Biamperometric determination of copper, silver and gold with ascorbic acid

Biamperometric determination of copper, silver and gold with ascorbic acid

Analytica Chimica Acta, 167 (1985) 399-401 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands Short Communication BIAMPEROMETR...

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Analytica Chimica Acta, 167 (1985) 399-401 Elsevier Science Publishers B.V., Amsterdam -Printed

in The Netherlands

Short Communication

BIAMPEROMETRIC DETERMINATION OF COPPER, SILVER AND GOLD WITH ASCORBIC ACID

S. N. JOSHI*s, Department

A. G. KULKARNI

of Chemistry,

and G. S. DESHMUKH

Ramnarain

Ruia College, Matunga, Bombay

400 019 (India)

(Received 3rd September 1983)

Summary. The simultaneous determination of milligram amounts of copper, silver and gold in mixtures is described. Ascorbic acid is added in excess and back-titrated biamperometrically with standard potassium iodate solution. Mixtures can be analyzed by using precipitation and masking.

Ascorbic acid has been quite widely used as a reducing titrant since its introduction by Erdey and Boder [l] for inorganic analysis. An interesting application is in the determination of noble metals. Copper can be directly titrated with ascorbic acid using 2,6-dichlorophenolindophenol as indicator [ 21. Silver can be titrated with ascorbic acid at 60” C; shortly before the endpoint the solution is buffered with sodium acetate [ 31. Gold(II1) in solutions at pH 1.6-3 can be titrated potentiometrically or amperometrically at 50°C [4]. An indirect coulometric determination of platinum(IV) has been based on the ascorbic acid/iodine system [5]. The methods reported for various noble metals are restricted to individual metal ions. A study of the simultaneous determination of two or more cations led to the development of the methods described here for indirect determinations of copper, silver and gold separately or when present together. Deshmukh and Bapat [6] reported the two-stage oxidation of ascorbic acid with potassium iodate. The first end-point, in 1 M hydrochloric acid, corresponds to the reduction of iodate according to Landolt’s reaction KI03 + 3C6H806 + KI + 3C6H606 + 3H,O. The second end-point, in >4 M hydrochloric acid, corresponds to formation of iodine monochloride as in the classical Andrew’s titration. The present method is based on addition of an excess of ascorbic acid to the metal ion solution, followed by back-titration in an acidic medium. Ascorbic acid was standardized against potassium iodate [6]. At low acidities its oxidation by iodate leads to dehydroascorbic acid and potassium iodide. Visual extractive end-points (carbon tetrachloride) are possible but it is easy to overstep the %esent

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o 1985 Elsevier Science Publishers B.V.

400

end-point. The biamperometric dead-stop end-point is advantageous when there is an iodine/iodide couple in solution [ 71. Experimental Reagents and apparatus. All the chemicals used were of analytical-reagent grade. A stock solution of copper was prepared by dissolving copper(I1) sulphate pentahydrate (12.484 g) in 1 1 of twice-distilled water containing sufficient acid to prevent hydrolysis. Silver nitrate (8.493 g 1-l) was dissolved in twice-distilled water. The gold solution was prepared by dissolving 3.033 g 1-l gold(II1) chloride in 0.5 M hydrochloric acid containing 10% (w/v) sodium chloride. Less concentrated solutions were prepared by successive dilution and concentrations were checked by classical methods [ 81. Ascorbic acid was used as an approximately 0.05 M solution; its titre was checked just before use and solutions were re-prepared as required. A standard 0.25 M solution of potassium iodate was prepared from the dried reagent. The manual biamperometric equipment had two similar platinum electrodes polarized at 100 mV; the solution was stirred magnetically. All the titrations were done at 25 + 1°C. Galvanometer readings were noted after each addition of reagent. Current values were plotted against volume of titrant added to locate the end-point graphically. Titration of ascorbic acid. An ahquot of ascorbic acid solution (10 ml of 0.05 M) was made 2 M in hydrochloric acid, in a total volume of 25 ml. The solution was titrated with standard iodate solution. A sudden increase in the current indicated the end-point (A ml of 0.25 M KI03). Determination of copper, silver or gold. An aliquot of copper (2-20 mg), silver (l-15 mg) or gold (l-15 mg) solution was mixed with a known excess of ascorbic acid (10 ml of 0.05 M). After 5 min, the hydrochloric acid concentration was adjusted to 2 M in a total volume of 25 ml. Higher acidities must be avoided so as not to overshoot the first end-point in Landolt’s reaction. Unreacted ascorbic acid was back-titrated with 0.25 M iodate solution as described above (B ml of KIOJ). The amount (A -B) ml of 0.25 M KIOJ was then related to the metal ion present. Results are listed in Table 1. Under the conditions used, copper(I1) is reduced to copper(I), and silver(I) and gold(II1) to the respective metal. TABLE

1

Single determinations

of copper, silver and gold

Copper (me)

Error (%)

Taken

Found

3.177 6.354 9.531 13.708

3.192 6.337 9.527 13.733

+0.47 -0.27 -0.04 + 0.15

Silver (mg) Taken

Found

2.696 5.393 8.089 10.786

2.668 5.414 8.102 10.840

Error (%) -1.03 +1.29 +0.24 + 0.50

Gold (mg) Taken

Found

1.575 3.151 6.302 9.453

1.574 1.148 6.296 9.444

Error (%) -0.063 -0.095 -0.095 -0.095

401

Determination of binary and ternary mixtures This study was restricted to analysis of gold alloys (for ornaments) containing copper and silver. Each of the metals interferes in the determination of the other. Procedures are given below to determine binary and ternary mixtures. However, the method suffers interference from other metal ions like Fe(III), V(V), Hg(II), Ce(IV), Se(IV), and Se(V1). Copper or gold and silver in a mixture. Silver immediately precipitates as a sulphate or chloride from acidic solution. The precipitate was dissolved in 5 ml of 0.1 M ammonia solution and a known excess of ascorbic acid (10 ml of 0.05 M) was added. After 5 min, the solution was neutralized and diluted to 25 ml with 2 M hydrochloric acid, and the unused ascorbic acid was backtitrated with the iodate solution. In another aliquot, the silver precipitate was left, and either copper or gold was determined as described above for the single elements. The first titration gives the total metal content, so that the amount of silver can be calculated. Copper and gold in a mixture. Both the metal ions were titrated by the usual procedure. Then, in another aliquot, copper was masked with 0.1 M disodium-EDTA and gold alone was titrated. Analysis of a ternary mixture. Accurately weighed gold alloy (about 3 50 mg) was dissolved in aqua regia, silver being precipitated as silver chloride, which was separated and dissolved in 100 ml of 0.1 M ammonia. The determination of silver was completed as for the binary mixture. The acidic filtrate was evaporated to dryness and the residue was dissolved in distilled water. Hydrochloric acid and sodium chloride were then added to make the solution 2 M and 10% (w/v), respectively, on dilution to 100 ml. Aliquots of this solution were used to determine copper and gold as described above. A gold alloy analyzed in this way was found to contain 62.5% gold, 22.5% silver and 15.0% copper. The analysis of the ternary mixture is of considerable technical importance. REFERENCES 1 2 3 4 5 6 7 8

L. L. L. L. A. G. C. A.

Erdey and E. Boder, Anal. Chem., 24 (1952) 418. Erdey and G. Siposs, Z. Anal. Chem., 157 (1957) 166. Erdey and I. Buzas, Acta Chim. Sci. (Hungary), 4 (1954) 195. Erdey and G. Rady, Talanta, 1(1958) 159. HuIanicki and W. Jedral, Anal. Chim. Acta, 100 (1978) 399. S. Deshmukh andM. G. Bapat, Z. Anal. Chem., 199 (1963) 367. W. Foulk and A. T. Bawden, J. Am. Chem. Sot., 48 (1926) 2045. I. Vogel, Quantitative Inorganic Analysis, 3rd edn., Longmans Green, London, 1975.