Investigation of Aluminate Solutions by water activity measurement

Isopiestic measurements were used to determine the change in water activity due to the variation of the aluminium content of alkaline aluminate solutions. This parameter was found suitable to indicate the concentration dependence of the species present in the solution. It was established that the increase of water activity due to the increased aluminium concentration can be attributed to two processes: 1. the coordination of hydroxide ion to aluminium, 2. dimerization connected with the dehydration of the monomeric species. On the basis of the Gibbs-Duhem equation the water activity data gave an indication of the concentration ranges in Which the sodium aluminate solution exists mainly as (I) NaOH + monomeric Al(OH)₄^? and ( H ) NaOH + dimeric Al₂ O(OH)₆ ^2? ions.