EFFECT OF THE ELECTROLYSIS POTENTIAL ON THE DEPTH OF THE ANODIC PEAK IN THE EXTRACTION APN METHOD

Yu. A. Karbainov, A. G. Stromberg

In [1] the dependence of the depth of the anodic peak on the electrolysis potential was studied theoretically for the case of a completely irreversible electrode process, using aqueous solutions as an example.

It was of considerable interest to study the character of the dependence of the current of the anodic peak on the electrolysis potential in the extraction APN method for three cases of electrode-process behavior at \(b \ll 0.1\) [2]:

a) a reversible electrode process;
b) an irreversible electrode process;
c) a completely irreversible electrode process.

For the case of a reversible electrode process, the dependence of the current of the anodic peak on the electrolysis potential was studied using Cd(2+) as an example; for the case of an irreversible electrode process, using Cu(2+) as an example, against a background of 1% oxyquinoline in chloroform \(+ 0.2N\) \(NH_4NO_3\) in methanol. The studied interval of potential changes was: for cadmium, \(-0.7 \div -1.4\) V; for copper, \(-0.3 \div -0.9\) V; for indium, \(-1.3 \div 2.1\) V relative to mercury at the bottom. The degree of reversibility of the cathodic process can theoretically be characterized by the formula [3]:

\[ \varphi_{n/2}^{k} - \varphi_{n}^{k} = 2.2 \frac{RT}{zF}. \]

Approximately, an analogous relation may be taken as valid for a reversible anodic peak:

\[ \varphi_{n}^{a} - \varphi_{n/2}^{a} = 2.2 \frac{RT}{zF}, \]

where \(\varphi_{n}^{k}\) and \(\varphi_{n}^{a}\) are the potentials of the cathodic and anodic peaks; \(\varphi_{n/2}^{k}\), \(\varphi_{n/2}^{a}\) are the half-peak potentials.

In our case, \(\varphi_{n}^{a} - \varphi_{n/2}^{a}\) for cadmium, copper, and indium are respectively equal to 0.036; 0.09 and 0.21 V. The calculated values (by the formula given above) are 0.03; 0.03 and 0.19 (V). On this basis, the anodic process in the case of cadmium is reversible, while in the cases of copper and indium it is completely irreversible.

The experimental conditions were as follows: the cathode was a stationary mercury droplet obtained electrolytically on a platinum contact sealed into a glass tube, from a saturated solution of mercurous nitrate (monovalent). In the experiments the radius of the droplet was ra-

was 0.044 cm. The anode was mercury at the bottom of the electrolyzer. The working volume of the solution was \(V = 10\) ml. The electrolysis time was \(\tau_{\mathrm e} = 5\) min. A constant stirring regime during electrolysis was maintained by a magnetic stirrer, whose rotation speed was 420 rpm.

The experimentally obtained dependence of the anodic-tooth current on the electrolysis potential for cadmium, indium, and copper is presented in Figs. 1, 2, 3. Each value of the anodic-tooth current on these graphs is the average of four measurements. As can be seen from the figures, the experimental curves of the dependence of the anodic-tooth current on the electrolysis potential have the form of a polarographic wave. In the case of cadmium, the experimental curve has the form of a reversible polarographic wave; in the case of indium, an irreversible one.

Fig. 1. Dependence of the anodic-tooth current on the electrolysis potential. Concentration of Cd(2+) \(9 \cdot 10^{-5}\) mol/l, sensitivity \(2 \cdot 10^{-7}\) A/mm, other conditions in the text.

To construct the theoretical curve \(I = f(\varphi_{\mathrm e})\) from equation (2), it is necessary to determine experimentally the half-wave potential, the transfer coefficient, and to know the limiting value of the anodic-tooth current; in other words, one must know three of the four characteristics entering into equation (2).

Such a check of the agreement between theory and experiment is, in our opinion, unsuccessful. It is better to compare the experimental curve of the dependence of the anodic-tooth current on the electrolysis potential directly with the cathodic polarographic wave, rather than with the curve described by equation (2). In this case, the experimental and theoretical waves (by the theoretical wave we shall mean polarographic wave (1)) are expediently represented in the coordinates

\[ \lg \frac{I}{I^* - I} - \varphi \quad \text{and} \quad \lg \frac{i}{I_k - i} - \varphi , \]

where

Fig. 2. Dependence of the anodic-tooth current on the electrolysis potential. Concentration of In(3+) \(9 \cdot 10^{-4}\) mol/l, sensitivity \(6 \cdot 10^{-7}\) A/mm. Other conditions in the text.

\(I\)—anodic-tooth current at the given electrolysis potential, μA;
\(I^*\)—limiting value of the anodic-tooth current corresponding to the limiting value of the electrolysis current, μA;

$i$ — instantaneous current value corresponding to the given potential (from the polarographic wave);

$I^{k}$ — height of the polarographic wave, $\mu\text{A}$.

Thus, it is advisable to compare the experimental data with the theoretical data by the value of $\varphi_{1/2}$ of both curves and by the slope coefficient of the straight lines obtained in the coordinates indicated above.

Fig. 3. Dependence of the anodic peak current on the electrolysis potential. Concentration of Cu (2+) $9 \cdot 10^{-5}$ mol/l, sensitivity $2 \cdot 10^{-7}$ A/mm. Other conditions are given in the text.

In our case, the dependence of the anodic peak current on the electrolysis potential for cadmium, indium, and copper in the coordinates $\lg \dfrac{I}{I^{*}-I}$ — $\varphi$ and the dependence $\lg \dfrac{i}{I^{k}-i}$ — $\psi$, obtained on the basis of the polarographic waves of these elements, gives the following results. For cadmium, with reversible course of the electrode process ($\tg \nu = 0.032$), the theoretical straight line practically coincides with the experimental one; for copper—with irreversible course of the electrode process ($\tg \nu = 0.084$). These straight lines differ somewhat ($\varphi_{1/2\,\text{theor}} = -0.48$ V, $\varphi_{1/2\,\text{exp}} = -0.51$ V). The angular coefficients of both straight lines practically coincide ($\tg \nu = 0.084$–$0.088$). In the case of a completely irreversible course of the electrode process for indium, the theoretical and experimental straight lines diverge noticeably; moreover, the theoretical straight line passes in the region of more positive potentials ($\varphi_{1/2} = -1.53$ V) than the experimental one ($\varphi_{1/2} = -1.64$ V). The reason for this discrepancy is not yet clear to us. The angular coefficients also differ somewhat ($\tg \nu = 0.240 \div 0.270$).

Thus, the results set forth above on the dependence of the anodic peak current on the electrolysis potential in the case of cadmium, indium, and copper against the background of an aqueous-free mixture: 1% oxine in chloroform + $0.2N$ $\mathrm{NH_4NO_3}$ in $\mathrm{CH_3OH}$, make it possible in general to conclude that, for reversible, irreversible, and completely irreversible course of the elec-

of the electrode process in the extraction APN method, the dependence of the current of the anodic peak on the electrolysis potential, just as in aqueous solutions, is expressed by a curve whose character resembles a cathodic polarographic wave.

REFERENCES

1. M. S. Zakharov. Zavod. laboratoriya, 30, 14 (1964).
2. A. G. Stromberg. Izvestiya SO AN SSSR, 5, 76 (1962).
3. P. Delahay. New Instruments and Methods in Electrochemistry, IL, Moscow, (1957).