On the Determination of the Solubility of Sb in Mercury by Accumulative Amalgam Polarography

L. F. Zaichko, M. S. Zakharov

(Presented by the scientific-methodological seminar of the Problem Laboratory of Microimpurities)

The solubility of metals in mercury is an important physicochemical property of amalgams. The results of various authors on the solubility of poorly soluble metals in mercury (G, Mn, Ni, Sb, etc.) sometimes differ considerably from one another (Table 1, Sb).

Table 1

Solubility of Sb in mercury according to literature data, g-atom/l

In the present communication, an additional study of antimony amalgams has been carried out by the method of accumulative amalgam polarography in order to confirm the true solution of antimony amalgam and to determine the solubility of Sb in mercury. It may be assumed that, during electrolytic accumulation, antimony does not form a true amalgam but is deposited on the surface of a stationary mercury spherical electrode as on an indifferent material. Taking the atomic radius of antimony as 1.61 Å, it can be calculated that, for the formation of a monolayer of antimony on the surface of the drop ($r_k = 0.04$ cm), approximately $\sim 1.3 \cdot 10^{14}$ atoms are required, which corresponds to $2.4 \cdot 10^{-8}$ g. If it is assumed that antimony forms a true solution with mercury, then upon reaching saturation [7] the drop will contain $\sim 2.2 \cdot 10^{-8}$ g of antimony. Thus, in both cases nearly identical values are obtained.

In this work the behavior of antimony amalgam was studied by measuring the equilibrium potential. The measurement of equilibrium potentials was carried out with a high-resistance potentiometer with a sensitivity of 0.0001 V. The electrode potential was measured relative to a saturated calomel electrode 3–4 times, and the average value was taken. Antimony amalgam in a stationary mercury drop was obtained by electrolysis for varying times in a stirred HCl solution containing $\mathrm{Sb}^{3+}$ at a definite concentration. After measurement of the equilibrium potential, the antimony was subjected to electro-dissolution at a linearly varying potential. The reproducibility of the results was checked from the magnitude of the maximum oxidation current and coulometrically. It was established beforehand that

dissolution of the antimony concentrated in the drop during measurement of the equilibrium potential does not occur.

The equilibrium potential of the mercury electrode was studied in the same way as in [6].

Fig. 1 shows the dependence of the equilibrium potential of the electrode on the duration of electrolysis at an antimony concentration in solution of \(6 \cdot 10^{-6}\) mole/l. The experimental data indicate that the equilibrium potential first changes and then assumes a constant value, which is associated either with attainment of the solubility of antimony in mercury or with coating of the mercury drop with a monolayer of antimony. Coulometric measurements showed that constancy of the equilibrium potential is attained when the concentration of antimony in the amalgam is (assuming that Sb dissolves in mercury) \(7\text{—}10^{-4}\) g-atom/l, which agrees with the solubility data obtained by us earlier [7].

Fig. 1. Dependence of the equilibrium potential of the electrode on the pre-electrolysis time at an antimony concentration in solution of \(5 \cdot 10^{-6}\) mole/l

From the graphical dependence of \(\varphi_p\) on the concentration of \(\mathrm{Sb}^{3+}\) in solution, it may be assumed that, after solubility is reached, antimony is not deposited on the surface of the mercury drop during pre-electrolysis.

Fig. 2. Course of the \(\lg i-\tau\) curves for the descending branch of the anodic peak of antimony. \(1 — r_k = 0.021\) cm, \(2 — r_k = 0.04\) cm

An experimental dependence of \(I\)—\(\tau\) was also obtained at a constant antimony concentration in solution (\(\tau\) is the time of electrolytic accumulation). A direct proportional dependence is observed under the conditions of the experiments up to attainment of the solubility of Sb (Fig. 2); calculations from the coulometric data of these measurements showed that the solubility of antimony is \(\sim 7 \cdot 10^{-4}\) g-atom/l. After attainment of the solubility of antimony, no strong scatter of points is observed, which may also indicate that the centers of crystall—

...ization of Sb are formed not on the surface, but inside the mercury drop. Upon reaching the solubility, the external appearance of the wave also changes noticeably. The wave becomes almost constant in height or even decreases somewhat, but its peak becomes rounded, and the half-wave width increases noticeably. This is evidently connected with partial dissolution of solid Sb during registration of the anodic antimony wave.

Confirmation that antimony forms a true amalgam with Hg (before saturation is reached) is also provided by the fact that the descending branch of the anodic peak for antimony obeys the theoretical equation [8], valid for the conditions of spherical diffusion (Fig. 2).

For drops of different radius, different slopes are observed for straight lines in the indicated coordinates, which indicates diffusion of antimony atoms to the surface of the mercury drop. It is of interest to compare the experimental data for antimony on stationary dropping and film amalgam electrodes.

Fig. 3. Dependence of the height of the anodic peak of antimony on the concentration of \(\mathrm{Sb}^{3+}\) in solution: 1 — for the film electrode and 2 — for the dropping electrode.

The surfaces of the indicated electrodes prepared by us differed by approximately a factor of 24. The surface of the drop was \(0.02\ \mathrm{cm}^2\), the surface of the film was \(0.479\ \mathrm{cm}^2\).

In one and the same solution, a direct proportional relationship between the concentration of antimony in solution and the current of anodic oxidation of Sb is observed for both the drop and the film for different concentrations of \(\mathrm{Sb}^{3+}\) in solution. The solubility of antimony, calculated from the area under the anodic wave for the film electrode, is \(6.6\ \mathrm{g\text{-}at}/\mathrm{l}\). This fact also indirectly indicates that, in determining it by the APA method, antimony is not deposited on the surface of the mercury electrode, but forms a true solution (Fig. 3).

Thus, the totality of the experimental data presented gives grounds to believe that, when determined by the APA method, antimony forms a true amalgam with mercury, and not a film on the surface of the drop. The data of work [9] also indicate that antimony with mercury, up to a certain concentration \((2.4 \cdot 10^{-4}\ \mathrm{g\text{-}at}/\mathrm{l})\), forms a true solution.

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