DEPENDENCE OF THE AMMONIUM AMALGAM EFFECT ON THE NATURE OF THE ANION AND THE PRESENCE OF CERTAIN CATIONS

A. V. KONKOVA

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

The ammonium amalgam effect (AAE) in amalgam polarography with accumulation (APA) is one of the promising methods for increasing the sensitivity of the determination of certain elements. The essence of the AAE is that, during electrolysis of solutions containing the ammonium ion, an unstable ammonium amalgam is formed on a stationary mercury drop at a sufficiently negative potential; upon decomposition according to the equation:

\[ \mathrm{NH_4(Hg) \to Hg + NH_3 + \frac{1}{2}H_2} \]

it converts the mercury drop into a drop of ammonium-amalgam foam. In this process the surface of the electrode increases by a factor of 10–20, as a result of which the rate of accumulation in mercury of the element being determined, whose ions are discharged at the electrode simultaneously with ammonium ions during the preliminary electrolysis, increases. When the anodic polarogram is recorded, the volume of the drop decreases to its initial size as soon as the cathode potential becomes more positive than the equilibrium potential of ammonium (because of decomposition of the ammonium amalgam). Thus, in an experiment with AAE the concentration of the element being determined in the mercury drop and the height of its anodic peak increase by a factor of 10–20 in comparison with an experiment without AAE under otherwise identical experimental conditions. Previous investigations of AAE in nonaqueous and aqueous solutions are described in works [1—4].

Fig. 1. Dependence of the radius of an ammonium amalgam drop on electrolysis time in 1 N solutions of ammonium salts with different anions. Experimental conditions: solution volume 10 ml; applied voltage 4 volts; temperature 16°C; pH — 5—6. 1 — \(\mathrm{NH_4F}\); 2 — \(\mathrm{NH_4Br}\); 3 — \(\mathrm{NH_4Cl}\); 4 — \((\mathrm{NH_4})_2\mathrm{SO_4}\); 5 — \((\mathrm{NH_4})_3\mathrm{PO_4}\); 6 — \(\mathrm{NH_4}\) tartrate and \(\mathrm{NH_4}\) citrate; 7 — \(\mathrm{NH_4NO_3}\) and \((\mathrm{NH_4})_2\mathrm{CrO_4}\).

Aqueous solutions of ammonium salts mixed with ammonium hydroxide, or without it, are the most widely used supporting media both in classical polarography and in the APA method. The nature of the supporting electrolyte, as is known, can have a substantial influence on electrochemical—

... processes of reduction of ions and oxidation of metals. This influence is associated with the effect of specific adsorption of background ions, the structure of the double layer, and complex formation on the rate of electrode processes.

The aim of the work is to investigate the influence of the nature of the background anion and the presence of certain cations on the kinetics of growth of the ammonium amalgam foam droplet and on the limiting radius of this droplet.

As the solutions studied, solutions of ammonium salts that are most often used in polarographic practice were taken:

\[ \mathrm{NH_4Cl,\ (NH_4)_2SO_4,\ NH_4F,\ NH_4Br,\ NH_4NO_3,\ (NH_4)_3PO_4,\ (NH_4)_2CrO_4,} \]

\[ \mathrm{(NH_4)_2C_4H_4O_6\ and\ (NH_4)_3C_6H_5O_7.} \]

The experiments were carried out in an electrolytic cell made of optical glass with plane-parallel walls. The cell was closed with a fluoroplastic lid with openings for the working electrode, the electrolytic key, and a tube for passing nitrogen through the electrolyte.

The cathode was a mercury drop on a platinum–silver contact; the anode was a saturated calomel electrode. The conditions under which the experiments were carried out are given in the figure captions.

The increase in the radius of the ammonium amalgam droplet during electrolysis was recorded with a MIN-2 microscope (×37, scale division value 0.05 mm).

The results of the experiments are presented in Fig. 1 by curves 1–7. From Fig. 1 it can be seen that the kinetics of growth of the ammonium amalgam droplet and its limiting radius depend to a considerable extent on the nature of the anion of the ammonium salt; moreover, there is a definite regularity in this dependence.

In terms of their influence on the kinetics of growth of the ammonium amalgam droplet and on its limiting radius, all the anions studied may be divided into three groups:

1) $\mathrm{F^-}$, $\mathrm{Br^-}$, $\mathrm{Cl^-}$ — anions capable of oxidation at the anode;

2) $\mathrm{SO_4^{2-}}$, $\mathrm{PO_4^{3-}}$, $\mathrm{C_4H_4O_6^{2-}}$, and $\mathrm{C_6H_5O_7^{3-}}$ — anions that give neither anodic nor cathodic waves;

3) $\mathrm{NO_3^-}$, $\mathrm{CrO_4^{2-}}$ — anions that are strong oxidizing agents and are capable of reduction at the cathode.

Fig. 2. Dependence of the radius of an ammonium amalgam droplet on electrolysis time in solutions of mixtures of chlorides of ammonium and alkali or alkaline-earth metals in a ratio of 9:1. Experimental conditions: solution volume 10 ml; voltage 4 V; pH 5–6; temperature 24°C. 1 — $\mathrm{NH_4Cl}$; 2 — $\mathrm{NH_4Cl + NaCl}$; 3 — $\mathrm{NH_4Cl + KCl}$; 4 — $\mathrm{NH_4Cl + LiCl}$; 5 — $\mathrm{NH_4Cl + SrCl_2}$; 6 — $\mathrm{NH_4Cl + BaCl_2}$; 7 — $\mathrm{NH_4Cl + CaCl_2}$.

The first group has almost no effect on the ammonium amalgam effect; the second group has a noticeable effect on the AAE, and the third group completely suppresses the AAE.

The suppression of the AAE in the presence of strong oxidizing agents (ions $\mathrm{NO_3^-}$ and $\mathrm{CrO_4^{2-}}$) should evidently be explained by the fact that the discharged radical $\mathrm{NH_4^0}$, which is a strong reducing agent¹, reacts with these

oxidizing agents, with formation of reduction products (\(\mathrm{NO_2}\), \(\mathrm{Cr^{3+}}\)), ammonia, and water.

The appreciable influence of oxygen-containing anions of the second group on the EAA also, apparently, should be explained by their oxidizing action.

The dependence of the EAA on the presence of cations was studied using certain alkali and alkaline-earth metals as examples. The cations investigated were: \(\mathrm{Li^+}\), \(\mathrm{K^+}\), \(\mathrm{Na^+}\), \(\mathrm{Sr^{++}}\), \(\mathrm{Ba^{++}}\), \(\mathrm{Ca^+}\). The kinetics of drop growth was studied in solutions of the following composition: \(0.9N\ \mathrm{NH_4Cl} + 0.1\ N\ \mathrm{MeCl}_n\). The experimental results are presented in Fig. 2. In this figure, curve 1 characterizes the kinetics of drop growth in a pure \(\mathrm{NH_4Cl}\) solution (\(0.9N\)), while curves 2–7 characterize the kinetics of drop growth in solutions of composition \(C_{\mathrm{NH_4^+}} = 9N;\ C_{\mathrm{Me}^{n+}} = 1N\). From Fig. 2 it is seen that the presence of cations slows the rate of drop growth, and in the presence of the calcium ion the drop does not grow at all.

According to their degree of influence on the kinetics of drop growth, the cations studied may be arranged in the following series:

\[ \mathrm{Na < K < Li < Sr < Ba < Ca.} \]

For the standard potentials of these metals we have the series:

\[ \mathrm{Na > Ca > Sr > Ba > K > Li.} \]

And for the half-wave potentials we have the series:

\[ \mathrm{Ba > Na > K > Sr > Ca > Li.} \]

The absence of parallelism in these three series indicates that the influence of the cations cannot be attributed to their simultaneous discharge with ammonium ions. Possibly, the dependence of the EAA on the presence of cations should be explained by the influence of the nature of the metal amalgam on the catalytic rate of decomposition of the ammonium radical into ammonia and hydrogen (on the electrode surface or inside the amalgam).

Conclusions

1. The dependence of the kinetics of growth of an ammonium amalgam drop and of its limiting radius on the nature of the anion of the ammonium salt and on the presence in the ammonium-salt solution of ions of alkali and alkaline-earth metals has been investigated.

2. It has been shown that the nature of the anion of the ammonium salt and the presence in solution of cations of alkali and especially alkaline-earth metals have a substantial influence on the kinetics of drop growth and on its limiting radius.

3. It has been shown that, in order to obtain the EAA in aqueous solutions, ammonium salts whose anions are strong oxidizing agents cannot be used as the supporting medium.

4. It has been shown that, despite the negative influence of cations of alkali and alkaline-earth metals on the kinetics of growth of the ammonium amalgam drop and on its limiting radius, the use of the EAA to increase the sensitivity of determining elements in solutions of mixtures of ammonium salts and salts of these metals (except calcium) is possible.

I express my deep gratitude to Professor A. G. Stromberg for valuable guidance in carrying out this work.

LITERATURE

1. Yu. A. Karbainov, A. G. Stromberg. ZhAKh, 20, 769, 1965.
2. A. G. Stromberg, A. V. Konkova. Electrochemistry (in press).
3. A. V. Konkova, A. G. Stromberg. Izv. TPI, vol. 164, 147, 1967.
4. A. V. Konkova. ZhAKh, vol. 23, issue 3, 451, 1968.

1. The reducing properties of the ammonium radical are due to the fact that it is a donor of atomic hydrogen. ↩