Effect of Inert Salts on Equilibrium and Rate Constants for Ionic Reactions

Abstract

Empirical evidence is given to show that individual ion activity coefficients can be written as logγt=Zt2G(S)+[ν1α1t+ν2α2t]C, where ν1 and ν2 are the number of ions of type 1 and 2 formed from one formula of the supporting electrolyte, C is the formal concentration of the electrolyte, Zt is the charge of the ion t, G(S) is a function of the medium only, α1t is a function of the ion-size parameter for ions 1 and t, and α2t is a similar function of the ions 2 and t. The equation is applicable only if the concentration of ion t is small in comparison to the concentration of electrolyte. Mayer's theory predicts that the above equation, when used to calculate experimentally observable quantities, should be correct to within good accuracy for ion-size parameters between 5.5 and 8.5 Å in a 1–1 supporting electrolyte and for higher-charged electrolytes at ionic strengths above about 0.2. Using this equation one can calculate the ratio of the rate constant for the bromacetate—thiosulfate reaction to that for the peroxydisulfate—iodide reaction at infinite dilution from data obtained in MgSO4, Mg(NO3)2, Na2SO4, and NaNO3 solutions at concentrations between 0.25 and 2F. The value obtained under these conditions is within experimental accuracy of the value obtained by extrapolation to infinite dilution from ionic strengths below 0.02.