STUDY OF INTERMETALLIC COMPOUNDS BY THE APN METHOD. IV. EXPERIMENTAL VERIFICATION OF THE THEORY IN THE CADMIUM—GOLD—MERCURY SYSTEM

P. A. MESYATS, P. P. MIKHEEVA

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

Earlier \[1, 2\], new methods were proposed for studying the chemical and phase composition of amalgams when intermetallic compounds (i.m.c.) are formed in them between two metals, using the APN method.

The aim of the present work was to study the ternary cadmium—gold—mercury system by the APN method. This system was studied by the potentiometric method by Hartmann and Scholzel \[3\], and it was shown that the i.m.c. formed, CdAu, is soluble in mercury. The dissociation constant of this compound at 20° averages \(2 \cdot 10^{-5}\). Measurements of the e.m.f. were carried out at different temperatures, and it was shown that at 90°C almost complete dissociation of the i.m.c. formed occurs. Hartmann and Scholzel carried out studies of the kinetics of formation of the i.m.c. CdAu and found that the compound AuCd is present in mercury in the dissolved state.

From the data of Hartmann and Scholzel \[4, 6\], Zebreva calculated the values of the solubility product of this compound and, on the basis of the large constancy of this quantity, concludes that it is present in mercury in the form of a sparingly soluble precipitate. There is no more detailed study of this system in the domestic literature.

We carried out a study of this system by one of the methods of the APN method—obtaining dependences of the depths of anodic notches on the electrolysis time at an unchanged concentration of the components in solution (the \(l—\tau\) curves). Our studies were carried out against a background of 0.1 M \((\mathrm{NH}_4)_2\mathrm{SO}_4\). On this background, the anodic notch of gold overlaps with the anodic notch of mercury; therefore the studies were limited only to observing the behavior of one cadmium notch. This, of course, makes it difficult to calculate the equilibrium constants of the i.m.c. formed.

The studies were carried out with a cadmium and gold concentration of \(\approx 5 \cdot 10^{-5}\) g-ion/liter in solution, in ratios of 1:0.5; 1:1; 1:2; 1:3. At this concentration, for cadmium separately and in the presence of various amounts of gold, the dependences \(l—\tau\) were constructed (Fig. 1). From the form of the dependences it may be concluded that the i.m.c. CdAu is present in mercury in the dissolved state \[1\].

When the dependences of the depth of the cadmium anodic notch on the electrolysis time are recorded in the presence of gold, an increase in the width of the cadmium notch is observed. Since the half-width of the notch changes, the dependences \(l—\tau\) were constructed with allowance for the areas under the notch, which are also proportional to the concentration of the metals, as are the heights of the notches. The broadening

tooth can be associated with two phenomena: 1) with dissolution of the solid phase of the intermetallic compound, if it is assumed that the intermetallic compound CdAu is present in mercury in the form of a sparingly soluble compound; 2) with partial dissociation of the compound, if the intermetallic compound CdAu is considered dissolved in mercury. In choosing between these two assumptions, we settled on the second, because it is more probable (the rate of dissolution of the solid precipitate is much lower than the rate of dissociation of the dissolved compound),

Fig. 1. Dependence of the depth of the anodic tooth of cadmium and of cadmium with gold on the electrolysis time. The concentration of cadmium and gold in solution is \(5 \cdot 10^{-5}\) g-ion/l. Curves: 1—for cadmium alone; 2, 3, 4, 5—for Cd with Au in the ratios 1:0.5; 1:1; 1:2; 1:3, respectively. Rate of change of potential \(0.016\) V/sec.

Fig. 2. Series of Cd teeth recorded in the presence of gold and without it. The width of the Cd half-tooth changes by a factor of 2.5 in the presence of gold. The rate of change of potential is the same. 1, 2, 3—teeth of Cd alone. Concentration \(5 \cdot 10^{-5}\) g-ion/l, recorded at times of 3, 6, and 9 min, respectively; 4, 5, 6, 7, 8—Cd teeth in the presence of an equivalent amount of gold after 6, 9, 12, 15, and 18 min.

and also because the data we obtained on the \(I—\tau\) dependences indicate the existence of the intermetallic compound in the liquid state. Fig. 2 presents the change in the shape of the cadmium tooth associated with dissociation of the intermetallic compound CdAu. To ensure that the results of the calculations are not affected by a shift of equilibrium in the system \(\mathrm{Cd}+\mathrm{Au}\rightleftharpoons \mathrm{CdAu}\) (liq.), we investigated this system at different rates of potential change using an oscillographic polarograph, which makes it possible to use very high rates of change of potential. With the aid of an additional attachment for reducing the sensitivity of the oscillographic polarograph, we selected a rate at which no noticeable dissociation of the compound is observed and the width of the cadmium half-tooth remains constant. At this rate of potential change, studies of the Cd—Au—Hg system were carried out for the same metal concentrations in solution, and analogous \(I—\tau\) dependences were obtained. The external form of the dependences confirms the assumption that the compound CdAu is soluble in mercury. For this case we calculated the dissociation constant of this compound. As indicated below, in systems where an anodic tooth of only one metal is obtained, calculation of the equilibrium constants is difficult. From our data we could not calculate the composition of the compound formed, and therefore we took it from the literature data [3, 4, 7]. Since

\(n=1\), i.e., the composition of the compound is CdAu; the formulas for calculating the dissociation constant are simplified [1].

\[ I_1 = K_1(1-p\alpha)\tau, \tag{1} \]

\[ \frac{1}{z}(1-p\alpha)(1-\alpha)^n = \left(\frac{Q}{z}\right)^n, \tag{2} \]

\[ Q = K g^{1/n}\cdot \frac{1}{n}\cdot \left(\frac{Q_1}{K_1}\right)^{1/n}\cdot \left(\frac{K_2}{Q_2}\right)^{\frac{1-n}{n}}. \tag{3} \]

Using the graphs for finding \(\alpha\) from [1] and formulas (1—3), we found that \(Kg = 9.8\cdot 10^{-6} \approx 1\cdot 10^{-5}\). This value does not coincide with the dissociation constant found by Hartman and Scholze [3], but, apart from the data of these authors, there are no others, and we may consider that, for the final establishment of this value, additional studies of the Cd—Au—Hg system by the potentiometric method must be carried out. To check for possible cementation of cadmium by gold, we performed experiments with a gold amalgam. For this purpose, a gold amalgam of a definite concentration was prepared, and a drop of this amalgam was suspended on the tip of a mercury electrode. Cadmium was accumulated from solution in the usual way. The obtained dependences \(I—\tau\) are analogous to those shown in Fig. 1 and indicate that cementation of cadmium by gold does not occur.

Fig. 3. Dependence of the depth of the anodic wave of cadmium (1) and cadmium in the presence of gold (2) on temperature.

Very interesting results were obtained in experiments for the Cd—Au—Hg system at different temperatures. For this purpose, the polarographic cell was placed in a thermostat, and the currents of anodic oxidation of cadmium in the presence of gold (ratio \(1:1\)) and of cadmium alone were recorded at 20°, 50°, 70°, and 90°C. The results are presented in Fig. 3. Indeed, at a temperature of 90°C there is almost complete dissociation of the intermetallic compound CdAu, and the current of anodic oxidation of cadmium in the presence of gold differs only slightly from the current of pure cadmium. The width of the half-wave also remains constant. These results confirm the data of Hartman and Scholze [3].

REFERENCES

1. M. S. Zakharov, A. G. Stromberg, N. A. Mesyats, Izv. TPI, (in press).

2. A. G. Stromberg, M. S. Zakharov, N. A. Mesyats, Izv. TPI, (in press).

3. H. Hartman, K. Scholze, Z. phys. Chem., 9, 1/2, 106, 1056.

4. A. I. Zebreva, Journal of Physical Chemistry, 35, 948, 1961; 36, 1822, 1962.

5. L. F. Kozin, Physicochemical Foundations of Amalgam Metallurgy. Alma-Ata, 1964.

6. Electrochemistry of Solutions and Metallic Systems. Proceedings of the Institute of Chemical Sciences, vol. IX, 1962.

7. M. Hansen, K. Anderko, Structure of Binary Alloys, vol. 1, 2. Moscow, 1962.