Influence of chloride ion on the rate of inhibited electrode processes

Abstract

In many cases, the addition of halide ions to the electrolyte leads to an increase in the rate of electrode processes. The depolarizing action is observed only at a definite relationship between the electrode potential and the point of zero charge of the surface. The influence of chloride ions on the cathodic deposition of metals on Hg and their electrocrystallization from solutions containing additions of effective inhibitors of electrode processes has been investigated. The depolarizing action of Cl − is very marked; in some cases the decrease in the polarization amounts to 0·5–·8 V. This effect is observed in the case of anodic ionization of metals as well. When surface-active substances of cationic nature are used as additives, the region of the depolarizing action of halide ions expands in the direction of more electronegative potentials, which according to Frumkin is the result of an increase in the anion adsorption. The accelerating effect of halide ions upon electrode processes is mainly the result of a decrease in the activation energy. Most likely, this effect can be attributed to the formation on the electrode of an intermediate complex of the type: electrode—Cl −—Me n+ , in which the chloride ion acts as an electronic bridge (Heyrovsky). A change in the rate of the electrode processes due to the formation of activated complexes of the type indicated is possible in the case of other electron transmitters as well. Thus, in the case of a simultaneous discharge of manganese and hydrogen ions, the cathodic evolution of hydrogen is determined by the kinetics of the discharge of Mn 2+ ions, which seems to result from the formation on the electrode of an intermediate H 2O-Mn 2+- e − (Me) complex. The depolarizing effect of halide ions is sharply decreased in complex electrolytes. If the ions of the metal deposited form part of the complex, anion, whereas the additive is cation-active, the inhibition due to the effect of halide ions somewhat increases. This is explained by the change in the ψ 1-potential and an increase in the density of the adsorption film caused by the adsorption of halide ions. The inhibiting effect of additives in the electrocrystallization of metals is not completely suppressed, as a rule, by the halide ions. The polarization decreases, but still remains greater than in the case of the electrolysis of chloride solutions without additives. This effect is of especial importance in the case of electronegative metals (Sn, Cd, Zn, Mn), where by proper selection of additives it is possible to improve the quality of the deposit without any considerable decrease in the current yield.