Aquametric microdetermination of hydration of ion-exchange resins

An aquametric microtechnique is proposed for measuring the pre-adsorbed (residual) moisture (DeltaW) and equilibrium moisture content (H) in macroporous weakly basic anion-exchangers and their gel analogues, during the derivation of water-vapour sorption isotherms by use of a modified McBain balance in a vacuum system. The absolute values of DeltaW and H are determined by direct Karl Fischer titration of weighed samples of the anion-exchanger after equilibrium hydration has been achieved at various relative water-vapour pressures (P/P(s). The relationship between DeltaW and the sample pretreatment conditions, characteristics of the polymer matrix (basicity and porosity), and the nature of the ionogenic groups in the ion-exchanger has been established. It has been shown that the hydration parameters, in terms of the BET theory and specific anion-exchanger surface area depend largely on DeltaW. For instance, in the case of the strongly basic anion-exchangers AB-171 and AB-17-8. which contain quaternary ammonium groups, the degree of hydration in the BET region may be well in excess of the apparent effective capacity (h) of the BET monolayer. It has been shown that the specific surface area for water (S(H(2)O), calculated from aquametric isotherms by using the BET equation, corresponds to the hydrophilic anion-exchanger surface area, as compared to the SH(2)O value calculated from gravimetric isotherms.