Influence of a Number of Factors on the Height and Shape of Anodic Peaks in the Extraction APS Method. Dependence of the Anodic Current Constant and Half-Peak Width on the Radius of a Stationary Mercury Drop

Yu. A. Karbainov, A. G. Stromberg

The influence of various factors (solution volume, electrolysis time, and radius of the mercury drop) on the height of the anodic peak was considered theoretically in [1]. The influence of various factors on the shape of anodic peaks is considered in another paper [2]. Our investigations were carried out in order to verify the validity of the conclusions of these works for nonaqueous solutions [3, 4].

Experiments on the influence of various factors on the height and shape of anodic waves in the extraction APS method were studied by us using as examples Cu(2+), Pb(2+), Cd(2+), In(3+), and Tl(+) in various nonaqueous systems: 1% oxine in CHCl$_3$ + 0.2 N NH$_4$NO$_3$ in CH$_3$OH (1:1); $5 \cdot 10^{-3}$% dithizone in CHCl$_3$ + 0.2 N NH$_4$NO$_3$ in CH$_3$OH (1:1); 2% sodium diethyldithiocarbamate in ethyl acetate + 0.2 N NH$_4$NO$_3$ in CH$_3$OH (1:1). Experimental conditions: a stationary mercury drop, obtained electrolytically on a platinum contact, was used as the cathode. The contact diameter was 0.15 mm. The drop size was determined by the weighing method from the formula:

\[ v_k = \frac{m}{d}, \]

where $d$ is the density of mercury (the drop size was calculated as the mean of three measurements). Mercury at the bottom of the electrolyzer served as the anode. The working volume of the solution was $v = 10$ ml. The solution was stirred during electrolysis with a magnetic stirrer. The electrolysis potential in the case of Cu(2+), Pb(2+) was $\varphi_{\text{э}} = -1.0$ V; for Tl and Cd(2+) $\varphi_{\text{э}} = -1.2$ V, for In(3+) $\varphi_{\text{э}} = -2.2$ V. The electrolysis time was $\tau_{\text{э}} = 5$ min. The sensitivity was $1.5 \cdot 10^{-7}$ A/mm. Background solutions were prepared as follows. The extractant and the indifferent electrolyte were prepared separately, after which they were mixed in a 1:1 ratio. The mixture prepared in this way was transferred into a cell for polarography; an addition of a standard solution of one of the elements under study, of definite concentration, was introduced into the mixture, after which the solution was purged with nitrogen for 15–20 min. The remaining experimental conditions will be given in the corresponding tables.

Table 1 presents the values of the anodic current constants and half-peak widths. As can be seen, the ratio of the anodic current constants for elements of the same valence on different backgrounds is inversely propor-

Table 1

nally, the ratio of their half-peak widths. On the other hand, the anodic process of Cu (2+) (first row) is most irreversible in the \(5\cdot10^{-3}\)% dithizone medium in \(\mathrm{CHCl_3}+0.2N\ \mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\) \(\left(\dfrac{K_{2,1}}{K_{2,2}}=1,68\right)\). In the medium of sodium diethyldithiocarbamate in ethyl acetate \(+0.2N\ \mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\), on the contrary, the reversibility of the anodic process of these two elements is approximately the same \(\left(\dfrac{K_{2,1}}{K_{2,2}}=1,09\right)\). The reversibility of the anodic process is also the same in the case of Pb (2+) and Cd (2+). Thus, the ratios \(\dfrac{K_{2,1}}{K_{2,2}}\) or \(\dfrac{\delta_2}{\delta_1}\) can serve, to some extent, as a criterion of the reversibility of the process. The inequality \(\dfrac{K_{2,1}}{K_{2,2}}\ne1\) may also be due to differences in the diffusion coefficients of the corresponding metals in mercury. Assuming that, with other conditions constant, the ratio \(\dfrac{K_{2,1}}{K_{2,2}}\) is proportional to the square root of the ratio of the diffusion coefficients of the metal atoms in mercury, i.e., \(\dfrac{K_{2,1}}{K_{2,2}}\sim\dfrac{D_1^{1/2}}{D_2^{1/2}}\), one can estimate the effect of the diffusion coefficients on the ratio \(\dfrac{K_{2,1}}{K_{2,2}}\). The values of co-

diffusion coefficients for lead, copper, and cadmium, taken by us from [5], are, respectively: \(1.17\cdot 10^{-5}\); \(0.93\cdot 10^{-5}\), and \(1.45\cdot 10^{-5}\ \mathrm{cm^2/sec}\) (at \(25^\circ\mathrm{C}\)). Their ratio is presented in Table 1. Thus, owing to the difference in diffusion coefficients, the anodic-current constants, as is evident from the table, differ by a factor of 1.12 in the case of lead and copper, by a factor of 0.9 for lead and cadmium, and by a factor of 1.25 for cadmium and copper.

Table 2

Table 2 gives other characteristics of the shape of the anodic peaks on different backgrounds. It has already been stated above that, for one and the same element on one background, the ratio \(\left(\dfrac{I}{i_{\mathrm{e}}}\right) \ll \dfrac{C_2}{C_1}\), since \(K_1 \gg K_2\).

The data obtained for Pb (2+), Cd (2+), and Cu (2+) clearly confirm this circumstance. The ratio \(\left(\dfrac{I}{i_{\mathrm{e}}}\right)\) for a given element on one background is constant, but differs on different backgrounds. This circumstance (see the table) can also serve as a criterion for a comparative evaluation of the reversibility of the anodic process for one and the same element on different backgrounds (with constant stirring regime of the solution, electrolysis time, and ion concentration in solution). For example, the anodic process in the case of Pb (2+), as is evident from the table, is more reversible on the background of 2% sodium diethyldithiocarbamate in ethyl acetate + \(0.2N\ \mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\) in comparison with the other backgrounds studied here. Table 3 presents data on the dependence of the anodic-current constant and the half-peak width on the radius of the stationary mercury drop for different elements on the background of 1% oxine in \(\mathrm{CHCl_3} + 0.2N\ \mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\), at a potential-sweep rate of \(1.16\cdot 10^{-2}\ \mathrm{V/sec}\). Experimental conditions:

Lead, conc. \(9\cdot 10^{-6}\ \mathrm{mol/l}\), sensitivity \(4\cdot 10^{-7}\ \mathrm{A/mm}\);

Copper, conc. \(9\cdot 10^{-5}\ \mathrm{mol/l}\), sensitivity \(4\cdot 10^{-7}\ \mathrm{A/mm}\);

Cadmium, conc. \(9\cdot 10^{-6}\ \mathrm{mol/l}\), sensitivity \(3\cdot 10^{-7}\ \mathrm{A/mm}\);

Table 3

Dependence of the anodic-current constant and the half-peak width on the radius of a stationary mercury electrode

Indium, conc. \(9 \cdot 10^{-4}\) mole/l, sens. \(4 \cdot 10^{-7}\) A/mm;
Thallium, conc. \(9 \cdot 10^{-5}\) mole/l, sens. \(1.5 \cdot 10^{-7}\) A/mm.

As can be seen from the table, for all elements, as the electrode radius increases, the anodic-current constant also increases. The half-peak width also increases. The coefficient \(v = \dfrac{\delta^{*}}{\delta}\), where \(\delta\) is the true value of the half-peak width (measured directly from the anodic peak) for a mercury dropping electrode, varies within the interval 1.2–1.5. In accordance with the theoretical expression, the ratio

\[ \frac{k_{2}\delta^{*}}{r} = \mathrm{const} \]

is in good agreement with the experimental data.

The magnitude of this ratio, as was to be expected, is proportional to the number of electrons participating in the electrode reaction.

Conclusions

1. The influence of a number of factors (the number of electrons participating in the electrode process; reversibility, etc.) on the height and shape of anodic peaks in the extraction APS method has been studied.

2. It has been shown that, under the conditions studied, for irreversible anodic waves the peak depth, as in aqueous solutions, is smaller, while the half-peak width, on the contrary, is larger than for reversible anodic waves; in the studied range of changes in the electrode radius, the anodic-current constant and the half-peak width increase with increasing radius of the stationary mercury drop, while the coefficient \(v\) varies within the interval 1.2–1.5.

REFERENCES

1. A. G. Stromberg. Izvestiya SO AN SSSR, 5, 76 (1962).
2. A. G. Stromberg, M. S. Zakharov. Zhurn. fiz. khimii, 38, 130 (1964).
3. A. G. Stromberg. Izvestiya Tomsk. politekhn. in-ta, 128, 13 (1965).
4. A. G. Stromberg. Proceedings of the 4th All-Union Conference on Electrochemistry, Academy of Sciences of the USSR, p. 213, 1959.
5. E. A. Zakharova. Candidate’s dissertation, Tomsk, 1965.