STUDIES ON THE DEVELOPMENT OF A METHOD FOR DETERMINING \(10^{-5}\)—\(10^{-7}\%\) ANTIMONY IN HIGH-PURITY LEAD BY THE APN METHOD

V. F. Yankauskas, V. M. Pichugina

(Presented by the Scientific-Methodological Seminar of the Chemical-Technological Faculty)

In high-purity lead, an antimony content of no more than \(2 \cdot 10^{-5}\%\) is permitted. For the determination of such small amounts, colorimetric and chemical-spectral methods are most often used. However, the latter are time-consuming and laborious.

We have carried out studies on the development of a method for determining antimony in high-purity lead without separation of the matrix, by amalgam polarography with accumulation (APN).

According to literature data [1], stable cathodic waves of antimony are obtained in acidic supporting electrolytes. Earlier [2] it was shown that, in a background of \(1\ \mathrm{M}\) hydrochloric acid solution, separate anodic peaks of antimony, copper, and bismuth can be obtained on a mercury film electrode. In this background a good anodic antimony peak is obtained with a peak potential of about \(-0.15\ \mathrm{V}\) (sat. cal. el.).

The peak height in the concentration range studied is directly proportional to the concentration of antimony in the solution [2]. Consequently, the antimony content can be determined by the method of additions.

Studies showed that the presence of \(\mathrm{NO_3^-}\) ions interferes with the determination of antimony: the antimony peaks decrease and disappear with time. It is assumed that trivalent antimony, in the presence of \(\mathrm{NO_3^-}\) ions, is converted into pentavalent antimony, which is not reduced in a \(1\ \mathrm{M}\) hydrochloric acid background. Therefore, after dissolving the lead sample in nitric acid, in order to remove the excess of the latter, the solution is evaporated to dryness. To convert nitrates into chlorides, the solution should be evaporated to dryness three times, after first adding hydrochloric acid. Reduction of antimony to the trivalent state is carried out with 10% tin chloride [3].

On the basis of the above studies, the following method is proposed for determining antimony in high-purity lead.

Method

\(1.0\ \mathrm{g}\) of finely divided lead is placed in a quartz beaker of \(150\ \mathrm{ml}\) capacity and dissolved in \(6\)—\(8\ \mathrm{ml}\) of \(6\ \mathrm{M}\) nitric acid on a boiling water bath. The contents of the beaker are evaporated to dryness. To the dry residue are added \(5\ \mathrm{ml}\) of diluted \((3:1)\) hydrochloric acid, and the mixture is evaporated to dryness. This operation is repeated three times. Then \(5\ \mathrm{ml}\) of diluted \((3:1)\) hydrochloric acid are added and the mixture is heated for a period of

5–10 min in a boiling water bath. Then 5 drops of 10% tin chloride are added, the mixture is heated for another 5 min and cooled. The cooled solution is diluted with 1 M hydrochloric acid to a volume of 10 ml. 1 ml of the solution being analyzed is transferred to a clean beaker, 2 ml of 1 M hydrochloric acid solution is added, and electrolytic accumulation is carried out at a potential of \(-0.5\) V (vs. SCE) for 5 min on a mercury-film electrode.

The antimony content is determined by the method of additions.

Table 1

Results of the analysis of high-purity lead for antimony content

Table 1 gives the results of analyses of high-purity lead. The sensitivity of the procedure under the developed conditions is \(1 \cdot 10^{-6}\%\).

LITERATURE

1. T. A. Kryukova, S. I. Sinyakova, T. V. Aref’eva. Polarographic Analysis, Moscow, 1959.

2. L. F. Zaichko, V. F. Yankauskas, M. S. Zakharov. Factory Laboratory, 31, 2, 1965.

3. Yu. Yu. Lur’e, N. A. Filippova. Factory Laboratory, 18, 1, 1952; 24, 2, 1858.