M. S. ZAKHAROV, V. M. PICHUGINA

STUDIES ON THE DEVELOPMENT OF A METHOD FOR DETERMINING MICROCONCENTRATIONS OF BISMUTH IN HIGH-PURITY TIN BY AMALGAM POLAROGRAPHY WITH ACCUMULATION

(Presented by the scientific seminar of the Department of Physical Chemistry)

In work [1], an alkaline solution of ethylenediamine was proposed as the supporting background for the simultaneous determination of microconcentrations of bismuth, copper, lead, and zinc in high-purity tin by amalgam polarography with accumulation. However, during prolonged storage ethylenediamine gradually decomposes (especially in the light), and as a result the bismuth and copper waves on the polarograms overlap one another. For this reason, it does not appear possible to determine the content of microconcentrations of bismuth and copper in high-purity tin. The copper content can be determined when polarographing on an alkaline background [2]; bismuth, however, does not give stable anodic waves on this background. At present, however, there is an acute need in technology for a highly sensitive method of determining microconcentrations of bismuth in high-purity tin.

The present work is devoted to developing a method for determining microconcentrations of bismuth in high-purity tin by amalgam polarography with accumulation.

According to the literature data [3], stable cathodic waves of bismuth are obtained on acidic backgrounds. We studied the amalgam-polarographic behavior of bismuth on backgrounds of sulfuric and hydrochloric acids of various concentrations. On the basis of comparing the polarographic behavior of bismuth in various acids, we settled on a background of 0.25 M HCl. In addition, hydrochloric acid can be readily purified from impurities of heavy metals by distilling it in a quartz distillation apparatus.

The studies showed that on a background of 0.25 M HCl a good anodic bismuth wave is obtained, with a peak potential of about 0.06 V (sat. cal. el.). The height of the wave in the concentration interval studied is directly proportional to the concentration of bismuth in solution (Fig. 1). Consequently, the bismuth content can be determined by the method of additions.

The potential of the copper wave on this background is only 0.16 V more negative than the potential of the bismuth wave; however, since the sensitivity for copper is lower than the sensitivity for bismuth, a 4–5-fold excess of it does not interfere with the determination of bismuth. In addition, usually in samples of pure tin the contents of bismuth and copper are approximately the same. The lower sensitivity for copper is possibly due to the formation of an intermetallic compound of copper with the mercury of the electrode [4].

To determine microconcentrations of bismuth, the excess tin must be removed. Tin is removed by evaporating it in the form of \(\mathrm{SnBr_4}\), as proposed in [2]. In order to determine whether microimpurities of bismuth can co-distill with tin, studies were carried out to establish the optimum temperature for evaporating tin.

Fig. 1. Calibration graph for bismuth.
Experimental conditions: \(\tau = 5\) min; \(\varphi_{\mathrm{e}} = -1.0\ \mathrm{V}\) (sat. cal. el.); \(V = 2\ \mathrm{ml}\); \(d_{\mathrm{k}} = 0.08\ \mathrm{cm}\); sensitivity \(3 \cdot 10^{-3}\ \mu\mathrm{A}/\mathrm{mm}\); background \(0.25\ M\ \mathrm{HCl}\).

The studies were carried out on a factory sample of tin containing bismuth. Evaporation was carried out at 300, 250, and \(220^\circ\). The results of the studies are presented in Table 1.

It is evident from the table that, at all the temperatures investigated, the bismuth content in the tin does not change. Consequently, at these temperatures no co-distillation of microconcentrations of bismuth occurs. At a temperature of \(300^\circ\), complete removal of tin takes place within 10–12 min, which fully satisfies the requirements for carrying out the analysis. Therefore, in subsequent experiments, evaporation of tin was carried out at temperatures of 250–\(300^\circ\).

Table 1

Results of studies to determine co-distillation of bismuth

On the basis of the studies we carried out, the following procedure is proposed for determining microconcentrations of bismuth in high-purity tin.

Procedure

Instruments: a polarograph with self-recording or photographic recording.

Reagents: hydrochloric acid, \(0.25\ M\) solution. The acid was purified from heavy metals by fourfold distillation. The final distillation was carried out in a quartz distillation apparatus.

A standard bismuth solution was prepared by dissolving metallic bismuth in nitric acid. \(0.5\ \mathrm{g}\) of finely powdered tin is washed with a weak solution of nitric acid and dissolved in a quartz-

4 ml of HBr and 1 ml of Br₂ are added to the same beaker. The contents of the beaker are evaporated in a sealed glass evaporation chamber at 250–300°. After evaporation, 2 ml of 0.25 M HCl (background) is added to the beaker and, at a potential of −1.0 V (vs. S.C.E.), impurities are accumulated on the mercury electrode in the electrolyzer described in [2] for 10 minutes. After this, polarography is carried out from a potential of −0.25–−0.5 V (vs. S.C.E.). The bismuth content is determined by the method of additions and calculated from the formula

\[ \% \mathrm{Bi}=\frac{h_1 \cdot v \cdot c \cdot 10^{-4}}{h_2 \cdot m}, \]

where \(h_2\) is the peak height from the bismuth addition;
\(v\) is the volume of the addition, ml;
\(c\) is the concentration of the addition, \(\gamma/\mathrm{ml}\);
\(h_1\) is the peak height from bismuth in tin;
\(m\) is the weighed portion of tin.

The maximum sensitivity of the procedure (at a peak height of 4 mm) for bismuth under the developed conditions (electrolysis time 10 minutes, electrolysis potential −1.0 V (vs. S.C.E.), diameter of the mercury drop 0.08 cm, instrument sensitivity \(3 \cdot 10^{-9}\) A/mm, with analysis performed on a 0.5 g tin sample) is \(2 \cdot 10^{-6}\%\).

Using the developed procedure, analyses were carried out on a series of high-purity tin samples (Table 2). Polarograms of one of the analyses are shown in Fig. 2.

We consider it a pleasant duty to express our gratitude to Prof. A. G. Stromberg for valuable advice during the performance and discussion of the work.

Table 2

Results of the analysis of high-purity tin for bismuth content

Experimental conditions: tin sample — 0.5 g; \(V = 2\) ml; \(\tau = 10\) min; sensitivity — \(3 \cdot 10^{-3}\) μA/mm; \(d_k = 0.08\) cm.

Fig. 2. Polarograms for analysis of a tin sample. Experimental conditions: \(\tau = 10\) min; \(\varphi_{\mathrm{e}} = -1.0\) V (vs. S.C.E.); \(V = 2\) ml; \(d_k = 0.08\) cm; sensitivity \(6 \cdot 10^{-3}\) μA/mm.
1. Polarogram of the tin sample. 2. The same, but with an addition of 0.05 \(\gamma/\mathrm{ml}\) of a standard bismuth solution.

Conclusions

A procedure has been developed for determining microconcentrations of bismuth in high-purity tin. The method is based on separating the excess tin by evaporating it in the form of SnBr₄, followed by determination of bismuth by the method of amalgam polarography with accumulation. Results are given for the analysis of several tin samples, characterizing their reproducibility.

References

1. V. E. Gorodovykh, M. S. Zakharov, V. N. Pichugina. Determination of microimpurities of Bi, Cu, Pb, and Zn in high-purity tin by the method of amalgam polarography with accumulation against the background of an alkaline EDTA solution. Plant laboratory (in press).

2. A. G. Stromberg, M. S. Zakharov, V. E. Gorodovykh, L. F. Zaichko. Determination of ultramicroimpurities of Cu, Pb, Zn in high-purity tin by the method of amalgam polarography with accumulation. Zavodskaya laboratoriya, 27, issue 5, 517 (1961).

3. G. A. Kryukova, S. I. Sinyakova, T. V. Aref’eva. Polarographic Analysis. Moscow, p. 326, 1959.

4. A. I. Zebreva. Electrochemistry of Solutions and Metallic Systems. Publishing House of the Academy of Sciences of the Kazakh SSR, p. 55, 1962.