STUDIES ON ION EXCHANGE BEHAVIOUR OF SOME HETEROPOLYACID SALTS AND THEIR USE AS ION SELECTIVE ELECTRODES

Abstract

The early ion exchangers were largely inorganic in origin. It is interesting to note that the first systematic studies on ion exchange were carried out on naturally occurring inorganic materials, viz., the clay fractions of the soil, which were investigated as long ago as 1850; furthermore, the first industrial applications of synthetic ion exchangers for water softening also employed inorganic materials. Lateron the ion exchange scene was dominated by the synthetic organic resins because of the ease with which reproducible preparations of appropriate mechanical and chemical stability could be made. In recent years,interest in the inorganic exchangers has been revived with the need for the high temperature separation of ionic components in radioactive wastes. In order to make such separations, highly selective exchangers are required which are not only stable at high temperatures but which also have exchange properties unaffected by acidity and high levels of radiation. Organic resins are unsuitable for such applications,as changes in selectivity and exchange capacity occur on exposure to radia tion, and degradation takes place at the high temperatures of interest. Ion Exchange Equilibria In ion exchange reactions, a reversible interchange of ions takes place between two components,one of which (the ion exchanger) is insoluble in the medium in which the exchange is carried out. A cation exchange process may be represented as: M - A+ + B+ —± M - B+ + A+ ... (1) where M represents the polymer network and A and B are both cations called counter ions. This equilibrium obeys the law of mass action. Two practical quantities which represent the extent to which exchange takes place are distribution 'B coefficient (K.) and selectivity quotient (KA ), both of which can be experimentally determined. For the calculation of the thermodynamic equilibrium constant of non-ideal systems, activity coefficients of the exchanging ions in both phases should be known. Distribution coefficient is defined as the number of milliequivalents of ion adsorbed per gram of the exchanger divided by the number of milliequivalents of that ion per mi in solution at equilibrium. Another characteristic quantity, the saturation capacity of an ion exchanger, is defined as the total number of milliequivalents of exchangeable ions per gram of the exchanger. The selectivity quotient arises from the reversible character of the exchange reaction. Obviously, the equilibrium in the Equation 1 may be displaced in either direction by adding an excess of either cation. When the exchanging ions have different charges e.g., Ca +/Na+ the ions of higher charge in dilute solutions are preferentially absorbed whereas in concentrated solutions this trend may be reversed. The preference of the exchanger for one ion B relative to the other A of a selected pair of similar ions (cations or anions) is quantitatively expressed by the selectivity quotient. It is the ratio of the concentration of two ions in solid phase divided by the concentration ratio in solution at equil ibrium. Thus, ,o !lB+3|"A+"] Selectivity quotient (KA ) a ^—^- =- (P) where the barred quantities refer to equilibrium concentrations in the exchanger phase and the others to the concentrations in solution. The present day knowledge of selectivity effects(1,2) is summarised in the following emperically established rules: 1. The lesser the solvated volume of an ion, the more strongly it is taken up by an exchanger. 2. In dilute solution, the ion exchanger prefers ions of higher valency. However,in concentrated solutions, the normal selectivity pattern is sometimes reversed. 3. The extent of exchange increases with the polarizability of exchanging ions. 4. The cation exchangers prefer those cations which associate with anions less strongly whereas anion exchangers prefer cations which form stronger complexes with anions.