Determination of 10-5–10-6% Impurities in High-Purity Silver by Amalgam Polarography with Accumulation
Abstract Generated abstract
A procedure is described for determining trace impurities in high-purity silver at levels of about 10^-5 to 10^-6 percent, addressing the limited sensitivity and reproducibility of existing spectral methods for some elements. The method uses amalgam polarography with accumulation after dissolution of a silver sample, precipitation and removal of silver as chloride, extraction separation of antimony with ethyl acetate, and polarographic determination on potassium chloride or hydrochloric acid backgrounds. Experiments on high-purity silver samples show that Pb, Zn, In, Bi, Sb, and Cu can be determined, with simultaneous measurement of Pb, Zn, In, Bi, and Cu and separate determination of Sb. The proposed procedure is reported to provide monitoring sensitivity down to 5 x 10^-6 percent for Pb and Zn and about 5 x 10^-5 percent for several other elements, with an estimated accuracy of 10 to 15 percent.
Full Text
DETERMINATION OF \(10^{-5}\)—\(10^{-6}\%\) IMPURITIES IN HIGH-PURITY SILVER BY AMALGAM POLAROGRAPHY WITH ACCUMULATION
V. S. UTENKO
(Presented by the scientific-methodological seminar of the CTF)
The analysis of high-purity silver is carried out by spectral methods: the spectral method proper, the hollow-cathode arc method, and the spectrochemical method. These methods have their own shortcomings—low sensitivity and poor reproducibility of results for certain impurities \((\mathrm{Sb}, \mathrm{Zn})\).
The stringent requirements for the accuracy and sensitivity of the analysis of high-purity silver confronted the laboratory with the need to develop a new method for analyzing silver, free of these shortcomings, as well as a method permitting control of high-purity silver standards. The method chosen for this purpose was amalgam polarography with accumulation.
On the basis of numerous experiments and literature data, the following scheme for the analysis of high-purity silver is proposed. A 3 g sample of silver is dissolved in 25–30 ml of nitric acid. The solution is evaporated 2–3 times to remove nitrogen oxides. After evaporation, the solution is diluted with 10–15 ml of distilled water. Silver is precipitated as the chloride by adding hydrochloric acid to the solution. The precipitate with the solution is boiled for 2–3 minutes to coagulate the silver chloride.
Silver chloride is separated from the solution by filtration through a paper filter; the AgCl precipitate is washed 3–4 times by decantation with hot distilled water containing 1% \(\mathrm{HNO_3}\), and then on the filter with pure hot distilled water. Analysis of the silver chloride for the presence of impurities gave a negative result. The filtrate, evaporated to a volume of 25–30 ml, is diluted with hydrochloric acid to a concentration of 10.5 g HCl in 50 ml of solution, which corresponds to \(6N\) HCl.
Antimony is separated from the remaining impurities by extraction with ethyl acetate. The ratio of the volumes of solution and extractant is 1:1. Experimental data show that the solution does not require repeated extraction. The raffinate and extract are evaporated to dryness.
To the beaker containing the evaporated raffinate, 50 ml of a background solution of \(0.1N\) KCl is added and the mixture is slightly heated. In this solution lead, zinc, copper, bismuth, and indium are determined simultaneously. Into the beaker containing the evaporated extract, 50 ml of \(2N\) HCl solution and 20 mg of sodium hypophosphite are poured in to reduce antimony to the trivalent state. In this solution \(\mathrm{Sb(III)}\) is determined.
Analysis of all high-purity silver samples was carried out on an SGM-9 polarograph at a sensitivity of \(1/10—28\cdot10^{-3}\) \(\mu\text{A}/\text{mm}\), with a preliminary electrolysis time of 15 min, an electrolysis potential of 1.2 V (vs. S.C.E.), and an electrode of the “lying drop” type.
The impurity concentration was determined by the comparison method.
The results of the sample analyses are presented in the table.
Table
Results of analysis of high-purity silver samples
| Parallel samples | Chemico-spectral method: Cu, content in % × 10⁵ | Chemico-spectral method: In, content in % × 10⁵ | Chemico-spectral method: Bi, content in % × 10⁵ | Amalgam polarography with accumulation: Cu, content in % × 10⁵ | Amalgam polarography with accumulation: In, content in % × 10⁵ | Amalgam polarography with accumulation: Bi, content in % × 10⁵ |
|---|---|---|---|---|---|---|
| 1 | 8,3 | 0,6 | 5 | 7,5 | — | 5,2 |
| 2 | 8,5 | — | 5,4 | |||
| 1 | 8,5 | 1,6 | 5 | 7,5 | — | 5,0 |
| 2 | 8,7 | — | 4,8 | |||
| 1 | 9,4 | 2,5 | 5 | 8,4 | — | 5,0 |
| 2 | 7,7 | — | 5,0 | |||
| 1 | 10,6 | 3,3 | 6 | 10,2 | — | 5,6 |
| 2 | 10 | — | 5,8 | |||
| 1 | 9,9 | 5,1 | 5,1 | 10 | 5,8 | 6,1 |
| 2 | 8,7 | 5,7 | 5,8 | |||
| 1 | 9,1 | 6,1 | 6 | 1,06 | 5,6 | — |
| 2 | 10 | 6,1 | — | |||
| 1 | 11,1 | 8 | 8 | 10 | 8 | — |
| 2 | 9 | 7,1 | — | |||
| 1 | 8,5 | 10 | 10 | 9,3 | 8 | — |
| 2 | 10 | 1,1 | — | |||
| 1 | 11,7 | 12 | 12 | 11,7 | 1,5 | — |
| 2 | 10 | 1,5 | — |
Conclusions
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The investigations showed the possibility of determining microimpurities in high-purity silver by the method of amalgam polarography with accumulation for six elements: Pb, Zn, In, Bi, Sb, and Cu. Pb, Zn, In, Bi, and Cu are determined simultaneously against a background of \(0.1\,N\) KCl; Sb is determined against a background of \(2\,N\) HCl.
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A drawback of this procedure is the mandatory separation and determination of copper and antimony on different backgrounds, since their anodic dissolution potentials are close (Cu — 0.15 V, Sb — 0.17 V).
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The proposed procedure can serve for monitoring high-purity silver with a sensitivity of \(5\cdot10^{-6}\%\) for Pb and Zn and \(5\cdot10^{-5}\%\) for Cu, Zn, Sb, and Bi; the accuracy of determination is 10–15%.