STUDY OF THE INTERMETALLIC COMPOUND OF CADMIUM WITH LITHIUM BY AMALGAM ACCUMULATION POLAROGRAPHY

A. A. KAPLIN, N. K. DZHABAROVA, A. G. STROMBERG

(Presented by the scientific seminar of the Department of Physical Chemistry)

The study of interactions in multicomponent systems, in particular the identification of the possibility of formation of intermetallic compounds (i.m.c.) in mercury, is an important task of amalgam polarography with accumulation (APA). This is connected with the fact that, upon formation of an i.m.c., the polarographic characteristics of the metal being determined \((h, \varphi_n, b_{1/2})\) may change substantially.

There are comparatively few data in the literature on complex amalgams of alkali metals [1, 2]. In this connection, a series of studies has been initiated on changes in the electrochemical properties of amalgams of alkali metals upon introduction into them of a third component that is readily soluble in mercury.

In the present work the Li–Cd–Hg system was studied by the APA method.

Experimental Part

The studies were carried out on an LP-60 polarograph and an OP-2 oscillopolarograph. A mercury film electrode with a film thickness of 10 μ and a surface area of 0.15 cm² was used as the indicator electrode. The reference electrode was bottom mercury. A 0.02 M solution of \((\mathrm{C_2H_4})_4\mathrm{NJ}\) in N,N-dimethylformamide (DMFA) was chosen as the indifferent electrolyte. On this background, separate anodic peaks of lithium and cadmium are recorded at potentials of −1.9 V and −0.3 V, respectively. All numerical values of potentials are given relative to bottom mercury. Standard solutions of lithium and cadmium were prepared by dissolving LiClO₄ salts (chemically pure) and CdJ₂ (analytical grade) in DMFA. The DMFA was twice distilled in a quartz apparatus. The salt \((\mathrm{C_2H_5})_4\mathrm{NJ}\) was twice purified by recrystallization from alcohol. Argon was used for stirring the solution.

In order to eliminate the influence of cementation and to make it possible to concentrate lithium and cadmium at different potentials, the method of electrode transfer was used [3]. For work in nonaqueous media with bottom mercury as the reference electrode, a new design of electrolytic cell was developed. The bottom mercury is transferred from one beaker to another in a closed circuit in a quartz cup; the working electrode is lowered into a cylindrical compartment with electrolyte located in the cup. Such transfer of electrodes, as experiments showed, excludes oxidation of alkali-metal amalgams. In electro-

Four quartz beakers with background solutions 2, 4 and with additions of cadmium 1 and lithium 3 were placed in the analyzer. In beaker 1, electrolysis of cadmium was carried out at a potential of \(-1.8\ \mathrm{V}\), and the anodic oxidation current of cadmium was recorded. The electrolysis was repeated under the same conditions, and after the electrolytic concentration of cadmium the electrodes were transferred to beaker 2 for washing the electrodes in the stirred solution. Then, in beaker 3, without dissolving the cadmium amalgam, electrolytic concentration of lithium was carried out at a potential of \(-2.2\ \mathrm{V}\), followed by dissolution and recording of the current-voltage curve of the complex amalgam. Next, lithium was concentrated and its amalgam was electrodissolved in beaker (3). The Li–Cd–Hg system was studied from the \(I\)—\(C\) and \(I\)—\(\tau\) curves [4,5].

In studying the dependence of the heights of the anodic peaks of the elements, at a constant lithium concentration in solution \((C_1 = 8 \cdot 10^{-4}\ \mathrm{g\text{-}ion/l})\), on the cadmium concentration in solution in the interval \(1.2 \cdot 10^{-4}\)—\(1.2 \cdot 10^{-3}\ \mathrm{g\text{-}ion/l}\), a decrease in the magnitude of the lithium anodic current with cadmium concentration was found. At the same time, a deviation from the linear dependence of the magnitude of the cadmium anodic current on concentration occurs. This indicates the presence of interaction between the elements in the amalgam. The rectilinearity of the plot in the coordinates \(\lg I_1\)—\(\lg I_2\) (Fig. 1) indicates the formation of a sparingly soluble compound of the type \(\mathrm{LiCd}_n\). The stoichiometric coefficient \(n\) of the compound formed is determined from the plot as the angular coefficient of the slope of the straight line. The solubility product of the i.m.c. is calculated from the relations [4]:

Fig. 1. For the study of the Li—Cd—Hg system from the \(I=C\) curves: \(I_1\) and \(I_2\) are the magnitudes of the anodic peaks of lithium and cadmium when jointly present in the amalgam, \(\alpha\); \(\tau_\mathrm{э}=3\ \mathrm{min}\); \(V_\mathrm{д}=1.2 \cdot 10^{-4}\ \mathrm{cm^3}\); \(W=400\ \mathrm{ml/min}\).

\[ \lg L=\lg N-\lg a_1-n\lg a_2; \tag{1} \]

\[ N=I_1\cdot I_2^n, \tag{2} \]

where \(I_1\) and \(I_2\) are the currents of anodic oxidation of lithium and cadmium when they are jointly present in the amalgam, \(\alpha\);

\(a_1\) and \(a_2\) are coefficients depending on the nature of the metal but not depending on their concentration in the amalgam. The coefficient \(a_1\) is calculated from the experimental data by the relation

\[ a_1=W\cdot F\cdot v\cdot \frac{Z_i}{э_i}. \tag{3} \]

where \(v\) is the volume of the amalgam, \(\mathrm{cm^3}\);

\[ \frac{\delta_1}{W} \]

— the effective width of the anodic half-peak, sec;

\(Z_1\) — the charge of the ions participating in the electrode reaction.

The value calculated from the data of three series of experiments was \(n = 0.62;\ 0.64;\ 0.65\), i.e., on average about \(2/3\). Consequently, the composition of the intermetallic compound formed can be represented by the formula \(\mathrm{LiCd}_{2/3}\) or \(\mathrm{Li}_3\mathrm{Cd}_2\). The values \(a_1\) and \(a_2\), calculated from (3), are 0.4 and 1.5 for lithium and cadmium. According to (1), the solubility product for \(\mathrm{Li}_3\mathrm{Cd}_2\) is \(1.2 \cdot 10^{-5};\ 1.4 \cdot 10^{-5};\ 0.9 \cdot 10^{-5}\ \left(\text{g-at}/\text{l}\right)^5\).

To confirm the results obtained, the dependences of the heights of the anodic peaks of lithium and cadmium, when they are present together in the amalgam, on the time of preliminary electrolysis at an unchanged concentration of the elements in solution were recorded. In this case, the equilibrium parameters of the intermetallic compound in mercury are calculated from the formulas [5]:

\[ n=\frac{a_1}{a_2}\times \frac{K_2}{K_1}; \tag{4} \]

\[ 1=\frac{K_1}{a_1}\times \left(\frac{K_2}{a_2}\right)^n \cdot \tau_*^{\,n+1}, \tag{5} \]

where \(K_1\) and \(K_2\) are the angular coefficients of the slopes of the \(J\)-\(\tau\) curves; \(\tau_*\) is the electrolysis time at which the concentration of the metals reached in the amalgam corresponds to the onset of formation of the intermetallic compound. The plots in the coordinates \(J\)-\(\tau\) for amalgams of the individual elements are linear (Fig. 2). When lithium and cadmium are present together in the amalgam, a bend is observed in the straight lines at an electrolysis time \(\tau = 6\) min, which indicates the formation of a sparingly soluble compound. From three series of experiments, according to the \(J\)—\(\tau\) plot and equation (4), \(n = 0.66;\ 0.64;\ 0.69\), i.e., on average \(2/3\). According to (5), the solubility product is \(0.2 \cdot 10^{-5};\ 0.5 \cdot 10^{-5};\ 0.1 \cdot 10^{-5}\).

The values of \(n\) over a wide range of rates of potential change \(W = 0.007;\ 0.016;\ 0.12\ \text{V/sec}\) coincide. This indicates that the equilibrium between the solid phase of the intermetallic compound and the liquid amalgam is practically not disturbed in the process of anodic dissolution of the metals from the amalgam.

Fig. 2. Study of the Li—Cd system by \(\lg I\)—\(\lg \tau\) curves: curves 1 and 2 are for lithium and cadmium separately; curves 1a, 2a are for lithium and cadmium when present together in the amalgam; \(C_{\mathrm{Li}} = 1 \cdot 10^{-4}\ \text{g-ion/l};\ C_{\mathrm{Cd}} = 2.5 \cdot 10^{-4}\ \text{g-ion/l}\).

The data obtained by the first method should be regarded as more reliable. Therefore, the value \((1.0 \pm 0.5)\cdot 10^{-5}\) may be taken as the mean value of the solubility product of the compound \(\mathrm{Li_3Cd_2}\).

Conclusions

1. By the AAP method, the formation in mercury of a sparingly soluble intermetallic compound of lithium with cadmium was detected.

2. Using established methods for studying intermetallic compounds by the AAP method, the composition of the compound formed was determined and its solubility product was calculated.

References

1. V. N. Korshunov, A. B. Grigor'ev. Elektrokhimiya, 6, 1204 (1970).

2. V. N. Korshunov, V. V. Selevin, K. B. Khlystova. Elektrokhimiya, 8, 912 (1972).

3. A. A. Kaplin, E. A. Zakharova. In: Preparation and Analysis of High-Purity Substances. Moscow, Nauka, (1966), p. 174.

4. M. S. Zakharov, A. G. Stromberg, N. A. Mesyats. Izv. TPI, 164, 165, 171 (1967).

5. A. G. Stromberg, M. S. Zakharov, N. A. Mesyats. Elektrokhimiya, 3, 1440 (1967).