The effect of some organic surfactants upon the reduction rate of In3+ in perchloric medium on mercury

The effect of Triton X-100, dodecylammonium, tetrabutylammonium, and dodecylsulphate upon the reduction rate of In³⁺ to indium amalgam in 0.1 M HClO₄ was investigated, both in the presence and in the absence of 1 M NaClO₄. After having accounted for the blocking effect upon the kinetics of In³⁺ reduction via the factor (1??), where ? is the surface coverage by the surfactant, the remaining electrostatic effect was compared with that predicted by the Frumkin theory for diffuse-layer effects. For this purpose the Gouy-Chapman-Stern theory was used to calculate the potential _d at the outer Helmholtz plane from measured values of the charge density q_M on the metal and of the charge density q_i due to the adsorbed surfactant. With the exception of Triton X-100, the electrostatic effect predicted by the Frumkin theory combined with the Gouy-Chapman-Stern theory was found to be much greater than that observed experimentally. This result was explained by the inadequacy of the Gouy-Chapman-Stern model of the double layer in the presence of bulky surface-active ions.