EFFECT OF TEMPERATURE ON ANODIC PEAKS IN THE APN METHOD DEPENDING ON THE NATURE OF THE SOLVENT

S. N. Karbainova, Yu. A. Karbainov

(Presented by the scientific seminar of the Department of Physical Chemistry)

The nature of the solvent has a substantial effect on the kinetics of electrode processes; therefore, finding general regularities in the influence of the nature of the solvent on the rate of electrode reactions is of great practical and theoretical interest.

The aim of the present work is to study the influence of the composition of a mixed solvent on the main characteristics of the anodic peak of lead at various temperatures of the analyzed solution on a stationary mercury drop electrode (SMDE).

As the mixed solvent, the water—ethanol system was chosen. The viscosity dependence of this system on the concentration of ethyl alcohol passes through a maximum [1]. The investigations were carried out using 20, 40, 60, 80, and 96% aqueous–ethanol mixtures. As the indifferent electrolyte, \(0.1N\) \(NH_4NO_3\) was used. Experimental conditions: concentration of lead ions in solution — \(C_0 = 2.8 \cdot 10^{-5}\) g-ion/l; radius of the mercury drop — \(r = 0.04\) cm; rate of potential change \(\omega = 6.65 \cdot 10^{-3}\) V/sec; electrolysis time — \(\tau = 2\) min; volume of the analyzed solution — \(v = 5\) ml; temperature variation interval — \(20—70^\circ C\). The reference electrode was bottom mercury.

The dependence of the height of the anodic peak of lead in 20, 40, 60, and 80% aqueous–ethanol mixtures on the concentration of its ions in solution at \(t = 22^\circ C\) was studied. According to these data, the sensitivity of lead determination decreases as the fraction of alcohol in the mixture increases in the interval 20–40%; in the interval 40–80% it increases, while the sensitivity of lead determination in 20 and 80% ethanol solutions is approximately the same. The change in the magnitude of the current of the anodic peak of lead corresponds quite well to the course of the viscosity–composition dependence curves for the aqueous–alcohol mixture: with increasing viscosity the sensitivity decreases, and vice versa. Consequently, possible changes in other properties of the solution with increasing ethanol concentration—namely, a decrease in dielectric permittivity [2]; a shift of the solution pH into the acidic region [3]; possible resolvation of lead ions—do not exert a substantial influence on the rate of the electrode reaction.

The potential of the anodic peak of lead, with increasing ethanol concentration, shifts somewhat into the positive region of potential change (by 20–30 mV). This experimental fact makes it possible to make the following assumptions.

1. If compounds of the lead ion with alcohol are formed, then they are unlikely to participate in the depolarization process. 2. Possible adsorption

organic molecules on the electrode, especially at high concentrations of the organic component, has practically no effect on the electrode process.

It is known [4] that the effect of increasing temperature as a means of increasing the sensitivity of the APN method manifests itself differently depending on the nature of the solvent. Therefore, it was of certain interest to study the effect of temperature on the characteristics of the anodic peaks of lead in water–ethanol mixtures. The experimental procedure is described in [5]. The experimental dependences of the anodic-peak current in the coordinates \(\lg I\)—\(1/T\) satisfactorily fall on straight lines, which makes it possible (in accordance with the Arrhenius equation) to find the experimental value of the activation energy of the overall electrolysis process in water–ethanol mixtures on a stationary mercury drop electrode, \(E_e\) (Table 1).

The value \(E_e\) in this case cannot be a characteristic of the electrode reduction process proper, since in the APN method the pre-electrolysis stage is carried out at the limiting-current potential. It may be assumed that the experimental values found for \(E_e\) indicate the nature of the temperature dependence of the diffusion coefficients of lead ions reduced on the stationary mercury drop electrode. The data in Table 1 show that the experimental values of the activation energy are in good agreement with the viscosity–composition curve of the water–ethanol mixture. The greater the viscosity and, consequently, the smaller the diffusion coefficient of ions in the solution, the more strongly its temperature dependence is manifested, the more sharply the anodic-peak current increases with increasing temperature, and the greater the activation energy of the overall electrode process.

Table 1

Values of the activation energy of the overall process of lead electrolysis on a stationary mercury drop electrode in water–ethanol mixtures

Table 2

Increase in the anodic-peak current of lead when the temperature is increased from 20 to 70°C

Table 2 gives numerical values of the quantity \(\mu\), characterizing the increase in the anodic-peak current with increasing temperature in the range 20–70°C.

As can be seen from the experimental data, increasing the temperature in the analysis of some aqueous–organic solvents makes it possible to increase the sensitivity of the determination by approximately an order of magnitude.

REFERENCES

1. Handbook of the Chemist, ed. by B. P. Nikol’skii, 3, 2nd ed., Moscow–Leningrad, 1965.
2. Landolt—Bornstein. Phys.-Chem. Tabellen II, Berlin, 1036, 1923.
3. I. Ya. Putnin, L. K. Lepin. Izvestiya AN LatvSSR, Chemical Series, 3, 270 (1968).
4. S. N. Karbainova. Abstract of Candidate’s Dissertation, Tomsk, 1971.
5. A. G. Stromberg, Yu. A. Karbainov, S. N. Karbainova. “Zavodskaya laboratoriya,” 3, 257 (1970).