INVESTIGATIONS INTO THE PHASE STATE AND SOLUBILITY OF NICKEL IN MERCURY BY THE APN METHOD

N. V. PODKORYTOVA, B. F. NAZAROV

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

The question of the magnitude of the solubility and the phase nature of nickel in mercury has not been resolved. In works [1, 3] the solubility of nickel was studied by measuring the height of the anodic nickel peak on a second drop of mercury, and it was assumed that on the first drop nickel is oxidized both from the surface and from the liquid phase of the amalgam. Opinions were expressed that the observed anodic nickel peaks are due to its dissolution from the amalgam [2] or from the surface of the electrode [11, 12]. The data on the magnitude of the solubility of nickel in mercury, determined by various authors [1, 3—9], are also contradictory.

The aim of our work is to study the phase nature of nickel under the conditions observed when determining it by the APN method, and to calculate from the data obtained the solubility of nickel in mercury.

The investigations were carried out on an OH-101 polarograph (Hungary) with a circulating electrolyzer for the AKPN method [10].

The working electrode was a mercury film electrode, and the reference electrode was a saturated calomel electrode. The mercury film on the electrode was obtained by electrolysis from a saturated solution of Hg₂(NO₃)₂; the amount of mercury was calculated by the coulometric method, assuming a current efficiency equal to 100%. The nickel content in the amalgam was also determined coulometrically from the area under the anodic wave. The background used in the work consisted of 4 ml of 0.1 M KSCN solution and 1 ml of 0.1 M KCl solution; the potential of the anodic nickel peak is equal to −0.45 V (relative to the saturated calomel electrode). The investigations were carried out at 17°C.

Fig. 1. Dependence of the amount of oxidized nickel on its concentration in solution (curve I) and dependence of the amount of dissolved nickel at the maximum on the amount of mercury \(q_{\mathrm{Hg}}\) (curve II). \(\varphi_{\mathrm{e}} = -1.1;\ \tau_{\mathrm{e}} = 1\) min

Dependences were obtained of the amount of oxidized nickel \((q_{\mathrm{Ni}})\) on the nickel concentration in solution for different amounts of mercury \((q_{\mathrm{Hg}})\) on the electrode; all curves were obtained at a constant electrolysis time. In Fig. 1 (curves I), as an example, dependences are presented for

$q_{\mathrm{Hg}} = 2.18$ mg and 1.87 mg. These dependences are linear in character, but at a certain concentration of nickel in the solution the amount of oxidized nickel decreases.

To study the phase nature of nickel in mercury, the dependence between the amount of oxidized nickel at its maximum value $q_m$ (Fig. 1, curves $I$) and $q_{\mathrm{Hg}}$ (Fig. 1, curve $II$) was studied. This dependence is linear, which proves the oxidation of nickel from the liquid phase of the amalgam.

To explain the dependences of $q_{Ni}$ on its concentration in the solution, the assumption was made that supersaturated nickel amalgams are formed. At a certain concentration of nickel in the amalgam, or after a certain holding time following electrolysis, its crystallization begins, which leads to a decrease in the anodic nickel peaks. In this case we have

\[ q = C_t V + q_{\mathrm{cr}}, \tag{1} \]

where $q$ is the total amount of metal on the electrode, g,
$C_t$ is the concentration of dissolved metal in the amalgam at the given time $t$, g/ml,
$V$ is the volume of mercury on the electrode, ml,
$q_{\mathrm{cr}}$ is the amount of crystalline phase in the amalgam at the given time $t$.

The expression for the crystallization rate is:

\[ W = \frac{dq_{\mathrm{cr}}}{dt} = K(C_t - C_{\mathrm{sat}}), \tag{2} \]

where $C_{\mathrm{sat}}$ is the concentration of the saturated amalgam. From (1) we express $C_t$:

\[ C_t = \frac{q - q_{\mathrm{cr}}}{V}. \tag{3} \]

Substituting (3) into (2) and taking into account that $C_{\mathrm{sat}}V = q_{\mathrm{sat}}$, we obtain the following expression:

\[ \frac{K}{V}(q_{\infty} - q_{\mathrm{cr}}) = \frac{dq_{\mathrm{cr}}}{dt}, \tag{4} \]

where $q_{\infty} = (q - q_{\mathrm{sat}})$ is the maximum amount of metal that can exist in the form of a solid phase.

Separating the variables and integrating over the limits from 0 to $t$:

\[ -\frac{K}{V}t = \ln\left(1 - \frac{q_{\mathrm{cr}}}{q_{\infty}}\right) \tag{5} \]

or

\[ q_{\mathrm{cr}} = q_{\infty}\left(1 - e^{-\frac{K}{V}t}\right). \tag{6} \]

Expression (5), plotted in the coordinates $\ln(1 - q_{\mathrm{cr}}/q_{\infty}) - t$, represents a straight line passing through the origin, with a slope equal to $-K/V$. To verify the theoretical relationships described above, studies were carried out measuring the anodic current and the amount of nickel after electrochemical concentration for different crystallization times (Fig. 2). Dissolution and crystallization of nickel were carried out in a narrow capillary, which eliminated errors due to loss or additional deposition of nickel.

Curves $I$, $II$, $III$ in Fig. 2 at large $t$ reach a limiting value, which characterizes the amount of nickel necessary for satu-

of this volume of mercury. The solubility of nickel in mercury calculated from these data is \(2.94 \cdot 10^{-5}\%\) by weight, which agrees with the data of [1].

Fig. 2. Dependence of the amount of oxidized nickel on the crystallization time. Initial amount of nickel on the electrode: curve I — \(2.8 \times 10^{-6}\) g; curve II — \(1.75 \cdot 10^{-9}\) g; curve III — \(0.9 \cdot 10^{-9}\) g; \(g_{\mathrm{Ng}} = 1.87 \cdot 10^{-3}\) g

Fig. 3. Dependence of

\[ -\lg\left(1-\frac{q_{\mathrm{cr}}}{q_{\infty}}\right) \]

on the crystallization time of nickel from the amalgam

In logarithmic coordinates all three curves (Fig. 2) fall on one straight line (Fig. 3) with a slope equal to

\[ \frac{K}{2.3\,V}=0.021;\qquad K=1.12\cdot 10^{-7}\ \mathrm{cm}^{3}\cdot \mathrm{s}^{-1}. \]

Consequently, the dissolution of nickel occurs from a supersaturated amalgam, which is unstable in time and tends toward equilibrium.

Conclusions

The phase nature of nickel in mercury has been studied; it has been established that nickel dissolves from the liquid phase of the amalgam. An equation describing the process of crystallization of the metal from a supersaturated amalgam has been derived, and the applicability of this equation to crystallization processes occurring in the nickel–mercury system has been shown. The solubility of nickel in mercury has been determined.

LITERATURE

1. I. E. Krasnova, A. I. Zebreva. Elektrokhimiya, 2, No. 2, 247–251 (1966).
2. W. Kemula, Z. Galus. Bull. Acad. Polon. Sci., 7, 553 (1959).
3. I. E. Krasnova, A. I. Zebreva, M. T. Kozlovskii. Doklady AN SSSR, 156, No. 2, 415–417 (1964).
4. G. Tammann, K. Kollman. Z. anorg. Chem., 160, 242 (1927).
5. G. Libl. Über die Amalgammetallurgische Herstellung von Reinstem Indium u. Antimon, München (1956).
6. G. Jangg, H. Palman. Z. Metallkunde, 54, 364 (1963).
7. N. M. Irvin, A. I. Russel. J. Chem. Soc. (L.), 891 (1932).
8. W. I. Van Heteren. Z. anorg. Chem., 24, 129 (1904).
9. E. Palmoer. Z. Electrochem., 38, 70 (1932).
10. V. I. Kuleshov. Dissertation. Tomsk Polytechnic Institute, Tomsk (1969).
11. V. F. Ivanov, Z. A. Iofa. Doklady AN SSSR, 137, 1149 (1961).
12. V. F. Ivanov, Z. A. Iofa. Doklady AN SSSR, 140, 1369 (1962).