USE OF THE TEMPERATURE-KINETIC METHOD FOR STUDYING CERTAIN PHYSICOCHEMICAL ISSUES IN THE METHOD OF AMALGAM POLAROGRAPHY WITH ACCUMULATION

S. N. KARBAINOVA

(Presented by the scientific-methodological seminar of the KhTF)

In work [1], a technique was proposed for carrying out preliminary electrolysis in the method of amalgam polarography with accumulation at temperatures close to the boiling point of the solution, which makes it possible to increase the sensitivity of the method by almost an order of magnitude. In works [2—3], certain theoretical questions concerning the influence of temperature on the height of the anodic peak in the APA method are considered; their experimental verification was carried out using cadmium and antimony against a background of 0.2 N NH₄NO₃ in ethyl alcohol. Therefore it is of definite interest to draw an unambiguous conclusion about the nature of the rate-limiting stage of the process of reduction of ions of the above-mentioned elements in this nonaqueous supporting electrolyte.

In the present work we applied the temperature-kinetic method, the essence of which consists in studying the dependence of the current density on the polarization potential at different temperatures [4]. In this case it is assumed that the activation energy of the electrode process does not depend on temperature. The values of the real activation energy of the process of reduction of metal ions in the supporting electrolyte under study can be found using the following equation [4]:

\[ \lg i = \mathrm{const} - \frac{E}{2{,}3R}\cdot \frac{1}{T}; \]

where \(i\) is the current density, A; \(E\) is the real activation energy of the electrode process, kcal/g-ion.

The magnitude of the activation energy makes it possible to judge the nature of the rate-limiting stage of the electrode process under study [5, 8]. Thus, for example, when the electrode process proceeds reversibly, the activation energy is small, amounting to 2—3 kcal/g-ion; moreover, its value is almost independent of the polarization potential. Conversely, high values of the activation energy (above 10 kcal/g-ion) indicate the chemical nature of polarization. In this case, as the polarization potential increases, the value of the activation energy decreases noticeably [5]. Investigations of the dependence of the activation energy on the stirring rate provide additional information on the nature of the rate-limiting stage of the electrode process.

The investigations were carried out on a Hungarian polarograph of the OH-101 type. A sealed glass cell equipped with a reflux condenser with a water seal was used in the work. The reference electrode was mercury at the bottom of the electrolyzer. The mercury drop serving as the cathode was grown electrolytically on a platinum contact from

solution of mercurous nitrate. The drop radius was 0.04 cm. Oxygen was removed from the solution with purified nitrogen.

The experiments were carried out without stirring the solution during the preliminary electrolysis and with stirring by a stream of nitrogen. The stirring rate was kept strictly constant. UT ultrathermostats (Poland) were used in the work. Temperature fluctuations did not exceed $\pm 0.5^\circ$C. The temperature-variation interval in our studies was 10–60°C. The experiments were repeated at least 3 times. The polarization potentials at which electrolysis was carried out were as follows: for cadmium, $-0.85$, $-1.0$, $-1.3$ V; for antimony: $-0.2$, $-0.3$, $-0.5$ V (Fig. 1).

Fig. 1. Dependence of the height of the anodic current on the electrolysis potential against a background of 0.2 N $NH_4NO_3$ in ethanol: 1—cadmium; 2—antimony.

In determining the real activation energy of the reduction process, electrolysis was carried out at different temperatures of the solution being analyzed. After electrolysis was completed, the nitrogen stream was stopped, the potential was lowered to a value more negative than the potential at which dissolution of the amalgam begins by 30 mV [9], and the cell was quickly transferred to another thermostat with a constant temperature equal to 20°C and kept there for 5 min. Control experiments established that no additional accumulation of metal occurred under these conditions. In addition, a holding time of 5 min is quite sufficient for the temperature of the solution in the cell to decrease to 20°C. After this, the anodic peak was recorded. The experimental conditions were as follows: ion concentration in the stirred solution—$6 \cdot 10^{-6}$ g-ion/l; metal-ion concentration in the absence of stirring—$2 \cdot 10^{-5}$ g-ion/l; volume of the solution analyzed—5 ml; electrolysis time—3 min; rate of potential change—$1.5 \cdot 10^{-2}$ V/s.

From the experimental data obtained, a plot was constructed in the coordinates

$\lg i = f\left(\frac{1}{T}\right),$

from which the values of the real activation energies were calculated. We studied the dependence of the magnitude of the activation energy of the reduction process of cadmium and antimony ions on the electrolysis potential. The results are presented in Table 1. From the data in the table

Table 1

It follows that the values of the activation energy of the reduction process for cadmium and antimony when the solution is stirred by a stream of nitrogen are lower than in the absence of stirring.

On the basis of the experimental data obtained on the values of the activation energy of the reduction process for cadmium and antimony at limiting-current potentials, and also on the dependence of the activation energies on the polarization potential and the influence of stirring on its value, a conclusion was drawn about the nature of the limiting stage of the process of reduction of metal ions in an alcoholic medium. The process of reduction of cadmium and antimony when the solution is stirred by a stream of nitrogen is reversible in character. The same may be said of the reduction of these ions in the absence of stirring of the solution by a stream of nitrogen.

Conclusions

The mechanism of reduction of cadmium and antimony ions in a background electrolyte of 0.2 N \(\mathrm{NH_4NO_3}\) in ethyl alcohol has been studied. It has been shown that the process of reduction of cadmium and antimony, both when the solution is stirred by a stream of nitrogen and in the absence of stirring, is reversible in character in the alcoholic medium studied.

Literature

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