La[Fe(CN)6] ion pairing in aqueous solution. A dielectric relaxation study

Complex permittivity spectra of aqueous La[Fe(CN)6] solutions at 25°C over the frequency range 0.2⩽ν/GHz⩽89 are presented. Because of the large association constant of lanthanum ferricyanide, KA=5300, and its high ion-pair dipole moment, µIP=77.7 D, dilute solutions in the range 0.0026⩽c/mol dm-3⩽0.0485 could be investigated. It is shown that the current models for the analysis of the ion-pair dispersion amplitude are reliable. The data reveal that the formation of contact ion-pairs is at a nearly diffusion controlled rate, and suggest that the ion-pair activity coefficient deviates from ideality. The effective solvation number of the electrolyte is determined from the solvent permittivity.