PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 174　　　　　　　　　　　　　　　　　　　　　　　　　1971

STUDY OF THE AMALGAM-POLAROGRAPHIC BEHAVIOR OF ANTIMONY IN AN ALKALINE MEDIUM

L. F. ZAICHKO, M. S. ZAKHAROV

(Presented by the scientific-methodological seminar of the problem laboratory of microimpurities)

Continuing the investigations on the study of the amalgam-polarographic behavior of antimony [1, 2], in the present work the amalgam behavior of antimony during the anodic oxidation of its amalgam in a strongly alkaline medium \((2N\ \mathrm{KOH})\) has been established and studied. In this medium, the formation of double anodic antimony peaks is observed. Experimental conditions: \(C_{\mathrm{Sb}} = 1 \cdot 10^{-6} — 1 \cdot 10^{-5}\ \mathrm{mol/l}\); \(v\) of solution \(= 5\ \mathrm{ml}\); \(\varphi_{\mathrm{e}} = -1.65\ \mathrm{V}\) (sat. cal. el.); \(r_k = 0.04\ \mathrm{cm}\); \(\tau_{\mathrm{e}} = 2\ \mathrm{min}\). The work was carried out both on different drops and on one and the same drop over the course of the working day. The results obtained were analogous. In the case of work on one and the same drop, the completeness of dissolution of antimony from the amalgam was checked by repeated recording of the polarogram.

Fig. 1. Calibration graph for Sb in \(2N\ \mathrm{KOH}\). Curve 1—for the first anodic peak; curve 2—for the second anodic peak.

In the literature there are data [3] and a detailed study of the nature of the double peaks of Cu and Pb in alkaline and carbonate media [4, 5] and of gallium in various media [6]. During the recording of the anodic polarogram of antimony in

In \(2N\) KOH two anodic notches with peak potentials of \(-0.76\) V (SCE) and \(-0.41\) V (SCE) are clearly noticeable. The dependence of the height of the anodic notches of antimony on the concentration of its ions in a \(2N\) KOH solution is shown in Fig. 1. The more negative notch (the first) is always greater in magnitude than the more positive notch (the second) and increases proportionally with the increase in the concentration of \(\mathrm{Sb}^{3+}\) in the solution. The second notch also grows with increasing concentration of \(\mathrm{Sb}^{3+}\) in the solution, although it is reproduced somewhat less well. A similar dependence was also observed on the electrolysis time. Variation of the background concentration in the range \(2\text{–}4N\) had no noticeable effect on the shape of the anodic notches of antimony. The following causes may be proposed for the formation of double anodic notches of antimony: a) formation of a passivating film of an antimony compound on the surface of the mercury drop, b) stepwise oxidation of the antimony amalgam. The suggestion that a sparingly soluble film of an antimony compound forms on the surface of the mercury drop is unlikely, since in a strongly alkaline medium \(\mathrm{Sb}^{3+}\) exists in the form of the readily soluble compound \(\mathrm{KSbO_2}\) [7].

Fig. 2. Double anodic notches of Sb in \(2N\) KOH.
1 — polarogram recorded without stirring the solution; 2 — with stirring for 1.5 min; 3 — second antimony notch when recording the polarogram from \(-0.63\) V.

The literature contains data [8, 9] according to which \(\mathrm{Sb}^{3+}\) in strongly alkaline solutions is reduced to \(\mathrm{Sb}^0\) and oxidized to \(\mathrm{Sb}^{5+}\). These data, as will be seen below, are also confirmed by the experimental investigations of the present work. Stirring the solution with a stream of nitrogen during recording of the anodic polarogram, under the experimental conditions, strongly affects the depth of the more positive antimony notch. It was found that a double notch is obtained when the anodic polarogram is recorded in the absence of solution stirring. If, however, during recording of the anodic voltammetric curve the potentiometric drum is stopped after the peak of the first notch has been reached for 1–1.5 min, and the solution is stirred with a stream of nitrogen directed at the stationary drop, then the second notch does not appear on the polarogram (Fig. 2). Evidently, when the polarogram is recorded without stirring the solution, the antimony amalgam is oxidized to \(\mathrm{Sb}^{3+}\)

\[ \mathrm{Sb}^0 + 4\mathrm{OH}' \rightarrow \mathrm{SbO_2}' + 2\mathrm{H_2O} + 3e, \]

which accumulates at the electrode surface, and the second notch is due to oxidation

\[ \mathrm{SbO_2}' + 2\mathrm{OH}' \rightarrow \mathrm{SbO_3}' + 2\mathrm{H_2O} + 2e. \]

When the solution is stirred during the stoppage of the potentiometric drum, the \(\mathrm{SbO_2}'\) ions are carried away from the electrode surface, as a result of which the second notch is not obtained.

Confirmation that the first notch corresponds to the three-electron oxidation of antimony amalgams is also provided by the fact that the area in coulombs under the first notch proved equal to the amount of electricity consumed for the reduction of \(\mathrm{Sb}^{3+}\) to \(\mathrm{Sb}^0\) in the cathodic cycle (Table 1).

In addition, a more positive peak is also obtained when recording an anodic polarogram without preliminary electrolysis, if the concentration of antimony \((\mathrm{Sb}^{3+})\) in the solution is of the order of \(10^{-4}\ \mathrm{mol/l}\), and the polarogram recording begins at \(-0.6\ \mathrm{V}\) (Fig. 2). This fact is an additional confirmation that the more positive peak is due to the oxidation of \(\mathrm{SbO_2'}\) to \(\mathrm{SbO_3'}\). Thus, the kinetics of formation of the second, more positive, antimony peak in \(2\ N\ \mathrm{KOH}\) is probably determined by the concentration of antimony \((\mathrm{SbO_2'})\) in the near-electrode layer.

Table 1

Determination of the number of electrons participating in the process of anodic oxidation of antimony amalgam in \(2\ N\ \mathrm{KOH}\). Experimental conditions: \(v = 5\ \mathrm{ml}\); \(r_k = 0.04\ \mathrm{cm}\); \(C_{\mathrm{Sb}} = 1 \cdot 10^{-4}\ \mathrm{mol/l}\); peaks were recorded without accumulation

Conclusions

It has been shown that during the anodic oxidation of antimony amalgam in a strongly alkaline medium, double peaks are formed. A possible mechanism is proposed for the formation of double peaks, caused by the stepwise oxidation of antimony.

References

1. L. F. Zaichko, M. S. Zakharov. In: Methods of Analysis of Chemical Reagents and Preparations, Moscow, IREA, 37, 1963.
2. L. F. Zaichko, M. S. Zakharov. Journal of Analytical Chemistry, 21, 65 (1966).
3. S. I. Sinyakova, I. V. Markova. Zavodskaya laboratoriya, 27, 521, 1961.
4. E. A. Zakharova. Proceedings of the Tomsk Polytechnic Institute, 128, 53, 1964.
5. E. A. Zakharova. Candidate’s dissertation, Tomsk State University, 1965.
6. O. S. Stepanova. Dissertation. Investigations in the Field of Amalgam-Polarographic Behavior of Ga and Ge by the Method of Amalgam Polarography with Accumulation, Tomsk, 1964.
7. G. Remy. Course of Inorganic Chemistry, Moscow, vol. 1, 717, 1963.
8. I. M. Kolthoff, R. L. Probst, Analyt. Chem., 21, 753, 1949.
9. D. Pavlov, D. Lazarev, Doklady AN SSSR, 118, 1, 103, 1958.