Dependence of the Ammonium Amalgam Effect on the Electrolysis Potential

A. V. Konkova, A. G. Stromberg

In considering the question of the influence of the electrolysis potential on the ammonium amalgam effect (AAE), it is first of all necessary to note certain features that occur in the presence of this effect.

As is known, when electrolysis is carried out in the absence of AAE, the ratio of the surfaces of the cathode and anode is selected in such a way that the current density (and, correspondingly, the polarization) of the anode is considerably less than the current density of the cathode. In this case it may be assumed that the anode is practically not polarized and that its potential remains constant. The magnitude of the voltage drop in the solution \((iR)\) is so small that it may be neglected. Then, instead of the equation

\[ E=\varphi_a-\varphi_k+iR \tag{1} \]

one may write:

\[ \Delta E=-\Delta\varphi_k \tag{2} \]

\((E\) is the voltage applied to the cell; \(i\) is the electrolysis current; \(R\) is the resistance of the solution; \(\varphi_a,\ \varphi_k\) are, respectively, the potentials of the anode and cathode).

Under AAE conditions the electrolysis current increases, in comparison with the current without AAE, by 2–3 orders of magnitude (because of the rather high concentration of discharging ammonium ions and the increase in the cathode surface); at the same time the ohmic voltage drop in the solution reaches a considerable value, and equality (2) is not obeyed. The potential of the anode (sat. cal. el.) under AAE conditions remains constant, which is ensured by its large surface of about \(12.5\ \text{cm}^2\).

Thus, if in APN practice the term “electrolysis potential” is usually understood to mean the voltage applied to the cell (and equal to the cathode potential), which is set and maintained constant during preliminary electrolysis, then under AAE conditions the electrolysis potential is understood as the value of the cathode potential relative to the second auxiliary reference electrode.

It is necessary to note that the magnitude of the cathode potential is determined in a definite way by the concentration of ammonium ions in the solution. In the general case, for the voltage applied to the cell we have expression (1), or

\[ -\varphi_k=E-\varphi_a-iR. \tag{3} \]

The discharge of ammonium on mercury is a reversible process; however, at small overvoltages for the ammonium current (considerably less than the limiting-current value), the expression

\[ i = K_7 C_n e^{-\frac{\alpha}{b'}\varphi_k}, \tag{4} \]

may be taken as valid, where \(K_7\) is the rate constant of the electrolytic reaction at \(\varphi_k = 0\); \(C_n\) is the concentration of ammonium ions in the solution; \(\alpha\) is the transfer coefficient; \(b' = \dfrac{RT}{F}\) (\(F\) is the Faraday number).

Substituting (4) into (3), we obtain

\[ (\varphi_k + E - \varphi_a)e^{\frac{\alpha}{b'}\varphi_k} = R K_7 C_n . \tag{5} \]

From this equation we see that, at constant \(E\) and \(\varphi_a\), the concentration of ammonium ions has a complex effect on the value of \(\varphi_k\), and this effect is intensified as the absolute value of \(\varphi_k\) decreases, i.e., as the voltage decreases.

Indeed, at an external voltage \(E = 2.5\) V, the cathode potential \((\varphi_k)\), measured with a Luggin capillary relative to the second auxiliary electrode (s.c.e.), in solutions with ammonium-ion concentrations of 0.1, 0.2, and 1.0 g-ion/l was found to be, respectively, \(-1.96\), \(-1.94\), and \(-1.88\) V¹. For the same ammonium-ion concentrations at a voltage of 4.0 V, the cathode-potential values are, respectively, \(-2.00\), \(-1.99\), and \(-1.97\) V. From comparison of these experimental data it can be seen that, at identical ammonium-ion concentrations in the solution, a larger value of the external voltage corresponds to more negative values of the cathode potential; moreover, with a change in concentration at the higher voltage, a weaker change in the cathode potential is observed. These experimental facts are qualitatively well explained by formula (5). With \(C_n\) constant in the right-hand side of equation (5), and with an increase in \(E\) in the left-hand side of the equation, in order to preserve equality the value of \(\varphi_k\) must become less positive (more negative).

Under the conditions of the AAE, the cathodic polarization of the mercury electrode is limited to a certain extent by the discharge of the background cations \((\mathrm{NH}_4^+)\), which, as a rule, are present in the solution in large amounts. Experiments carried out by us to clarify the relationship between the external voltage \(E\) and the cathode potential show that, in the voltage interval from 1 to \(\sim 2\) volts, when the ammonium ion is not yet discharged, there is direct proportionality between \(E\) and \(\varphi_k\). With a further increase in voltage, when the cathode potential becomes more negative than the equilibrium potential of the ammonium ion in the solution of the given concentration, the process of reduction of \(\mathrm{NH}_4^+\) ions proceeds on the mercury drop, and the value of the current \(i\) increases. In this case, an increase in the voltage now shifts the cathode potential only slightly toward negative values, causing mainly an increase in the ohmic voltage drop in the solution.

It should also be noted that the cathode potential during electrolysis under AAE conditions shifts somewhat in the positive direction.

When electrolysis is carried out under AAE conditions, the surface of the mercury drop \((S)\) increases with time; the current density at the cathode

in this case decreases (at \(i \simeq \mathrm{const}\)). In turn, the decrease in current density causes a decrease in the cathode overvoltage \((\eta)\), which is related to the current density by the equation

\[ \eta = \frac{RT}{azF}\ln \frac{i}{S}. \tag{6} \]

Fig. 1. Dependence of the cathode potential on the electrolysis time in solutions of \(\mathrm{NH_4Cl}\) of different concentrations. Curve 1—\(C_{\mathrm{NH_4^+}} = 0.01\ \mathrm{N}\); curve 2—\(C_{\mathrm{NH_4^+}} = 0.1\ \mathrm{N}\); curve 3—\(C_{\mathrm{NH_4^+}} = 1.0\ \mathrm{N}\).

Fig. 2. Dependence of the drop radius on the electrolysis time at different cathode potentials: 1—\(\varphi_{\mathrm{K}} = -1.99\ \mathrm{V}\); 2—\(\varphi_{\mathrm{K}} = -1.97\ \mathrm{V}\); 3—\(\varphi_{\mathrm{K}} = -1.95\ \mathrm{V}\); 4—\(\varphi_{\mathrm{K}} = -1.93\ \mathrm{V}\).

With an increase in the concentration of ammonium ions in the solution, the rate of growth of the ammonium amalgam drop increases [1]. Accordingly, the rate of change of the current density and of the cathode potential magnitude also increases.

also must increase with increasing \(C_{\mathrm{NH}_4}\). This proposition is confirmed experimentally. In Fig. 1, curves 1, 2, and 3 present the results of measurements of the cathode potential at various times during electrolysis carried out in 0.01, 0.1, and 1.0 N solutions of \(\mathrm{NH_4Cl}\) at an external voltage of 4.0 volts. Comparison of these curves readily shows that the greatest change in the cathode potential with time, and at the greatest rate, occurs in the most concentrated solution.

The influence of the electrolysis potential on the growth kinetics of an ammonium amalgam drop is presented in Fig. 2. The experiments studying this influence were carried out under the following conditions:

\[ C_{\mathrm{NH}_4^{+}} = 1.0\ \text{g-ion/l};\quad V = 10\ \text{ml};\quad \mathrm{pH} = 9,\quad t = 16^\circ\mathrm{C}. \]

It is seen from Fig. 2 that, as the value of the electrolysis potential increases, the growth rate of the ammonium amalgam drop increases. However, as the experiments showed, only the time required for the ammonium amalgam drop to reach its limiting radius depends on the electrolysis potential, and not the magnitude of this radius itself.

1. The potential difference between the cathode and the second auxiliary electrode was measured with a PPTV-1 potentiometer with an accuracy of \(\pm 0.005\) V. ↩