STUDY OF INTERMETALLIC COMPOUNDS OF GALLIUM WITH GOLD, COBALT, AND NICKEL BY AMALGAM POLAROGRAPHY WITH ACCUMULATION

O. S. STEPANOVA

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

Earlier [1], the possibility was shown of studying the composition of intermetallic compounds of gallium and germanium with copper, as well as of cadmium with gold, by the method of amalgam polarography with accumulation. Methods for determining the composition of intermetallic compounds by amalgam polarography with accumulation are set forth in works [2, 3].

It is known that gallium forms intermetallic compounds with gold, cobalt, and nickel of composition \((\mathrm{Ga}_4\mathrm{Au}_5)\mathrm{Hg}_x\), \((\mathrm{Ga}_7\mathrm{Co}_{10})\mathrm{Hg}_x\), \((\mathrm{Ga}_7\mathrm{Ni}_{10})\mathrm{Hg}_x\) [5] and \(\mathrm{GaAu}\), \(\mathrm{GaCo}\), \(\mathrm{GaNi}\) [4]. It is of interest to study the intermetallic compounds of gallium with gold, cobalt, and nickel by the method of amalgam polarography with accumulation and to confirm their composition. The advantages of this method over the potentiometric method and others are discussed in work [1].

According to the method set forth in work [2], determination of the composition of an intermetallic compound is carried out from the ratio of the concentrations of the elements at which the anodic notches of the latter disappear completely. Determination of the composition of an intermetallic compound \(AB_n\) by the method proposed by A. G. Stromberg and V. E. Gorodovykh [3] is carried out by the formula

\[ \frac{n+1}{n}=\lg \frac{x_m''}{x_m'} \bigg/ \lg \frac{c_1'}{c_1''}, \tag{1} \]

where \(n\) is the number of atoms of the added element in the intermetallic compound (Au, Co, Ni); \(c_1'\), \(c_1''\) are the concentrations of metal ions in solution (Ga) for two series of experiments. To determine the value \(\lg \dfrac{x_m''}{x_m'}\), a graph is constructed for two series of experiments in the coordinates \(\lg y - \lg x\), where

\[ y=\frac{I}{^{\circ}I_1}; \qquad x=\frac{c_2}{c_1}. \tag{2} \]

Here \(^{\circ}I_1\) and \(I\) are the depth of the anodic notch of the element whose concentration in solution remains constant (Ga), respectively without addition of the second element to the electrolyzer and with additions; \(c_2\) is the concentration of metal ions in solution whose concentration changes (Au, Co, Ni) during the experiment.

The values \(^{\circ}I_1\), \(I\), and \(\dfrac{c_2}{c_1}\) are determined from experiment. At \(\lg y_m = 0\), one finds

are obtained from a plot of the values \(\lg x_m''\) and \(\lg x_m'\), and \(n\) is determined from equation (1).

Experimental part\(^1\)

The investigations were carried out on a 7-77-4B polarograph with an electrolyzer whose design is described in [6]. An acetate buffer (pH 4.6) was used as the supporting electrolyte. The anodic gallium peak in this supporting electrolyte occurs at a potential of \(-0.75\) V (sat. cal. el.). The content of impurities of heavy metals in these solutions was less than \(1 \cdot 10^{-8}\) g-ion/l. Two series of experiments were conducted: in the first series, the gallium concentration was \(1 \cdot 10^{-4}\) g-ion/l; in the second series it was \(1.6 \cdot 10^{-4}\) g-ion/l and \(2 \cdot 10^{-4}\) g-ion/l. Solutions of gold, cobalt, and nickel were added to the electrolyzer during the investigation until the gallium peaks disappeared completely. To obtain reproducible results, the stirring conditions in all experiments were kept constant. The electrolysis time was 300 sec. The electrolysis potential during accumulation of gallium with gold, cobalt, and nickel was \(-1.4\) V (sat. cal. el.). At these potentials, the diffusion current is reached for all the indicated elements. The volume of the solution was 5 ml; the radius of the stationary mercury drop was \(r = 0.04\) cm.

Fig. 1. Plot of the dependence \(\lg y\)—\(\lg x\).
\(C^2G_1\)—\(aAu_n\), \(C_{\mathrm{Ga}}\cdot 10^{+4}\) g-ion/l: (1)—1.0; (2)—2.0; (3,4)—Ga \(Co_n\), \(C_{\mathrm{Ga}}\cdot 10^{+4}\) g-ion/l: (3)—1.0; (4)—2.0; (5,6)—Ga \(Ni_n\), \(C_{\mathrm{Ga}}\cdot 10^{+4}\) g-ion/l: (5)—1.0; (6)—1.6.

After the electrolysis was completed, nitrogen was shut off and, after 1 min, the anodic polarogram was recorded.

The results of the investigations are presented in Tables 1 and 2 and in Fig. 1. Each measurement was repeated at least four times [7].

\(^1\) The experimental part was carried out with the participation of diploma student O. K. Tikhonova.

Table 1

Results of studies of the interaction of gallium with gold, cobalt, and nickel

Table 2

Calculated data for determining the composition of intermetallic compounds by the method of A. G. Stromberg and V. E. Gorodovykh [3]

Discussion of the Results

It is seen from Table 1 that, at values of the ratios

\[ \frac{[\mathrm{Ni}]}{[\mathrm{Ga}]} = 1, \]

\[ \frac{[\mathrm{Au}]}{[\mathrm{Ga}]} = 1 \quad \text{and} \quad \frac{[\mathrm{Co}]}{[\mathrm{Ga}]} = 1 \]

the anodic peaks of gallium disappear completely. The values of \(n\) (Table 2) for the intermetallic compounds \(\mathrm{GaAu}_n\), \(\mathrm{GaCo}_n\), and \(\mathrm{GaNi}\) are respectively equal to 1.

Consequently, the intermetallic compound of gold with gallium has the composition \(\mathrm{GaAu}\), which is in agreement with the data of [8]; these metals form an intermetallic compound of the same composition in alloys without mercury [4]. The intermetallic compounds of gallium with cobalt and nickel have the compositions \(\mathrm{GaCo}\) and \(\mathrm{GaNi}\) and are in agreement with [9] and in contradiction with work [5].

For the intermetallic compounds \(\mathrm{GaCo}\) and \(\mathrm{GaNi}\), the solubility product (\(Lp\)) was calculated. The latter was found from equation (16) of work [3]

\[ \lg Lp = n \lg x_m + (n+1)\lg c_1^0 - n \lg k', \tag{13} \]

where \(\lg x_m\) is obtained from the \(\lg x\)—\(\lg y\) plot; \(c_1^0\) is the concentration of gallium atoms in the amalgam in the absence of cobalt and nickel; \(c_1^0\) is calculated from the equation

\[ c_1^0 = \frac{i \cdot \tau}{zFv_k}, \]

where \(i\) is the accumulation current of gallium in the absence of cobalt and nickel; \(\tau\) is the accumulation time, sec; \(z\) is the valence of gallium ions; \(F\) is Faraday’s number; \(v_k\) is the drop volume, \(\mathrm{cm}^3\)

\[ k' = \frac{k_2}{k_1}, \]

where \(k_2\) is the electrolysis constant for cobalt and nickel in the absence of gallium, \(k_1\) is the electrolysis constant for gallium in the absence of cobalt and nickel; \(k_2\) and \(k_1\) were determined from the following equations:

\[ k_1 = \frac{i_1}{zFc_1} \quad \text{and} \quad k_2 = \frac{i_2}{zFc_2}, \]

where \(i_1\) is the electrolysis current of Ga, μA; \(i_2\) is the electrolysis current of Co and Ni, μA.

The solubility products for the intermetallic compounds \(\mathrm{GaCo}\) and \(\mathrm{GaNi}\) are respectively equal to \(2.6 \cdot 10^{-16}\) and \(3.90 \cdot 10^{-16}\ \mathrm{g\text{-}atom}^2\mathrm{ml}^{-2}\).

For the intermetallic compound \(\mathrm{GaAu}\), \(Lp\) cannot be calculated, since neither an anodic peak of gold nor a cathodic wave of gold could be obtained, because the potential of the anodic peak is more positive than the potential of anodic dissolution of Hg, and the reduction of gold ions in acetate begins directly from the potential of mercury dissolution [10, 11]. It should be noted that, in studying the interaction of gallium and gold, the shape of the anodic peak changes. The anodic peak of gallium becomes anomalous in appearance: both the electrochemical and diffusion branches of the anodic peak are distorted; the peak decreases in height, broadens, becomes more obtuse, bifurcates, and, finally, at a ratio of gallium and gold concentrations of \(1:1\), the anodic peak of gallium disappears completely and a new anodic peak appears at a potential of \(-0.25\ \mathrm{V}\) (sat. cal. el.) (Fig. 2). Figure 2 presents polarograms from which the influence of gold on the anodic peak of gallium is evident. The anodic peak at a potential of \(-0.25\ \mathrm{V}\) (sat. cal. el.), apparently, co-

corresponds to the oxidation of the intermetallic compound GaAu. At the same time, it may be assumed that during work copper contamination entered the electrolyzer and the test solutions, and that the anodic wave at a potential of \(-0.25\) V (vs. S.C.E.) corresponds to the oxidation of the latter. However, first, copper on the background used (acetate buffer with pH 4.6) appears at a potential of \(-0.1\) V (vs. S.C.E.); second, we would not observe the anodic wave of copper in the presence of gallium at such a concentration \((1 \cdot 10^{-4}\ \text{g-ion/l})\), since Ga and Cu interact with one another and give the intermetallic compound GaCu [1]. Consequently, it remains to assume that the anodic wave with a peak at \(-0.25\) V (vs. S.C.E.) corresponds to oxidation of the intermetallic compound. We added copper at a concentration of \(1 \cdot 10^{-4}\ \text{g-ion/l}\) to the electrolyzer containing the test solution with acetate buffer and solutions of gallium and gold at a concentration ratio of the latter of \(1:1\), in order to clarify the nature of the anodic wave with a peak potential of \(-0.25\) V (vs. S.C.E.). The anodic wave at this potential disappeared, and no peaks appeared on the polarogram (Fig. 2, д). This apparently can be explained by the fact that copper entered into reaction with molecules of the intermetallic compound GaAu

Fig. 2. Polarogram of anodic oxidation of gallium and gold:
a) background, б) anodic wave of gallium \((1 \cdot 10^{-4}\ \text{g-ion/l})\), в) Ga with addition of Au, г) Ga and Au \((1:1)\), д) Ga and Au \((1:1)\) + Cu \((1 \cdot 10^{-4}\ \text{g-ion/l})\).

\[ \mathrm{GaAu} + \mathrm{Cu} \rightarrow \mathrm{GaCu} + \mathrm{Au} \]

\[ \begin{array}{cc} \downarrow & \uparrow \\ \mathrm{GaAu}\downarrow & \end{array} \]

Unfortunately, there are no numerical data on the dissociation constants of GaAu and GaCu that would allow one to assert precisely such a reaction mechanism; but on the basis of the experimental data it may be assumed that the dissociation constant of the intermetallic compound GaCu is smaller than the dissociation constant of GaAu.

There are indications in the literature [12, 13] that molecules of an intermetallic compound are present in the liquid phase of complex amalgams. Proceeding from this assumption regarding the existence in mercury of molecules of an intermetallic compound, Tammann and his coworkers [13], on the basis of deviations of the potentials of complex amalgams from the potentials of the corresponding simple amalgams, calculated the dissociation constants of intermetallic compounds in mercury. At the same time, there are reports [14—16] that, when an intermetallic compound dissolves in mercury, it undergoes complete dissociation into its constituent components. The concentration in the liquid phase of the amalgam of free atoms of the metals entering into the compound is determined not by the dissociation constant of the intermetallic compound, but by the value of the solubility product of this compound in mercury [17].

We shall study this question in detail in the future, and here we shall not dwell on it further. The values of \(n\) and \(Lp\) make it possible to estimate the maximum values of the concentrations of ions in solution of both metals forming the sparingly soluble intermetallic

compound in mercury, at which the solubility product of this compound (under the given experimental conditions) is still not reached and, consequently, no error is introduced in the joint determination of both metals in the analysis.

The maximum concentrations can be calculated from equation (17) of work [3]

\[ c_2 c_2=\frac{Lp}{\gamma_1\gamma_2^n} =\frac{Lp}{(k')^n\left(\gamma_1^{\,n+1}\right)}, \tag{4} \]

where \(\gamma_1\) and \(\gamma_2\) are the accumulation coefficients, which are respectively equal to:

\[ \gamma_1=\frac{\kappa_1\tau}{v_k},\qquad \gamma_2=\frac{\kappa_2\tau}{v_k}. \tag{5} \]

If the product of the concentrations is smaller than the constant in the first part of equation (4), then there will be no mutual influence of the metals in their joint determination by amalgam polarography. From formula (4) we obtain, for the maximum permissible concentration of gallium and cobalt ions in solution (a), and also of gallium and nickel in solution (b) (at \(\tau=1800\) sec):

\[ \text{a) } c_1c_2=0{,}4\cdot10^{-24}\ \text{g-ion}^2/\text{ml}^2; \]

\[ \text{b) } c_1c_2=0{,}8\cdot10^{-24}\ \text{g-ion}^2/\text{ml}^2. \]

Thus, at equal concentrations of gallium and cobalt ions, and also of gallium and nickel ions in solution (\(c_1=c_2\)), and under the given experimental conditions (\(\gamma\mathrm{Ga}=2{,}66\cdot10^{-3}\)), the maximum permissible concentration of these ions in solution, at which an intermetallic compound between gallium and cobalt in mercury (a), and also between gallium and nickel in mercury (b), is not yet formed (the value of the solubility product is not reached), is respectively equal to:

\[ \begin{aligned} \text{a) }& c_1=c_2 && \sqrt{0{,}4\cdot10^{-24}}=0{,}638\cdot10^{-12}\ \text{g-ion}/\text{ml};\\ & c_2=10c_1; && c_1=0{,}2\cdot10^{-12}\ \text{g-ion}/\text{ml};\\ & c_2=100c_1; && c_1=0{,}638\cdot10^{-13}\ \text{g-ion}/\text{ml}; \end{aligned} \]

\[ \begin{aligned} \text{b) }& c_1=c_2 && \sqrt{0{,}8\cdot10^{-24}}=0{,}9\cdot10^{-12}\ \text{g-ion}/\text{ml};\\ & c_2=10c_1; && c_1=0{,}30\cdot10^{-12}\ \text{g-ion}/\text{ml};\\ & c_2=100c_1; && c_1=0{,}9\cdot10^{-13}\ \text{g-ion}/\text{ml}. \end{aligned} \]

If the product \(c_1c_2\) exceeds the permissible limiting value \(Lp/\gamma^{\,n+1}(k')^n\) under the given conditions of the analysis, then the value of \(\gamma\) must be decreased, that is, \(\tau\) and \(\kappa_1\) must be decreased and \(v_k\) increased (see formula 5).

Conclusions

1. By the method of amalgam polarography with accumulation, the composition of the intermetallic compounds of gallium with gold, cobalt, and nickel in mercury was determined.

2. The solubility products of the intermetallic compounds of gallium with cobalt and nickel in mercury were calculated.

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