As can be seen from the table, the exchange current and the reaction rate constant on the neutral background, which corresponds to the flat orientation of $\beta$-naphthol molecules at the interface /2/, decrease with increasing $\beta$-naphthol concentration within one order of magnitude, whereas on the acidic background (inclined orientation of the adsorbate molecules) they decrease by two and a half orders of magnitude. These data confirm the conclusion drawn above regarding the irreversible character of the discharge–ionization reaction of cadmium on the acidic background and the quasi-reversible character on the neutral background.

Using the values of the degree of coverage of the electrode surface ($\theta$) /2/ found by us from adsorption measurements, dependences of the degree of inhibition of the exchange current $\Delta i_o/i_o$ on $\theta$ were constructed for the neutral and acidic backgrounds. On the neutral background, the degree of inhibition of the reaction at all degrees of coverage is less than $\theta$, and at $\theta = 0.8$–$0.9$ the exchange current reaches an almost constant value (see the table). This indicates that, in the case of flat orientation of the adsorbate molecules, the cadmium discharge–ionization reaction proceeds at an appreciable rate on the occupied surface.

On the acidic background, the dependence $\Delta i_o/i_o$ has an S-shaped form, and at $\theta > 0.5$ the degree of inhibition becomes greater than the degree of coverage, while the exchange current decreases sharply. Such a sharp difference in the dependences of $\Delta i_o/i_o$, in all probability, is explained by the strengthening of the adsorption film of $\beta$-naphthol at the interface as a result of an increase in the fraction of vertically oriented molecules upon transition from the neutral to the acidic background.

Literature

1. B. B. Damaskin. Principles of Modern Methods for Studying Electrode Reactions, Moscow State University, 1965.
2. Yu. N. Oblivantsev, V. E. Gorodovykh, present collection.

STUDY OF THE EFFECT OF BENZYLAMINE ON ELECTRODE PROCESSES INVOLVING CADMIUM, COPPER, ZINC, AND LEAD IONS

V. E. Gorodovykh, L. M. Smolova

The presence in the solution being analyzed of organic surface-active substances (SAS) most often leads to a decrease

nor the peak depth of the element being determined. It has been shown /1/ that this effect is mainly associated with inhibition of the cathodic process at the stage of preliminary concentration of the metal in the region of surfactant adsorption. It follows that substantiated conclusions about the mechanism by which the anodic peak is inhibited by individual surfactants can be made provided the following set of experimental studies is carried out: 1) study of surfactant adsorption on the electrode; 2) study of the effect of surfactants on anodic–cathodic polarographic waves; and 3) investigation of the effect of surfactants on the depth of the anodic peak under the conditions of the stripping-voltammetry method.

This work is a continuation of studies /2/ on the influence of organic surfactants of the aromatic series on the anodic peaks of certain metals. The simplest representative of aromatic amines—benzylamine—was chosen as the object of study.

Experimental procedure

The adsorption of benzylamine on mercury was studied by the method of measuring the interfacial tension. Electrocapillary curves were recorded using a capillary electrometer of Gouy in a 0.1 M sodium sulfate solution with various additions of benzylamine (a total of 9 concentrations in the range from \(2 \cdot 10^{-3}\) M to 0.1 M) at \(20^\circ\)C. A saturated calomel electrode served as the reference electrode.

Anodic–cathodic polarographic waves at an amalgam dropping electrode were recorded by the usual procedure. The metal amalgam was obtained by electrolysis of a solution of the salt of the metal under study with a mercury cathode and a soluble anode in an atmosphere of inert gas (high-purity nitrogen with an oxygen content \(<0.01\%\)).

Anodic peaks at a mercury film electrode were obtained using an LP-7 polarograph against a background of 0.1 M sodium sulfate. The accumulation potential was \(-1.4\) V (relative to the saturated calomel electrode). The metal concentration in solution was \(3 \cdot 10^{-7}\) g/ml, and the preconcentration time was 3 min. Oxygen was removed from the solution by purging with purified nitrogen. All solutions were prepared with tridistillate distilled with \(KMnO_4\) to remove organic impurities. Sodium sulfate was recrystallized twice.

Experimental results and discussion

Electrocapillary measurements showed that, with an increase in the concentration of benzylamine in the solution, the surface tension of mercury decreases appreciably, beginning at a potential of \(-0.1\) V. The decrease in surface tension at the maximum of the electrocapillary curve, at a benzylamine concentration of \(0.1\) M, is \(59\ \mathrm{dyn/cm^2}\). The maximum of the electrocapillary curve shifts into the region of positive potentials, from \(-0.5\) V in the pure solution to \(-0.4\) V in the most concentrated benzylamine solution. Desorption of benzylamine is observed on the positive branch of the electrocapillary curve in the potential interval \(+0.2 \div 0.1\) V. On the basis of the above experimental data it may be considered that benzylamine is a surface-active substance of the cationic type.

According to the electrocapillary measurements, on the basis of the Gibbs equation,

\[ \Gamma = -\frac{1}{RT}\left(\frac{\partial \sigma}{\partial \ln C}\right)_{\varphi}, \]

where \(\Gamma\) is the adsorption, \(\mathrm{mol/cm^2}\); \(\sigma\) is the surface tension, \(\mathrm{dyn/cm^2}\); \(C\) is the concentration of the surfactant in solution, the adsorption values of benzylamine on mercury were calculated with the aid of the “Promin” computer, and adsorption isotherms \((\Gamma-C)\) were constructed at various values of the electrode potential, as well as dependences in \(\Gamma-\varphi\) coordinates at various values of the benzylamine concentration. The maximum adsorption values at benzylamine concentrations in solution of \(9.4 \cdot 10^{-3}\) M; \(1.3 \cdot 10^{-2}\) M; \(6.3 \cdot 10^{-2}\) M and \(8 \cdot 10^{-2}\) M, according to our measurements, are respectively \(4.6 \cdot 10^{-10}\); \(9.6 \cdot 10^{-10}\); \(14.8 \cdot 10^{-10}\) and \(18 \cdot 10^{-10}\ \mathrm{mol/cm^2}\). From the dependences in \(\Gamma-\varphi\) coordinates it follows that benzylamine is noticeably adsorbed on mercury over a wide interval of potentials, beginning at \(-0.1\) V. This makes it possible to investigate its influence on the kinetics of discharge–ionization of a number of elements whose half-wave potentials are located in this region.

The results of the kinetic measurements showed that, in the presence of benzylamine in the solution, a considerable decrease in the cathodic current of cadmium is observed against a background of \(0.1\) M sodium sulfate. The maximum decrease in the limiting current is \(70\%\) rel. The limiting current of the anodic polarographic wave of cadmium in the presence

under the influence of benzylamine decreases to a lesser extent (maximum decrease 30%). At the same time, the slope of the anodic–cathodic cadmium wave does not change, and the half-wave potentials remain practically constant. According to the criteria proposed by A. G. Stromberg /3/, this gives grounds to suppose that benzylamine inhibits the discharge–ionization reaction of cadmium mainly as a result of the appearance of additional hindrances at the stage of supply and removal of the reaction participants to and from the electrode surface.

The results of experiments under the conditions of amalgam polarography with accumulation indicate a strong influence of benzylamine on the height of the anodic peaks of cadmium, copper, and zinc. When the benzylamine content in the solution is \(2 \cdot 10^{-2}\) M, the degree of inhibition of the anodic peaks of zinc, copper, and cadmium is, respectively, 100%, 70%, and 73%. Such a significant decrease in the height of the anodic peaks is associated with inhibition of the cathodic process at the stage of preliminary concentration of the indicated elements under the influence of benzylamine, and with a decrease in the concentration of the metal in the stationary mercury electrode before the beginning of the electro-dissolution stage.

It is interesting to note that the anodic peaks of lead under these same conditions are inhibited to a much lesser extent (only by 20%). This fact agrees with literature data /4/, according to which electrode processes involving lead ions are only weakly affected by individual surfactants.

References

1. T. V. Gomza. Candidate’s dissertation, Tomsk, 1973.

2. Advances in Polarography with Accumulation. Publishing House of Tomsk Polytechnic Institute, Tomsk, 38, 100, 188, 1973.

3. A. G. Stromberg, V. E. Gorodovykh, T. V. Gomza. In: The Double Layer and Adsorption on Solid Electrodes, Tartu, 3, 235, 1972.

4. V. V. Orlenko. Abstract of Candidate’s dissertation. Dnepropetrovsk, 15, 1970.