The Exchange of Water Between Hydrated Cations and Solvent

Abstract

The rate of exchange of water between the hydrated forms of Al+++, Cr+++, Fe+++, Ga+++, and Th+4 and the solvent water has been measured in solutions of varying acidity. With the exception of chromic ion, and of ferric ion at low acidity, the exchange is complete in ca 3 min at 25°. For chromic ion, the half‐time for exchange is independent of acidity over a wide range and is ca 40 hr at 1 M salt concentration. The coordination number of chromic ion for water over the entire acidity range is 6.0. The initial hold‐back of oxygen by ferric ion at low acid is slight and is attributed to the presence of polynuclear hydrolyzed species. For all the cations, the distribution of oxygen isotopes in the final solution is noticeably different from random, the cations showing a greater affinity for H2O18 over H2O16. The rates of exchange are correlated with the electronic structure, which influences the energy required to form the transition state.