Dependence of the tensammetric peak height on the concentration of surface-active substances

Abstract

In AC polarography many surface-active substances show nonfaradaic waves, which are caused by the adsorption-desorption process of the surfactant at the electrode surface. Breyer school has given the name of tensammetric peak to this type of wave and pointed out that the tensammetric peak height obeys the Langmuir adsorption isotherm, shown by eqn.(1), where
a=amount of adsorbed substance per unit area of electrode surface,
Z=the maximum number of adsorption sites per unit area,
ω=adsorption coefficient,
C=concentration of surfactant in the bulk of solution.
If the tensammetric peak height i_p is proportional to the amount of adsorbed substance a, as Breyer states, then the reciprocal of peak height plotted against the reciprocal of concentration should give a straight line, as eqn. (2) shows.
On the other hand, upon investigating the concentration dependence of the tensammetric peak potential, Senda and Tachi report that i_p plotted against log C shows a roughly linear relationship.
Employing the nine species of surface-active substance, as tabulated in Table I, the concentration dependence of the tensammetric peak height is measured. The concentration range is so chosen that the peak height dose not exceed 500 MΩ^-1; otherwise the effect of circuit resistance is not negligible. In general, a surfactant shows two peaks, namely, the peak at a potential more positive than the electrocapillary maximum potential i_p⁺ and the peak at a more negative potential i_p^-. In the present investigation, the peak height of i_p⁺ is measured with exception of benzyl alcohol with which both i_p⁺ and i_p^- are measured.
The method of linear regression is applied to the 1/i_p vs. 1/C and the i_p vs. log C plots, and the corresponding two correlation coefficients r₁ and r₂ are evaluated. The difference between r₁ and r₂ is tested according to the method given in the review of Doerffel by means of eqn. (3). As shown in Figs. 2, 3, 4 and 5, and also in Table I, the linear relationship between 1/i_p and 1/C holds, in general, much better than that between i_p and log C does.
In the plot of 1/i_p vs. 1/C, however, with several surface-active substances, the straight line crosses the ordinate at a negative value of 1/i_p, as illustrated in Figs. 2 and 4. If 1/C tends to zero, 1/i_p should show a positive value on the basis of Langmuir adsorption isotherm. This contradiction remains theoretically unsolved.