Kinetics of rehydration of crystalline anhydrides. Manganous formate

Abstract

The anhydride resulting from the dehydration of manganous formate dihydrate is crystalline and its skeletal structure is distinct from the dihydrate. Under the proper conditions of temperature and water vapour pressure the freshly dehydrated anhydride rehydrates readily and completely to the dihydrate. Rehydration against time curves have been obtained for anhydrides prepared from the dihydrate which was in the form of individual single crystals, crystalline powder, ground powder and reactor-irradiated powder. The pressure, time and temperature dependences of the rehydration process have been determined. The extent of rehydration is described by a square-root dependence on time. This suggests that the rehydration is diffusion-controlled. The rate of rehydration increases markedly with pressure. Below a supersaturation ratio of approximately 10, rehydration does not occur; rather there is an appreciable sorption. The rate of sorption is diffusion-controlled with an energy of activation for diffusion of 12.0 kcal /mol. Isotherms have been determined for the sorbed water.