STUDY OF INTERMETALLIC COMPOUNDS BY THE APN METHOD. IV. EXPERIMENTAL VERIFICATION OF THE THEORY ON THE COPPER—MERCURY—ZINC SYSTEM

N. A. MESYATS, M. S. ZAKHAROV

(Presented by the scientific seminar of the problem laboratory of physicochemical methods of analysis)

In previous works [1, 2], new methods were proposed for studying the chemical and phase composition of intermetallic compounds (i.m.c.) between two metals in mercury using the APN method.

The aim of the present work was to study the ternary copper—zinc—mercury system by the APN method. The data available in the literature concerning this system are contradictory. Thus, Zebreva [3] believes that the i.m.c. CuZn is present in mercury in the form of a sparingly soluble compound. Stromberg and Gorodovykh [5], assuming a priori that the i.m.c. CuZn$_n$ is present in mercury in the form of a sparingly soluble compound, established by the APN method that copper with zinc forms an i.m.c. of composition CuZn and that its solubility product is equal to $5 \pm 1 \cdot 10^{-8}$. The values of the solubility product $(L_p)$ of this compound, calculated by Kozin [4] from potentiometric data [3], depending on the zinc concentration in the amalgam, varied from $2.82 \cdot 10^{-5}$ to $7.1 \cdot 10^{-8}$. Kozin holds the opinion that equilibrium in this system is characterized by the value of the dissociation constant, since it is more constant and is equal to $2.6 \cdot 10^{-3}$ [4].

We studied this system by two variants of the APN method, since copper and zinc are reduced on the mercury electrode and give separate waves [1]. The i.m.c. formed before the potential of mercury dissolution gives no wave.

The first method is based on obtaining dependences of the depths of the waves of zinc and copper on the electrolysis time at an unchanged concentration of the components in solution (curves $I—\tau$). The second method consists in obtaining dependences of the depth of the anodic waves of the elements at a varying concentration of one of the elements and a constant concentration of the other (curves $I—C$).

The characteristic form of the $I—\tau$ curves can serve as evidence of the phase composition of the amalgam [2], i.e., indicate in what form (a sparingly soluble precipitate or a soluble compound) the i.m.c. exists in mercury.

In the study by the first method, two series of experiments were carried out. In the first series, the concentrations of zinc and copper in solution, and consequently also in the amalgam (preliminary accumulation of the metals in mercury was carried out at the limiting current of zinc and copper), were identical and equal to $5 \cdot 10^{-5}$ g-ion/l (in solution). In the second series of experiments, the concentration of copper in solution, and consequently also in the amalgam, was twice the concentration of zinc.

Polarography was carried out against a background of 0.1 M tartaric acid and 0.25 M ammonium acetate. The results of the studies are given in Fig. 1. It follows from the figure that the intermetallic compound CuZn is present in mercury in the form of a sparingly soluble compound [2]. From the experimental data one can calculate the solubility product of this compound by the formulas [2]:

\[ L_{\mathrm p}=\frac{K_1}{a_1}\cdot\left(\frac{K_2}{a_2}\right)^n\cdot \tau_*^{\,n+1} \tag{1} \]

\[ L_{\mathrm p}=\frac{I_1^*}{a_1}\cdot\left(\frac{I_2^*}{a_2}\right)^n, \tag{2} \]

Fig. 1. Dependence of the depth of the anodic notches of copper and zinc on electrolysis time in their simultaneous presence in solution. Equivalent concentrations of copper and zinc \(5\cdot 10^{-5}\) g-ion/l. Curves 1, 2—\(I\)–\(\tau\) dependences for copper and zinc taken separately, found experimentally; 3—4—theoretical and experimental \(I\)–\(\tau\) curves for copper and zinc taken together in equivalent concentrations \(=5\cdot 10^{-5}\) g-ion/l; \(\circ\)—experimental points for copper, ×—experimental points for zinc; 5—6—experimental \(I\)–\(\tau\) curves for copper and zinc taken in the ratio 2 : 1.

where \(K_1\) and \(K_2\) are angular coefficients, determined from the \(I\)–\(\tau\) plots for the elements taken separately; \(a_1\) and \(a_2\) are coefficients determined from the expression \(a=w\cdot F\cdot v\cdot \frac{z}{\rho}\) [1]; \(I_1^*\) and \(I_2^*\) are the values of the currents of the elements corresponding to equilibrium metal concentrations, A; \(\tau_*\) is the time from which precipitate formation begins, sec. The solubility product calculated from the experimental data is \(2\cdot 10^{-6}\). From these same data the composition of the intermetallic compound (\(n\)) \( \mathrm{CuZn}_n \) was calculated by the formulas [1]:

\[ n=\frac{I_2^0-I_2}{I_1^0-I_1}\,\beta, \tag{3} \]

\[ n=\frac{a_1}{a_2}\cdot\frac{I_2^*}{I_1^*} =\frac{a_1}{a_2}\cdot\frac{K_2}{K_1}, \tag{4} \]

where \(\beta=\frac{a_1}{a_2}\); \(I_1^0, I_2^0\) are currents corresponding to the initial concentrations of metals in the amalgam, A; \(I_1, I_2\) are the same, but after interaction of the elements with formation of an intermetallic compound, i.e., currents proportional to the equilibrium concentrations.

The value \(n\), calculated by these formulas, is 0.9–0.93; consequently, one may take \(n=1\). Thus, according to our data,

the I.M.C. between copper and zinc in mercury has the composition CuZn, which agrees with the literature data. In addition, this is a confirmation of the validity of the method for studying I.M.C., proposed in [2]. In addition to the \(I—\tau\) curves, \(I—c\) curves were obtained (Fig. 2). The zinc concentration in the solution in these studies remained constant and equal to \(5 \cdot 10^{-4}\) g-ion/l; the copper concentration varied within the limits \((5—9)\cdot 10^{-5}\) g-ion/l. The form of the graph in the coordinates \(\lg I_1—\lg I_2\) represents

Fig. 2. Graph of equation (5) for the Zn—Cu system.
The zinc concentration is constant and equal to \(5 \cdot 10^{-4}\) g-ion/l;
the copper concentration varies in the range \((5—9)\cdot 10^{-5}\)
g-ion/l in solution. Rate of potential change
\(W = 0.014\) V/sec

a straight line (Fig. 2). This indicates that the I.M.C. \(\mathrm{CuZn}_n\) is present in mercury in the form of a sparingly soluble precipitate [1]. Using this graph and formulas [1]:

\[ \lg I_1 = \lg M - n \lg I_2, \tag{5} \]

where

\[ M = L_p a_1 a_2^n, \tag{6} \]

one can find the composition \((n)\) and the solubility product \((L_p)\) of the I.M.C. The slope coefficient of the straight line (equal to \(n\)) described by equation (5) is 0.93, i.e., the I.M.C. of copper with zinc in mercury has the composition CuZn. The solubility product, calculated by formula (6), proved to be equal to \(1 \cdot 10^{-6}\).

To make sure that a possible shift of equilibrium during the removal of the anodic teeth did not affect the results of the calculations of \(n\) and \(L_p\), we carried out investigations at different rates of potential change. (The data in Fig. 2 were obtained at a potential-change rate of \(0.014\) V/sec.) The results of the calculations of \(n\) and \(L_p\), from data obtained at different rates of potential change, coincided, which indicates the absence of displacement of the phase equilibrium during the removal of the anodic teeth.

The authors thank Professor A. G. Stromberg for his attention to the work.

Conclusions

The Cu-Zn-Hg system has been studied by the APN method. It has been shown that in this system an intermetallic compound of composition CuZn is formed, which

which is a sparingly soluble compound. The solubility product of this compound has been calculated to be \((1–2)\cdot 10^{-6}\).

REFERENCES

1. M. S. Zakharov, A. G. Stromberg, N. A. Mesyats. Izvestiya TPI, (1963) (in press).

2. A. G. Stromberg, M. S. Zakharov, N. A. Mesyats. Izvestiya TPI (1963) (in press).

3. A. I. Zebreva, M. T. Kozlovskii. Journal of Physical Chemistry, 30, 1553 (1956).

4. L. F. Kozin. Physicochemical Foundations of Amalgam Metallurgy. Nauka Publishing House, Alma-Ata, 1964.

5. A. G. Stromberg, V. E. Gorodovykh. Journal of Inorganic Chemistry, 8, 2355 (1963).