Some studies of selectivity in uni-bivalent anion exchange reactions.

A statistical thermodynamic treatment of uni-bivalent exchange reactions is used to determine the free energy and enthalpy changes of such reactions with both laboratory prepared and commercially available strongly basic resin exchangers. It is shown that the plot of in [KCf2AC/fB] (where KC is the mass action constant for the exchange reaction, C is the total equivalent ionic concentration, ϕ, the volume capacity of the exchanger and fA and fB are the activity coefficients of the ions in solution) against the ionic fraction of the bivalent ion in the exchanger at equilibrium is not always linear. The degree of curveture of this plot varies with temperature, with cross-linking and with resin structure. The van't Hoff isochore is used to calculate values of DeltaH from data obtained by equilibrium experiments. In general good agreement is obtained between the enthalpies obtained in this manner and the values measured directly in a non isothermal reaction calorimeter, thus supporting the theoretical treatment which is used to describe the ion exchange process. An empirical correction term is introduced which explains deviations from linearity of the selectivity plots for chloride-oxalate exchange in terms of non uniformity of crosslinking of the ion exchange gel. This correction is not as successful in the treatment of results for chloride-sulphate exchange and it is concluded that this is possibly due to differences in structure of the oxalate and sulphate ions. The application of an iterative procedure to the values of o (an energy term associated with placing two bivalent ions on adjacent exchange sites) and (the thermodynamic equilibrium constant) obtained by experiment, enables the ion exchange isotherm to be calculated with accuracy. The implication is that an approximate knowledge of the values of o and KT, for particular exchanging ions and resin allows the exchange isotherm to be predicted