The 'Anomalous' Association of Thallous Salts in Aqueous Solution

The failure of simple ionic theories to account for association of thallous salts in aqueous solution is discussed. Accepting experimental evidence that the associated species are ionic, it is shown that on a dielectric continuum model neither dielectric saturation of the solvent nor polarization of the ions can account for the association. Consideration of the thermodynamics of association shows that hydration effects cannot be neglected and it is suggested that Tl+ differs from alkali metal ions in association reactions because of the tendency of Tl+ to distort in an unsymmetrical field.