ON THE DETERMINATION OF THE SOLUBILITY OF GALLIUM AND GERMANIUM IN MERCURY BY THE APN METHOD

O. S. STEPANOVA, M. S. ZAKHAROV

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

The value of the solubility of metals in mercury is very important in the separation of metals by methods of amalgam metallurgy [1—6], as well as in the analysis of microconcentrations of elements by the method of amalgam polarography with accumulation [7].

Unfortunately, the data available in the literature on the solubility of metals (including gallium and germanium) in mercury are not sufficiently complete and are not distinguished by accuracy and reliability. In addition, there are a number of metals whose solubility is unknown and, for the present, cannot be determined by direct methods because of its small magnitude and the insufficient sensitivity of analytical methods.

The present work is devoted to determining the solubility of gallium and germanium in mercury by the method of amalgam polarography with accumulation. The method of amalgam polarography with accumulation has a number of advantages over existing methods for determining the solubility of metals in mercury. First, oxidation of the amalgam is excluded, since the work is carried out in an inert medium from which even traces of oxygen have been removed; second, there is no need to thoroughly purify a large mass of mercury (the mercury drop, obtained by electrolysis at a current of 30 mA from a solution of mercurous nitrate, grade ch.d.a., “absolutely pure”); third, the sensitivity of the method of amalgam polarography with accumulation is much greater than the sensitivity of potentiometric, chemical, and thermal methods of analysis.

Experimental Part

The investigations were carried out on a 7-77-4B polarograph with a recorder. The electrolytic cell used in the work is described in [8]. The vessels used in the work (flasks for storing standard solutions of gallium and germanium, beakers for polarography, etc.) were made of quartz.

A mercury drop of radius 0.04 cm, in order to avoid low results in the case of formation of intermetallic compounds of Ga and Ge with the electrode contact, was suspended on a Pt contact when working with germanium solutions and on an Ag contact in the case of gallium. Although it is known that Ge with Pt gives an intermetallic compound, we did not observe any influence of the platinum contact on the anodic peak of germanium even during electrolysis lasting an hour. Before polarography, the solution under investigation was thoroughly purged of traces of oxy-

with purified nitrogen. The mode of stirring with nitrogen was kept constant in all experiments.

Experimental conditions: anodic peaks of gallium were obtained against the background of an acetate buffer (pH 4.6), and of germanium—against the background of \(0.2\,M\ \mathrm{Na_2CO_3} + 0.025\,M\) Trilon B (pH 10.3). The electrolysis time for germanium in our investigations was, respectively, 300 and 600 sec. The preliminary accumulation potential for gallium was \(-1.4\ \mathrm{V}\) (sat. cal. el.), and for germanium \(-1.8\ \mathrm{V}\) (sat. cal. el.). At these values, the diffusion current is reached for gallium and germanium. The investigations were carried out at \(25^\circ\mathrm{C}\).

Each measurement was repeated at least four times [9].

Fig. 1a. Dependence of the depth of the gallium anodic peak on the concentration of the latter in solution.

The dependence of the anodic current of the metal on the concentration of the latter in solution was investigated. Then, knowing the amount of electricity released in the anodic cycle (the area under the anodic peak in coulombs), the concentration of metal atoms in the mercury drop was calculated by the formula [10]:

\[ c_2=\frac{q}{zFv_k}, \]

where \(q\) is the amount of electricity, coulombs;
\(z\) is the number of electrons participating in the electrode reaction;
\(F\) is the Faraday number, 96500 coul.;
\(v_k\) is the volume of the mercury drop, \(\mathrm{cm^3}\).

The experimental data are presented in Table 1 and in Figs. 1a, b and 2a, b.

Table 1

Experimental data for determining the solubility of Ga and Ge in Hg

Fig. 1b. Dependence of the concentration of gallium atoms in mercury on the concentration of the latter in solution.

Fig. 2a. Dependence of the depth of the anodic notch of germanium on the concentration of the latter in solution.

Fig. 2b. Dependence of the concentration of germanium atoms in mercury on the concentration of the latter in solution.

Discussion of Results

From the experimental data (Fig. 2a) it is evident that, at first, as the concentration of the metal in the solution increases, the magnitude of the anodic diffusion current of germanium increases proportionally. Proportionality between the metal content in mercury and the magnitude of the anodic current is observed in the case of germanium amalgam up to \(1.8 \cdot 10^{-4}\) g-at/l (Fig. 2b), with an accuracy of 10–15%. A further increase in the concentration of the metal in the solution does not affect the value of the anodic current, i.e., the depth of the anodic peak of germanium remains constant or even decreases somewhat. This phenomenon is associated with reaching the solubility limit of germanium in mercury. The limiting oxidation current of these amalgams is determined only by the dissolved portion of the metal in the amalgam [11]. The presence of a solid phase is not reflected in the magnitude of the anodic current: apparently, the rate of transition of the solid phase into the liquid is incomparably lower than the rate of diffusion of metal atoms in mercury.

The slight decrease in the magnitude of the anodic current after the solubility limit is reached is, in all probability, explained by an increase in the viscosity of the amalgam.

On the calibration plot (2a), two intersecting straight lines are obtained. At the point of intersection the solubility of the given metal in mercury is reached. From plot (2b) one can accurately determine the solubility of the metal in a mercury drop. For germanium at 25°C this value is \(1.8 \cdot 10^{-4}\) g-at/l, or 0.000096 wt% (\(1 \cdot 10^{-4}\) wt%).

It should be noted that L. F. Kozin [12] calculated, by an empirical formula, the solubility of germanium in mercury, and it amounted to \(1.3 \cdot 10^{-18}\) at.%. This value must be treated with great caution, since extrapolation of an empirical dependence established for most metals in the concentration range from \(10^{-1}\) to \(10^{-4}\) at.% into the region of substantially lower concentrations (from \(10^{-5}\) to \(10^{-20}\) at.%) is in no way justified [13]. If the solubility of germanium in mercury were actually such, then this element could not be determined by the method of amalgam polarography with accumulation.

The data we obtained on the solubility of germanium in mercury confirmed A. G. Stromberg’s prediction [7, 14] that the solubility of germanium in mercury is of the same order as that of antimony and that, consequently, germanium can be determined by the APN method. Indirect confirmation of the correctness of the value we obtained for the solubility of germanium in mercury is seen from the experimental data of Stashkova and Zelyanskaya [15]. They obtained, by electrolysis, a germanium amalgam with a content of the latter of 0.1 mm/l or \(0.5 \cdot 10^{4--}\) wt% (there was no saturation of the amalgam).

For gallium, on the \(I\)—\(c\) curve (Fig. 1 a, b), a break in the straight line was observed at a gallium amalgam concentration of \(2.5 \cdot 10^{-2}\) g-atom/l, which corresponds to \(1.3 \cdot 10^{-2}\) wt%. According to literature data, some authors consider gallium to be practically insoluble in mercury or soluble at less than \(10^{-3}\)% [16]; other authors [17, 18, 19, 20], using potentiometric and chemical methods, established that the solubility of gallium in mercury is 1.3 wt%. Thus, the value of the solubility of gallium in mercury determined by us and by the latter authors differs. The reasons for the underestimated solubility of gallium in mercury determined by the method of amalgam polarography with accumulation are as yet unknown.

Conclusions

1. By the method of amalgam polarography with accumulation, the solubility of germanium in mercury has been determined.

2. Apparently, by the method of amalgam polarography with accumulation it is not possible to determine the solubility of elements that are readily soluble in mercury.

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