The estimation of lattice energies of ionic crystals: A critical examination of the method of determination and use of lyotropic numbers

Abstract

The estimation of lattice energies for crystals for which structural parameters are unavailable depends on the use of (a) wholly-or semi-empirical equations, such as the Kapustinskii equation, or (b) correlations between solution thermodynamic quantities, especially hydration enthalpies and lyotropic numbers. Approach (b) depends on the experimental measurement of lyotropic numbers previous measurements of this kind are limited in number and are experimentally ill-defined. In the present work lyotropic number determinations have been carried out for selected anions using reproducible gelatin indicator and with photometric end-point determination using a commercial turbidimeter. The shape of the graphs obtained is discussed and from them the lyotropic numbers of the anions have been evaluated. The influence of changes of conditions, involving variations in temperature, ph, ionic strength and Indicator concentration, has been investigated and the effects assessed. lyotropic number is related to ion hydration enthalpy and, there sufficient data exist, combined ion hydration enthalpy values have been used to estimate the lattice enthalpies of Group i and Group ii salts. Some values from the present work are compared to existing values for several Ionic crystals the lattice enthalpy values now found are new. The application of lyotropic methods to the estimation of lattice energies is discussed.