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°–V^e–V^c where V° is the volume of the metal ion, V^e is the volume reduction due to electro-striction, and V^c 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 V^e for all these ions when calculated with due allowance for V^c obey approximately the electrostriction rule V^e=Az²/.