EFFECT OF β-NAPHTHOL ON ELECTRODE PROCESSES

AT A CADMIUM AMALGAM ELECTRODE

T. V. GOIMZA, V. E. GORODOVYKH

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

A theoretical consideration of the effect of adsorption of organic surfactants on the kinetics of electrode processes has been carried out on the basis of the following model of the mechanism of the electrode reaction.

It is assumed that the electrode surface is covered with adsorbed organic molecules, whose dimensions are significantly (for example, by a factor of 5–10) greater than the dimensions of the depolarizer ion. In order to accept an electron, the reacting ion must approach the electrode surface to a distance approximately equal to the ionic radius. Thus, the electrochemical reaction proper is preceded by the passage of the depolarizer ion to the electrode surface between the adsorbed organic molecules. Upon adsorption of a surfactant, the depolarizer ion, which is in the intermediate state of the reaction, may be located at a different distance from the electrode surface than in the absence of surfactant; its interaction with the surrounding molecules also changes, which will cause a change in the activation energy of the electrochemical stage proper of the reaction and in the electron-transfer coefficient. In addition, the presence of organic molecules in the electrical double layer causes a change in the $\Psi'$ potential [1–3].

For such a mechanism of the influence of surfactants, on the basis of the regularities of formal kinetics we have obtained an equation for the limiting kinetic current.

In the general case

\[ I_{\text{kin}} = K_{\text{kin}} \cdot C_0 \tag{1} \]

\[ K_{\text{kin}} = B_{\text{kin}} l \, \frac{W_{\text{kin}}}{RT} \tag{2} \]

where $C_0$ is the concentration of ions in solution;
$B_{\text{kin}}$ is a pre-exponential factor depending on the number of collisions of ions with the free electrode surface, i.e., on the blocking of the surface by adsorbed organic molecules;
$W_{\text{kin}}$ is the activation energy of the process of delivery of the reacting ion to the electrode surface (in our case, the activation energy of the process of the hindered penetration of reacting ions through the layer of adsorbed organic molecules).

For the limiting kinetic current in the presence of a surfactant we write the following expression:

\[ I_{\mathrm{kin}}=K(1-\Theta)e^{-\frac{W_{\mathrm{kin}}}{RT}}C_{\mathrm{e}} \tag{3} \]

where \(\Theta\) is the degree of coverage of the electrode surface by surfactant molecules.

If adsorption of the surfactant causes only the appearance of a kinetic stage of retarded penetration of the reacting ion to the electrode surface, or a blocking effect, then, as the surfactant concentration increases, in accordance with equation (3), the wave heights within the adsorption-potential interval should decrease, while the half-wave potentials of both waves remain constant.

The form of the dependence of the relative decrease in limiting current on the degree of coverage of the electrode surface by the surfactant \((\Delta I/I-\Theta)\) can serve as an experimental criterion for the mechanism of the decrease in limiting current. If this dependence is linear, then the probable cause of the decrease in \(I_{\mathrm{kin}}\) consists in blocking of the surface by organic molecules [4–5]. The appearance of hindrance to the passage of ions between surfactant molecules will cause the indicated dependence to deviate from linearity.

Considering the electrochemical stage proper of the process within the framework of our model of the mechanism of the electrode reaction in the presence of a surfactant, for the exchange current one may write the following expression:

\[ i_0\Theta=zFC_0\exp\left\{\frac{1}{RT}\left[W_0+\Delta W_0+\left(\alpha zF(\varphi_{\mathrm{p}}-\psi'_{\mathrm{p}})_\Theta-zF\psi'_{\mathrm{p},\Theta}\right)\right]\right\}, \tag{4} \]

where \(\Delta W_0\) is the change in the activation energy of the discharge process; \(\psi_{\mathrm{p}}\) and \(\psi'_{\mathrm{p}}\) are the equilibrium potential and the corresponding value of the \(\psi'\)-potential;

the index \(\Theta\) denotes quantities measured in the presence of a surfactant.

Since the change in the exchange current may be the result both of a change in the activation energy of the charge-transfer reaction and of the \(\psi'\)-effect, we have proposed an experimental criterion that makes it possible to establish the role of the \(\psi'\)-effect.

The theoretical basis of the criterion is provided by the relations following from Stromberg’s theory of anodic–cathodic polarographic waves [6, 7]:

\[ \varphi_{\frac12,\kappa}=\psi_{\mathrm{p}}-\frac{RT}{\alpha zF}\ln\frac{I\kappa}{i_0^*} +\frac{1-\alpha}{\alpha}\,\psi'_{\frac12,\kappa}, \tag{5} \]

\[ \varphi_{\frac12,a}-\varphi_{\mathrm{p}}=\frac{RT}{\beta zF}\ln\frac{Ia}{i_0^*} +\psi'_{\frac12,a}, \tag{6} \]

where \(i_0^*\) is the exchange current at \(\psi'=0\).

If adsorption of the surfactant causes only the appearance of the \(\psi'\)-effect, then in the coordinates \((\varphi^1_2-\varphi^{\mathrm{p}})-\psi'^{1/2}\) straight lines with slopes \((1-\alpha)/\alpha\) and 1, respectively, should be obtained. The influence of other factors on the charge-transfer reaction will cause these dependences to deviate from linearity.

A decrease in the exchange current causes a change in the half-wave potentials of the anodic and cathodic waves, while the wave height remains constant. Thus, a change in the shape of the polarographic waves makes it possible to establish which stage of the electrode process is affected by adsorption of the surfactant.

Experimental verification of the considerations set forth above was carried out by us using as an example the influence of \(\beta\)-naphthol on the discharge–ionization of cadmium at an amalgam dropping electrode. The experimental procedure was similar to that described in [8]. The amalgam was prepared electro-

through the capillary into the lower electrolyzer, in which the polarization measurements were carried out. The lower part of the apparatus was a polarographic cell with replaceable cups. The polarization curves were recorded with an OH-101 polarograph. The adsorption of β-naphthol on mercury had been studied by us earlier [9]. It was established that, depending on the concentration of β-naphthol in solution and on the electrode potential, the orientation of β-naphthol molecules relative to the electrode surface changes.

The effect of β-naphthol adsorption on the anodic–cathodic waves of cadmium was manifested in a shift of the half-wave potentials of both the cathodic and anodic waves and in a decrease in the cathodic limiting current (Fig. 1). The dependence

Fig. 1. Anodic–cathodic waves of cadmium, recorded against the background of 0.1 N sulfuric acid (1) and with additions of β-naphthol equal to: 2 — \(2.44 \cdot 10^{-4}\) M; 3 — \(3.66 \cdot 10^{-4}\) M; 4 — \(4.88 \cdot 10^{-4}\) M; 5 — \(7.32 \cdot 10^{-4}\) M; 6 — \(9.76 \cdot 10^{-4}\) M; 7 — \(1.22 \cdot 10^{-3}\) M.

of the relative decrease in the cathodic limiting current on the degree of coverage of the electrode surface by β-naphthol molecules \((\Delta i/i—\Theta)\) is nonlinear (Fig. 2), which corresponds to the presence of ad-

Fig. 2. Dependence of the potentials of the anodic \(S^{1/2}a\) and cathodic \(\varphi_{1/2}k\) half-waves and of the total shift of the half-waves \(\Delta\varphi^{0}_{1/2}ak\) on the concentration of β-naphthol in solution.

of the hindered penetration of the cadmium ion to the electrode surface between adsorbed organic molecules. In the range of β-naphthol concentrations at which reorientation of the adsorbed molecules to a vertical position begins [9], a bend is observed on the \(\nabla/i, \Theta\)-curve, and the increase of \(\Delta i/i\) is accelerated. Apparently, with an inclined orientation of β-naphthol molecules, the hindrance to penetration of the cadmium ion to the electrode surface increases.

The change in the half-wave potentials of the anodic and cathodic waves occurs differently. Use of equations (5, 6) as an experimental criterion indicates a change in the activation energy of the transition reaction: in the coordinates \((\varphi^1_{1/2}-\varphi_p)-\varphi'_{1/2}\), curves of complex form were obtained (Fig. 3).

Fig. 3. Dependence of \(\varphi'_{1/2}-\varphi^p\) on the \(\psi^1\)-potential for the cathodic (1) and anodic (2) processes.

REFERENCES

1. A. N. Frumkin, V. S. Bagotskii, Z. A. Iofa, B. N. Kabanov. Kinetics of Electrode Processes. Moscow, Moscow State University Press, 1952.

2. A. G. Stromberg, L. S. Zagaynova. DAN SSSR, 97, 107, 1954.

3. A. N. Frumkin. DAN SSSR, 85, 302, 1952.

4. W. Müller, W. Lorenz. Z. phys. Chem., N. F., 27, 23, 1961.

5. V. K. Venkatesan, B. B. Damaskin, N. V. Nikolaeva-Fedorovich. ZhFKh, 39, 129, 1965.

6. A. G. Stromberg. ZhFKh, 29, 214, 409, 1955.

7. A. G. Stromberg, L. S. Zagaynova. ZhFKh, 31, 1042, 1957.

8. A. G. Stromberg, ZhFKh, 31, 1704, 1957.

9. Yu. N. Oblivantsev, T. V. Gomza, V. E. Gorodovykh. Study of the adsorption of β-naphthol on mercury. Present collection.