Phase diagram calculation of molten salt hydrates using the modified BET equation

From water activity data within the concentration range of molten salt hydrates the parameters ri and εi of the modified Brunauer, Emmett and Teller (BET) equation have been determined for several salts i. The parameters were used to model the solid–liquid phase diagrams and to extract the temperature functions of the Gibbs energy of formation of all occurring solid phases with respect to pure liquid water and anhydrous molten salts as reference states. Applying the multi-component formulation of the BET model according to Ally and Braunstein, phase diagrams of aqueous ternary systems composed of the salts LiNO3, NaNO3, Mg(NO3)2, Ca(NO3)2, Zn(NO3)2, LiCl, CaCl2, LiClO4, and Ca(ClO4)2 were predicted and compared with available experimental data. Good agreement with the experimental data was found, when the presumptions of the BET model in respect to water activity and chemical interactions are satisfied. Based on the BET calculations, a list of new eutectic temperatures and compositions of salt hydrate mixtures is given.