Amalgam-Polarographic Behavior of Gallium

O. S. Stepanova, M. S. Zakharov

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

Continuing the work on amalgam polarography [1–4], we studied the amalgam-polarographic behavior of gallium in various electrolytes. The investigations were carried out on a polarograph of the 7-77-4E type in the electrolysis cell described in [5]. Standard gallium solutions were prepared by dissolving a weighed portion of metallic gallium in minimal amounts (sufficient for dissolution) of 4–5 N hydrochloric acid at a temperature of 18°C. After dissolution of the weighed portion, the contents of the flask were brought up to the mark with triply distilled water. Solutions of \(1 \cdot 10^{-5}\)—\(1 \cdot 10^{-8} M\) were prepared by successive dilution and acidified with 2 ml of 1 N HCl.

It is known [2] that the anodic wave of gallium is obtained against a background of 0.1 N NaSal + 0.1 N KCl. As background solutions we tested the following: 0.1 N KCl; 0.1 N KNO\(_3\); acetate buffer (pH 4.6); 0.1 N NaCl; 0.1 N KCNS (pH 4.6). On the backgrounds listed, anodic waves of gallium were obtained for the first time (Table 1).

Table 1

Amalgam-polarographic behavior of gallium on several backgrounds

The accumulation of gallium was carried out at a potential of \(-1.4\) V (sat. cal. el.). At this value of the potential, the diffusion current is reached for gallium (Figs. 1, 2).

To establish the reversibility of the process of anodic oxidation of gallium on the backgrounds studied, we used the expression which, for a reversible anodic process in the case of linear semi-infinite diffusion, has the following form [11]:

Fig. 1. Effect of the electrolysis potential on the height of the anodic peak of gallium in acetate buffer background, pH 4.6.

Fig. 2. Effect of the electrolysis potential on the height of the anodic peak of gallium in 0.1 N KNO\(_3\) background, pH.

\[ \varphi_{\text{p}}^{a} - \varphi_{\text{p}/2}^{a} = 2.2\,\frac{RT}{ZF}. \]

In our case, where spherical diffusion from a finite volume is observed at a slowly varying potential, the use of this equation apparently does not introduce any noticeable error into the calculation results. If gallium were oxidized reversibly, then \(\varphi_{\text{p}}^{a} - \varphi_{\text{p}/2}^{a} = 0.0177\ \text{V}\). From the experimental data, on all backgrounds \(\varphi_{\text{p}}^{a} - \varphi_{\text{p}/2}^{a} \gg 0.0177\). Consequently, on all backgrounds gallium is oxidized irreversibly.

When anodic polarograms are recorded in the concentration range \(1 \cdot 10^{-6}\) g-ion/l—\(3 \cdot 10^{-4}\) g-ion/l, on all backgrounds at pH \(3 \div 5\) a double peak of gallium is obtained (Fig. 3). One peak is obtained at a potential of \(-1\) V, and the other at a potential of \(-0.8\) V. The results of the studies show that the peak at the more negative potential is due to anodic dissolution of gallium with the formation of a sparingly soluble film of gallium hydroxide \( \mathrm{Ga(OH)_3} \) on the surface of the mercury drop, which forms and exists in the interval pH \(= 3 \div 9\) [6], preventing further oxidation of gallium. With large anodic polarization, apparently, penetration of gallium atoms through the film occurs, which—

Fig. 3. Double gallium peak in 0.1 N KCl + 0.1 N NaSal background;
\(C_{Ga} = 1 \cdot 10^{-4}\) g-ion/l; \(r = 0.01\) cm;
\(\tau = 300\) sec; \(i_x = 6 \cdot 10^{-8}\) A/mm;
\(\varphi_{\text{el}} = -1.4\) V (sat. cal. el.); \(\omega = 800\) mV/min.

which are oxidized, and as a result a second anodic notch is obtained at a more positive potential. The notch at the more negative potential, caused by the formation of the sparingly soluble film \(Ga(OH)_3\), remains practically constant in the concentration range \(1\cdot10^{-6}\)—\(1\cdot10^{-4}\) g-ion/l of gallium in solution during an electrolysis time of 10 min on different supporting electrolytes (Figs. 4, 5).

As can be seen from Figs. 4, 5, the curves obtained are analogous; only at pH 4.6 is the anodic notch at the more negative potential somewhat smaller in magnitude than the same notch at pH 3.2. This is quite understandable, since at pH 4.6 more gallium hydroxide precipitates than at pH 3.2, since at this pH value it only begins to form. The constancy in magnitude of the anodic notch at the more negative potential, caused by the \(Ga(OH)_3\) film, can be explained by the fact that the kinetics of the process is determined by the rate of diffusion of anions to the electrode surface.

Fig. 4. Change in the height of the double gallium notch with the concentration of the latter in solution against an acetate buffer background.

Fig. 5. Change in the height of the double gallium notch with the concentration of the latter in solution against a background of 0.1 N \(KNO_3\) + \(n\) 1 N \(HCl\) (pH 3.2).

In the work of E. A. Zakharova [12], double notches of copper in certain supporting electrolytes were studied, caused by the formation of sparingly soluble copper compounds on the surface of the mercury drop, and new methods and procedures were proposed for studying their nature.

Of great interest is the use of the computational method [7] in amalgam polarography. We calculated the polarographic characteristics and certain coefficients for gallium on the above-mentioned supporting electrolytes.

The studies were carried out in a supporting-electrolyte volume of 5 ml; the rate of potential change was 666 mV/min. An important condition for obtaining correct results is the complete removal from the solution of oxygen purified with hydrogen or nitrogen.

For the current of the anodic notch \((I_2, a)\), for the current of preliminary electrolysis with stirring of the solution \((I_1, a)\), and for the current of the cathodic peak \((I_3, a)\), we have the relations [7], where \(C_1\) and \(C_2\) are the concentrations of metal ions in solution (g-ion/l) and of metal atoms in the amalgam (g-at/l); \(K_1\), \(K_2\), \(K_3\) are the constants of electrolysis, the anodic notch, and the cathodic peak (A·cm·mol\(^{-1}\)); \(S\) is the surface area of the mercury microelectrode, cm\(^2\). Between the currents \(I_2\), \(I_1\), \(I_3\), three relations were obtained:

\[ \gamma_{\mathrm{p}}=\frac{I_1}{I_3}=\frac{K_1}{K_3}, \]

\[ \gamma_{\mathrm{e}}=\frac{I_2}{I_1}=\frac{K_2}{K_1}\cdot\frac{C_2}{C_1}, \]

\[ \gamma_{\mathrm{ch}}=\frac{I_2}{I_3}=\frac{K_2}{K_3}\cdot\frac{C_2}{C_1}. \]

In addition, one can also write three relations between the constants and the concentrations:

\[ \gamma_{\mathrm{f}}=\frac{K_2}{K_3};\qquad \gamma_{0}=\frac{K_2}{K_1};\qquad \gamma_{\mathrm{k}}=\frac{C_2}{C_1}; \]

where \(\gamma_{\mathrm{p}}, \gamma_{\mathrm{e}}, \gamma_{\mathrm{ch}}, \gamma_{\mathrm{f}}, \gamma_{0}, \gamma_{\mathrm{k}}\) are the coefficients of stirring, electrolysis, sensitivity, form, attenuation, and concentration. For the convenience of comparing the amalgam-polarographic properties of gallium with those of other elements, the polarographic characteristics obtained should be reduced to selected standard conditions. The following were chosen as standard conditions:

\[ K_1^0=500\ a.s.m.\ mol^{-1}, \]

\[ \gamma_{\mathrm{k}}^0=100; \]

\[ \omega^\circ=10^{-2}\ v/sec, \]

\(t^\circ=25^\circ\mathrm{C}\), where \(\omega\) is the rate of change of potential. Then

\[ \gamma_{\mathrm{p}}^0=\frac{K_1^0}{K_3^0}=\frac{500Z}{K_3^0};\quad \gamma_{\mathrm{f}}=\gamma_{\mathrm{f}}^0;\quad \gamma_{0}^0=\frac{K_2^0}{500Z};\quad \gamma_{\mathrm{e}}^0=\frac{K_2^0}{500Z}\cdot100;\quad \gamma_2^0=\frac{K_2^0}{K_3^0}\cdot100. \]

The experimental and calculated data are presented in Tables 2–4.

From the data obtained it may be concluded that, with identical stirring and identical concentration, the sensitivity of the determination of gallium against a background of 0.1 N KCl is greater than that against a background of 0.1 N KNO\(_3\).

For gallium in one and the same electrolyte solution, the half-width \((\delta)\), as our investigations have shown, does not depend on the concentration of gallium in the solution and in the amalgam (Table 2). These results agree with those obtained in [7] for other elements.

Moreover, the greater the half-width, the smaller the depth of the peak and the smaller the values of the constant \(K_2''\). Thus there are grounds for considering that the constant of the anodic peak will be inversely proportional to the half-width.

We have calculated the values

\[ \frac{K_{2,1}''}{K_{2,2}''} \quad \text{and} \quad \frac{\delta_2}{\delta_1}, \]

where \(K_{2,1}''\) and \(\delta_1\), \(K_{2,2}''\) and \(\delta_2\) are the constant of the anodic peak and the half-width in the first and second electrolytes (Table 4).

It is seen from Table 4 that the ratios between the constants approximately coincide with the ratios of the half-widths. This fact indicates that the difference between the constants \(K_2\) is due to the different degree of irreversibility of the electrode process (different half-width).

The influence of the pH of the medium on the anodic and cathodic currents of gallium on the indicated backgrounds was investigated. It was established that in an acidic medium (pH 1–2.8) the reduction of gallium is masked by the discharge of hydrogen ions.

Table 2

Amalgam-polarographic characteristics of the anodic peaks of gallium

*) The lowered values of \(K_2\) and \(K''_2\) are explained by the fact that against this gallium background there is a double peak, the total area of which under the anodic peak curve is 37.9 coulombs, while the peak height was measured only for the positive gallium peak.

hydroxonium \((\mathrm{H_3O^+})\); in a neutral medium gallium solutions undergo hydrolysis; in an alkaline medium the existing gallium ions are not reduced in the mercury drop over the potential interval studied (down to \(-2\) volts). Polarography of gallium must be carried out in a buffer solution with \(\mathrm{pH}\ 3\text{–}5\).

Table 3

Standard values of constants and polarographic characteristics of anodic teeth

Calibration curves for gallium over a wide concentration interval \(1 \cdot 10^{-9}\text{–}1 \cdot 10^{-6}\) g-ion/l against the background of an acetate buffer with pH 4.6 are rectilinear, which is not observed in unbuffered solutions. The minimum concentration of gallium determined by the APN method under the experimental conditions has been established:

Table 4

Values of ratios for gallium on different backgrounds

\[ r = 0.04\ \mathrm{cm}; \]

\[ v_\phi = 5\ \mathrm{ml}; \]

\[ \varphi_{\mathrm{el}} = -1.4\ \mathrm{V}\ (\text{saturated calomel electrode}); \]

\[ \tau = 800\ \mathrm{s}; \qquad K_1 = 9 \cdot 10^3; \]

\[ i_x = 3 \cdot 10^{-9}\ \mathrm{a/mm}. \]

Under these conditions, from a gallium concentration of \(1 \cdot 10^{-9}\) g-ion/l an anodic tooth of \(7\ \mathrm{mm}\) is obtained. The sensitivity for gallium can be increased by thorough purification of nitrogen from traces of oxygen, if the electrolysis time is increased (to \(\tau = 3600\ \mathrm{s}\)), the volume of the background solution is decreased, and the instrument is operated at maximum sensitivity. Taking into account the ability of gallium to give intermetallic compounds with Pt, Au, the mercury drop must be suspended on a silver wire. The sensitivity for gallium can be increased with the aid of a film electrode [9], if a minimum value of the residual current is achieved [10].

LITERATURE

1. M. S. Zakharov. ZhAKh, vol. 18, No. 4, 450, 1963.
2. O. S. Stepanova, M. S. Zakharov. Izv. vysshikh uchebnykh zavedenii (Chemistry and Chemical Technology), No. 2, 184, 1964.
3. O. S. Stepanova, M. S. Zakharov. ZhAKh, vol. XX, No. 2, 153, 1965.

1. A. G. Stromberg, M. S. Zakharov, O. S. Stepanova, S. F. Gurskaya. Methods for the Analysis of Chemical Reagents and Preparations, issue 5–6, IREA, Moscow, 1963.

2. V. I. Kuleshov. Methods for the Analysis of Chemical Reagents and Preparations, issue 5–6, IREA, Moscow, 1963.

3. M. Bréant, J.-K. Merlen, Compt. rend., 255, 41, 1729, 1962.

4. M. S. Zakharov, A. G. Stromberg. ZhFKh, 38, 130, 1964.

5. O. S. Stepanova, M. S. Zakharov. Izv. TPI (in press), vol. 148.

6. V. A. Itolinsky. Candidate dissertation, Tomsk, 1963.

7. A. G. Stromberg. Izv. TPI, vol. 128, 13, 1965.

8. P. Delahay. New Instruments and Methods in Electrochemistry, IL, 135, 1947.

9. E. A. Zakharova. Izvestiya TPI, vol. 128, 53, 1965.