bipyridine at pH 3 and 5O”C, and titration of the unused cobalt(I1) complex with iron(II1) chloride solution_ Many metal ions can be

tolerated; Ag(1) and Pd(I1) are eliminated by precipitation with sodium chloride and l,lO-phenanthroline or 2,2’-bipyridine, respectively, but Hg(II), Fe(II1) and Pt(IV) interfere. The method is applied to the determination of gold in alloys.

Gold(II1) can be reduced quantitatively to the metal with cobalt(I1) nitrate at pH 2-6.5 and 50°C in solutions containing l,lO-phenanthroline or 2,2’-bipyridine fl] . Gold (2.0-40.0 mg) can be determined by oxidation of the unreacted cobalt(I1) with iron(II1) and visual or potentiometric titration of the resulting ferroin or the iron(IIk2,2’-bipyridine complex with cerium(IV) sulphate solution [l] _ Lower amounts of gold (0.25-2.5 mg) can be determined similarly by titrating the unconsumed cobalt(I1) complex with cerium(IV) to visual, potentiometric or biamperometric end- points [2] _ In all these methods, filtration of the precipitated gold metal was essential to avoid interference with the titrations.

The aim of the work resorted here was to work out a rapid redos proce- dure without resorting to the filtration of gold, and without the intermediate reaction of excess of cobalt(I1) with iron(II1). Detailed investigations showed that the precipitated gold does not react with iron(II1) in weakly acidic medium and that cobalt(I1) can be titrated with iron(II1) without inter- ference from gold. Another favourab!e factor for simplification of the procedure is that the optimum conditions for the gold(II1 jcobalt(I1) reaction [l, 21 and the iron(III)-cobalt(II) reaction [3, 41 are practically identical. It has been shown that cobalt(I1) can be titrated at 50°C with iron(II1) in the presence of l,lO-phenanthroline [3] or 2,2’-bipyridine [4] at pH 3. In the method presented here, gold(II1) is precipitated as the metal with excess of cobalt(I1) in l,lO-phenanthroline or 2,2’-bipyridine medium at pH 3 at 5O”C, and tine unconsumed cobalt(I1) complex is titrated potentiometricaliy with standard iron(II1) chloride solution.

0003-2670/82/0000-0090/$02.75 0 1982 Elsevier Scientific Publishing Company

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Experinzen tal _-1pparatrts. A universal potentiometer (Leeds-Northrup type K-3) was

used wit.11 a platinum-SCE electrode pair. The pH values were measured with a Metrohm pH meter with a combination glasscalomel electrode_

Reagents_ Stock solutions of gold(III) chloride and cobalt(H) nitrate were prepared from Spec-pure gold and high-purity cobalt (Johnson- blatthey). Working solutions (0.005 M gold(III), and 0.01 M and 0.02 M cobalt(H)) were obtained by accurate dilution. An approsimately 0.02 M iron(II1) chloride solution was prepared from the tetrahydrate (Merck) and distilled water weakly acidified with hydrochloric acid. After standardiza- tion with potassium dichromate, this solution was accurately diluted to give 0.005 M and 0.01 M iron(II1) chloride solutions_ l,lO-Phenanthroline and 2,2’-bipyridine solutions (0.1 M) were prepared from Merck reagents. Analyt- ical-grade reagents were used to prepare solutions for the interference studies.

Recommended procedure. Into a 150-ml beaker, transfer a solution con- taining l-10 mg of gold(III), add a suitable volume of 0.01 M cobalt(I1) nitrate solution such that the cobalt(I1) concentration is at least five times higher than that of gold(III), and then add a calculated volume of 0.1 M l,lO-phenanthroline or 2,2’-bipyridine such that its concentration is at least 7-S times higher than that of cobalt(I1). Adjust the pH of the solution to about 3 with dilute sodium hydrosicie and add 10 ml of 0.1 M sodium

acetate-acetic acid buffer (pH 3.0). Maintain the temperature at 50°C and titrate with standard 0.005 M-O.01 M iron(II1) chloride solution. Near the end-point, particularly with 0.005 LM iron(II1) solution, wait for 2 min between the additions to allow the potential to stabilize. The time required is about 30 min.

Analysis of alloys. Dissolve a known weight of alloy in 5 ml of aqua regia, evaporate nearly to dryness and add 5 ml of hydrochloric acid (1 + 4). Cool to room temperature and dilute the solution to the mark in a lOO-ml volu- metric flask. Transfer an aliquot containing about 10 mg of gold to a 150-ml beaker. Add 12.50 ml of 0.02 M cobalt(I1) nitrate and 30 ml of 0.1 M l,lO- phenanthroline or 2,2’-bipyridine solution_ Adjust the pH to about 3 and continue with the above procedure, titrating with 0.01 M iron(II1) chloride solution_ -4 sharp potential increase of 150-200 mV indicates the complete- ness of the reaction_

Results Results obtained for gold(II1) in pure solutions are summarized in Table 1.

The maximum error obtained in the lowest concentration range was 2%, whereas at the highest level the error did not exceed 0.4%. The standard deviation for the determination of 3.94 mg of gold was 0.022 mg (n = 6).

Effect of diverse ions. The effects of various ions were studied by adding them separately to a fixed amount of gold (9.85 mg) in solution. The deter- mination was not affected by 10 mg of Cu(II), by 25 mg of Ni(II), Zn(II), Pb(II), Mn(II), Ca(II), Mg(II), Ra(II), Al(III), Cr(II1) or Se(IV), 5 mg of

337

TABLE 1

Potentiometric determination of gold in pure solutions

Gold present

(mg)

Gold found (mg)”

l,lO-Phen

method

Error 2,2’-Bipy

(70) method

Error

(S)

0.985 0.995 + 1.0

1.97 1.95 -1.0 3.94 3.96 i-o.5

5.91 5.94 i- 0.5 7.88 ‘7.91 + 0.4

9.85 9.82 -0.3

=Each value is the mean of 6 determinations.

0.995 i 1.0

1.99 + 1.0 3.92 -0.5

5.95 + 0.6 7.85 ---0.4

9.83 --0.2

Cd(H), Ti(IV), W(W) or Mo(VI), or by 50 m g of phosphate_ Most of these ions do not react with the complesing agent. Others such as Cu(II), Ni(II), Zn(II), Cd(I1) and Mo(V1) form complexes with l,lO-phenanthroline and 2,2’-bipyridine, but any suppressing effect on the formation of the cobalt comples was avoided by the addition of a large excess of completing agent. Silver(I), palladium(II), mercury(H) and platinum( IV) made the determina- tion impossible even when present in low concentration because of precipitate formation. However, the effect of 20 mg of Pd(II), and 25 mg of Ag(I), could be overcome by quantitative precipitation with l,lO-phenanthroline or 2,2’-bipyridine, and sodium chloride, respectively, followed by filtration_ Even small amounts of iron(II1) caused significant interference because cobalt(I1) reduced iron(II1) to iron(I1) under the experimental conditions.

Application of the method to alloys. As the results obtained when common ions generally encountered in gold alloys, such as Cu(II), Ni(II), Zn(I1) and

Cd(II), were present, were quite satisfactory, the proposed method was applied to the determination of gold in gold-containing alloys. The results presented in Table 2 are in good agreement with the values obtained by the well-known hydroquinone method [ 53 _

TABLE 2

Potentiometric determination of gold in alloys

Type of alloy Gold found (70)

Hydroquinone

method [ 5 ]

I,1 0-Phen method”

2,2’-Bipy methoda

Au-Pd’= 55.0 54.92 5-1.88 Au-cu ‘79.4 79.32 79.56

Au--AgC 80.5 80.56 80.68 Au-Ni-Zn 68.6 68.48 68.68

=Average of 5 determinations. bPd removed as its phenanthroline or bipyridine complex. CAg removed as silver chloride.

Discussion The normal oxidation-reduction potential of the Au(III)/Au(O) system is

E, = 1.4 V. Cobalt(H) cannot be oxidized because the redox potential of the Co(II)/Co(III) system is l-79 V. The formation of complexes with l,lO- phenanthroline or 2,2’-bipyridine alters the oxidation potential of the couple to values of 0.37 V or 0.31 V, respectively. Under appropriate experimental conditions, gold(III) oxidizes cobalt(I1) to the respective cobalt(II1) com- plexes and is reduced to the metal, e.g.,

Au3+ i 3[Co(l,10-phen)3] *+ = Au + 3[Co(l,lO-phen),] 3e

The excess of cobalt(I1) can then be titrated, in the presence of the gold, with iron(II1) chloride solution_ This proposed method is more convenient and faster than the cerium(IV) methods [ 1,2] _

The author is indebted to Dr. D. P. Lahiri, Deputy Director, Defence Metallurgical Research Laboratory, for valuable suggestions and help in the preparation of the manuscript_

REFERENCES

1 B. V. Rao, S. V. Athavale and S. L. N. Acharyulu, Chim. Anal, 53 (1971) 323. 2 B. V. Rao, D. V. Rao and D. P. Lahiri, Mikrochim. Acta, II (1977) 201. 3 F. Vydra and R. Pribil, Talanta, 5 (1960) 44; Collect_ Czech. Chem. Commun., 26

(1961) 2169. 4 F. Vydra and R. Pribil, Talanta, 8 (1961) 824. 5 J. Seath and F. E. Beamish, Ind. Eng. Chem., Anal. Ed., 9 (1937) 373.