An ultraviolet spectrophotometric study of ion association in solutions of cupric sulphate in 80% aqueous methanol

Abstract

About forty years ago, Debye, Hűckel P.Debye and E.Hűckel, Phys.Z.,24, 185 (1923).and Onsager L.Onsager ibid., 28, 277 (1927). proposed formulae for the effect of long-range coulombic interaction between ions upon the thermodynamic and transport properties of electrolyte solutions. These formulae have been found successful for dilute aqueous solutions of ionic strength c.0.1 of many salts when it is assumed that the concentrations of free ions are the same as their stoicheiometric concentrations. The derived equations do not predict the properties of many other dilute solutions of salts, especially in non-aqueous solvents. In these cases it is thought that ions of opposite charge approach sufficiently close together to make the energy of their mutual electrical attraction considerably greater than their thermal energy. In this way, they form new entities in solution which are sufficiently stable to withstand several thermal collisions with solvent molecules; because of the ephemeral nature of these entities, only the time-average effect of ion association can be treated theoretically or determined experimentally. The most common product of ion association is an ion-pair although higher products may also form. The ion association product may be charged or un-uncharged depending upon its ionic constitution and the individual charges of the ions. This close approach of ions of opposite charge leading to ion-pair formation has been studied theoretically by Bjerrum N.Bjerrum, K. danske vidensk. Selsk., 7, No.9 (1926). and Fuoss R.M. Fuoss, J.Amer.Chem.Soc. 80, 5059 (1958).who treat ion-pairs as separate species in equilibrium with the free ions. They derive formulae for the dissociation constants of ion-pairs as a function of universal constants, the dielectric constant of the solvent, the temperature and the distance of closest approach of the ions.