Molar volumes of ions which form long-lived solvation complexes with water

Abstract

It is shown that the volume occupied by the water in the molecular hydration complex of Al3+ is less than the volume of an equal amount of bulk water. Thus the apparent molar volume ϕ° of this ion is obtained from the sum of three terms, ϕ°=V°–Ve–Vc where V° is the volume of the metal ion, Ve is the volume reduction due to electro-striction, and Vc is the volume loss due to complexing or capture of water. The concept is applied also to the ions Fe3+, Cr3+, Mg2+, and Be2+ and it is shown that values of Ve for all these ions when calculated with due allowance for Vc obey approximately the electrostriction rule Ve=Az2/.