EFFECT OF CERTAIN FATTY ACIDS ON ELECTRODE PROCESSES AT A CADMIUM AMALGAM DROPPING ELECTRODE

V. E. GORODOVYKH, A. V. SEMENOVA

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

In the work of A. G. Stromberg [1] it was theoretically shown that, in the course of a reversible electrode process at an amalgam dropping electrode, a single anodic-cathodic wave should be obtained, characterized by the equation:

\[ \varphi = \varphi_{1/2\,\mathrm{rev}} - b \lg \frac{I_a + i}{I_k - i}. \]

In the case of an irreversible electrode process, a split anodic-cathodic wave should be obtained, characterized by the equations:

\[ \Delta \varphi_{1/2k} = \varphi_{1/2\,\mathrm{rev}} - \varphi_{1/2k} = \frac{b}{\alpha}\lg \frac{I_k}{i_0} + b \lg \frac{I_a}{I_k}; \]

\[ \Delta \varphi_{1/2a} = \varphi_{1/2a} - \varphi_{1/2\,\mathrm{rev}} = \frac{b}{\beta}\lg \frac{I_a}{i_0} + b \lg \frac{I_k}{I_a}. \]

The smaller the exchange current, the greater the difference between the potentials of the anodic and cathodic half-waves.

In works [2, 3] it was shown that the presence of kinetic hindrances cannot cause changes in the reversibility of the process, i.e., splitting of the anodic-cathodic waves. The presence of kinetic hindrances may cause a manyfold decrease in limiting currents.

As shown in works [4, 5], adsorption of surface-active substances can cause a change in the reversibility of the electrode process. Thus, the reversible anodic-cathodic wave of cadmium in the presence of gelatin and camphor splits, which, in the opinion of the authors of the works, indicates the presence of hindrances at the transition stage associated with a change in the adsorption \(\Psi'\)-potential.

With the aim of further practical verification of this theory and obtaining experimental material, a study was carried out of the effect of certain organic acids on the anodic-cathodic waves of cadmium at an amalgam dropping electrode.

Apparatus and method of work

The study was carried out on an OH-101 type polarograph. To obtain the amalgam and record the anodic-cathodic waves, the setup described in [6] was used. Cadmium amalgam of concentration

$2 \cdot 10^{-3}$ g-atom/liter was prepared by electrolysis with a mercury cathode and a soluble cadmium anode from a CdSO$_4$ solution of concentration 100 g/liter. To prevent oxidation of the amalgam, nitrogen preliminarily purified of oxygen was continuously passed through the cell. For preparing the solutions, water was used after double distillation in quartz apparatus in the presence of a 3% KMnO$_4$ solution. Chemically pure CdSO$_4$ salt was additionally recrystallized and calcined.

To free the solution of oxygen, it was purged with nitrogen purified of oxygen. Electrode potentials were measured relative to a mercury–sulfate electrode. The equilibrium potentials of the amalgam were determined on a stationary amalgam with a PPTV-01 potentiometer.

The following organic acids were chosen as the objects of study: enanthic, caprylic, and benzoic. The studies were carried out in a solution of 0.5 M H$_2$SO$_4$ + $2 \cdot 10^{-3}$ mol/l CdSO$_4$. In view of the low solubility of the organic acids in water, their concentration in the solution was taken in fractions of saturation ($d$ sat.).

Results of the experiments and their discussion

From Fig. 1 and Fig. 2 it is seen that the character of the effect of benzoic, enanthic, and caprylic acids on the anodic–cathodic waves of cadmium is different. Thus, with an increase in the concentration of benzoic acid, the height of the anodic and cathodic waves does not change; the half-wave potentials of the anodic and cathodic waves shift, respectively, in the positive and negative directions. An increase in the concentration

Fig. 1. Effect of organic acids on the height of the anodic and cathodic waves of cadmium:
1—1′ — benzoic; 2—2′ — enanthic;
3—3′ — caprylic.
Conventional symbols:
o — cathodic wave; Δ — anodic wave

Fig. 2. Dependence of the potential of the anodic and cathodic half-waves of cadmium on the concentration of organic acids in the solution: 1—1′ — caprylic;
2—2′ — enanthic; 3—3′ — benzoic.
Conventional symbols:
o — cathodic wave; Δ — anodic wave

of enanthic and caprylic acids leads to a decrease in the height of the anodic and cathodic waves of cadmium and to a shift of the half-wave potential. It is characteristic that these changes are manifested to a greater degree for caprylic acid, with the longer hydrocarbon chain.

Electrocapillary measurements on mercury, carried out at various concentrations of the named organic acids by the drop-counting method, showed that all of them lower the surface tension in the region of the potentials of cadmium discharge–ionization on mercury. Moreover, the degree of lowering of the surface tension increases in the series: benzoic—

...ic < enanthic < caprylic at identical concentrations of acids in fractions of saturation.

The nature of the change in the anodic-cathodic cadmium wave in the presence of various concentrations of caprylic acid is presented in Fig. 3.

Fig. 3. Polarization curves on the cadmium amalgam dropping electrode in a solution of composition \(2\cdot10^{-3}\) M \(CdSO_4\); \(0.5\) M \(H_2SO_4+x\)—caprylic acid.
\(1—0.00;\quad 2—\dfrac{1}{20};\quad 3—\dfrac{1}{10};\quad 4—\dfrac{1}{5};\quad 5—\dfrac{2}{5};\)
\(6—1.00\)—fractions of saturation, \(7\)—residual-current curve.

In the region of adsorption of caprylic acid, as its concentration in the solution is increased, a decrease in the limiting currents and a broadening of the anodic-cathodic wave are observed.

Upon reaching the desorption potential (about \(1.6\) V), a second cathodic wave is observed, whose limiting current approaches the limiting current of cadmium in the absence of organic acids. In a solution saturated with caprylic acid (curve 6), the cathodic wave is strongly shifted in the negative direction and practically merges with the hydrogen-evolution wave.

It is characteristic that the degree of decrease in height and the shift of the half-wave potential for the cathodic waves are greater than for the anodic waves.

On the basis of the theoretical conclusions and the experimental data obtained by us, the following assumption can be made about the nature of the influence of the indicated acids on the processes of discharge-ionization of cadmium.

In the case of benzoic acid, its adsorption evidently affects only the transfer reaction. Adsorption of enanthic and caprylic acids causes difficulty both in the transfer reaction and in the stage of delivery-removal of the depolarizer.

Conclusions

The influence of adsorption of benzoic, enanthic, and caprylic acids on the anodic-cathodic waves of cadmium in a solution of \(0.5\) M \(H_2SO_4+\) \(2\cdot10^{-3}\) M \(CdSO_4\) was investigated.

It was shown that, upon adsorption of benzoic acid, splitting of the anodic-cathodic wave occurs without a decrease in its height. Upon ad-

upon sorption of enanthic and caprylic acids, splitting of the anodic–cathodic wave and a decrease in its height occur.

Assumptions have been put forward concerning the nature of the influence of these acids on the stages of discharge–ionization of cadmium.

REFERENCES

1. A. G. Stromberg. Journal of Physical Chemistry, 27, 1287, 1953.
2. A. G. Stromberg. Journal of Physical Chemistry, 29, 409, 1955.
3. A. G. Stromberg. Journal of Physical Chemistry, 29, 2142, 1955.
4. A. G. Stromberg and M. S. Guterman. Journal of Physical Chemistry, 27, 993, 1953.
5. L. S. Zaiganova and A. G. Stromberg. DAN, 105, 747, 1955.
6. A. G. Stromberg. Journal of Physical Chemistry, 31, 1704, 1957.