The Temperature Dependence of the Fractionation of Tritiated Water in an Equilibrium between Crystalline Alum and Its Aqueous Solution

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The temperature dependence of the fractionation of tritiated water (HTO) was examined during the crystal growth of alum from its tritiated aqueous solution. When the ratios of the mole fractions of HTO in the water of crystallization to that of the mother liquid, S, at various temperatures are compared, the following order is established: S19.90≈S29.50>S36.52>S41.95>S47.43>S53.70≈S69.50, where the suffixes represent the crystallization temperatures, and where S19.90=0.975 and S69.50=0.949. The concept of the hydration of ions developed recently was applied to the problem of the crystal growth from the solution. Two kinetic parameters, such as the relaxation time of the aluminum ion for entering the surface, τc, and that of the hydrated aluminum ion, Al(OH2)63+, for the exchange of the water with bulk water, τe, were taken into account to explain the fractionation. A surface-diffusion mechanism was suggested as essential in the crystal growth of alum from the solution. The activation free energy for the crystal growth from an unstirred solution was found to be 18.17 kcal/mol, a value which was in good agreement with the value given by Bennema.
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