Superheated solutions in dual-temperature ion exchange separations

The problem of experimental determination of the differential enthalpy of ion exchange is considered. The determination of the enthalpy from experimental equilibrium coefficients is characterized by a high degree of error. A considerably more accurate method of determination of the differential enthalpy of ion exchange on selective ion exchangers is proposed. The method is based on the analysis only of the composition of the solution in the equilibrium system at two temperatures. The influence of temperature on the exchange of mono- and divalent ions on polyacrylic and polymethacrylic cation exchangers within the range from 273 K to 400 K is studied. It is shown than for all exchangers under study the differential enthalpy linearly increases with temperature. The role of the increase of ΔHn¯ with temperature in the process of a single-step dual-temperature separation is estimated. It is found that the increase of the upper temperature boundary leads to a very sharp increase of the degree of dual-temperature separation. This effect is most pronounced in superheated solutions. It is shown that when predicting changes in selectivity with temperature and the extent of purification of solutions of alkali metal salts from admixtures of divalent ions, it is necessary to take the temperature dependence of the ion exchange enthalpy into account.