STUDY OF Li—Cd AMALGAMS BY AMALGAM POLAROGRAPHY WITH ACCUMULATION

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

(Presented by the scientific and technical seminar of the Department of Physical and Colloid Chemistry)

By the method of amalgam polarography with accumulation (APA), several intermetallic compounds (i.m.c.) in mercury have been studied [1—6]. In the literature there are no data on the investigation by this method of complex amalgams including alkali and alkaline-earth elements.

In the present work, using the APA method, the Li—Cd—Hg system was studied. The fundamental possibility of the formation of an i.m.c. between lithium and cadmium is based on the presence in the lithium atom of an unfilled \(s\)-shell [7]. As the indifferent electrolyte, a \(0.02\ M\) solution of \((\mathrm{C}_2\mathrm{H}_5)_4\mathrm{NJ}\) in dimethylformamide was selected. On this background, separate peaks of lithium and cadmium are recorded at potentials \(-1.9\) and \(-0.3\ \mathrm{V}\), respectively*). This made it possible to carry out a study of Li—Cd amalgams by the APA method in two ways [8, 9].

Fig. 1. Electrolytic cell with transfer of electrodes: 1 — glass beaker, 2 — quartz beaker, 3 — Teflon lid, 4 — mercury film electrode, 5 — gas-supply tube, 6 — compartment with electrolyte, 7 — quartz spoon with mercury

The investigations were carried out on an \(Lp\)-60 polarograph and a PO-2 oscillopolarograph. As the indicator electrode, a mercury film electrode with a film thickness of \(10\ \mu\mathrm{m}\) on a silver substrate with a surface area of \(0.15\ \mathrm{cm}^2\) was used. In the work, the technique of electrode transfer was used [6]. For work in nonaqueous media with bottom mercury as the reference electrode, in order to conduct the concentration of lithium and cadmium at different potentials and in separate electrolytes, we developed a new design of the electrolytic cell (Fig. 1). The bottom mercury is transferred from one beaker to another, with the circuit closed, in a quartz cup; the working electrode is thereby lowered into the cylindrical compartment with electrolyte located in the cup. Such transfer of electrodes,

*) All listed potential values are given relative to bottom mercury.

as special experiments showed, excludes oxidation of the alkali-metal amalgams. Four quartz beakers (Fig. 1), of volume 20 ml, containing background solutions (2, 4) and additions of cadmium (1) and lithium (3), are placed in the electrolyzer. In beaker 1, cadmium is electrolyzed at a potential of \(-1.8\) V and the anodic oxidation current of cadmium is recorded. The electrolysis is repeated under the same conditions and, after electrolytic concentration of cadmium, the electrodes are 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 is carried out at a potential of \(-2.2\) V, followed by dissolution and recording of the current–voltage curve of the complex amalgam**). Next, lithium is concentrated and its amalgam is electro-dissolved in beaker 3.

Purification of the salts \((\mathrm{C_2H_5})_4\mathrm{NI}\), lithium, and cadmium is carried out by the procedure described in [10]. Dimethylformamide is distilled twice in a quartz apparatus [11]. Argon is used for stirring the solutions.

Fig. 2. Dependence \(I—C\):
\(C_{\mathrm{Li}} = 8 \cdot 10^{-4}\ \mathrm{g\text{-}ion/l} = \mathrm{const}\);
\(C_{\mathrm{Cd}} = 1.6 \cdot 10^{-4}—1.2 \cdot 10^{-3}\ \mathrm{g\text{-}ion/l}\);
\(V_{\mathrm{Hg}} = 1.2 \cdot 10^{-4}\ \mathrm{cm^3}\);
\(W = 400\ \mathrm{mV/min}\); \(\tau = 3\ \mathrm{min}\).

Figure 2 presents the results of studying the intermetallic compound by the first method, based on obtaining the dependence of the heights of the anodic peaks of the elements at constant concentration in the amalgam of the more electronegative element and changing concentration of the more electropositive element. The stoichiometric coefficient \(n\) of the compound formed is determined from the graph in the coordinates \(\lg I_1—\lg I_2\) as the angular coefficient of the slope of the straight line, and the solubility product of the intermetallic compound is calculated from the relations [8]:

\[ \lg \Pi_{\mathrm{p}} = \lg N - \lg a_1 - n \lg a_2 \qquad (1); \qquad N = I_1 \cdot I_2^n, \qquad (2) \]

where \(I_1\) and \(I_2\) are the heights of the anodic peak of lithium and cadmium; \(a\), \(a_1\), and \(a_2\) are calculated from the experimental data according to the relation

\[ a_i = W \cdot F \cdot V \cdot \frac{Z_i}{s_i}, \qquad (3) \]

where \(v\) is the volume of the amalgam, \(\mathrm{cm^3}\); \(\dfrac{s_i}{W}\) is the effective width of the anodic half-peak, V; \(Z_i\) is the charge of the ions participating in the electrode reaction.

The graph in the coordinates \(\lg I_1—\lg I_2\) is linear, which indicates the formation of a sparingly soluble compound of the type \(\mathrm{LiCd}_n\). In three series of experiments, \(n = 0.62;\ 0.64;\ 0.65\), i.e., on average about \(2/3\). Consequently, the composition of the intermetallic compound formed may be represented by the formula \(\mathrm{LiCd}\) or \(\mathrm{Li_3Cd_2}\). According to formula (1), the solubility product for \(\mathrm{Li_3Cd_2}\) is \(1.2 \cdot 10^{-5};\ 1.4 \cdot 10^{-5};\ 0.9 \cdot 10^{-5}\ (\mathrm{g\text{-}at/l})^5\).

By the second method, based on obtaining the dependence of the heights of the anodic peaks of the components, when they are jointly present in the amalgam, on the time of preliminary electrolysis at unchanged concentra-

**) To avoid contamination of the lithium solution with cadmium ions, it is advisable to dissolve the complex amalgam in beaker 4.

the concentrations of the elements in solution, the equilibrium parameters of the intermetallic compound in mercury are calculated by formulas [8]:

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

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

where \(K_1\) and \(K_2\) are the slope coefficients of the \(I\)—\(\tau\) curves; \(\tau_*\) is the electrolysis time at which the concentration of metals corresponding to the onset of formation of the intermetallic compound is reached in the amalgam. The plots in the coordinates \(I\)—\(\tau\) for amalgams of the individual elements are rectilinear (Fig. 3). When lithium and cadmium are jointly present 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 between lithium and cadmium. From three series of experiments, according to equation (4), the graph \(I\)—\(\tau\) gave \(n=0.66;\ 0.64;\ 0.69\), i.e., on average \(2/3\). According to formula (5), the solubility product is \(0.2\cdot 10^{-5};\ 0.5\cdot 10^{-5};\ 0.1\cdot 10^{-5}\ \left(\text{g-at}/\text{l}\right)^5\).

Fig. 3. Dependence \(I\)—\(\tau\): curves 1, 2 for lithium and cadmium separately; curves 1a, 2a in their joint presence: \(C_{\mathrm{Li}}=1\cdot10^{-4}\ \text{g-ion/l};\ C_{\mathrm{Cd}}=2.5\cdot10^{-4}\ \text{g-ion/l}\).

The values of \(n\) over a wide range of rates of potential change \((W=0.007;\ 0.0016;\ 0.12\ \text{V/sec})\) coincide. This indicates that the equilibrium between the solid phase of the intermetallic compound and the liquid amalgam is practically not disturbed during the anodic dissolution of the metals from the amalgam.

The data obtained by the first method should be considered more reliable. Therefore, for the average value of the solubility product of the compound \(\mathrm{Li}_3\mathrm{Cd}_2\), the value \((1.0\pm0.5)\cdot10^{-5}\) may be accepted.

LITERATURE

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