241 Exchange of Some Divalent Metal Cations and Their Effect on the Dissociation of Iron(III) Phosphate S. Mustafa, * A. Naeem, N. Rehana and T. Hussain National Centre for Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan. (Received 29 June /995; accepted 25 October /995) ABSTRACT: Potentiometric titrations of iron(I1I) phosphate have shown an apparently weak monobasic acid behaviour towards alkali, alkaline earth and divalent transition metal ions. The selectivity sequence and dissociation of the exchanger was found to increase in the order Cu 2 + > Zn 2 + > Ni 2 + > Ca 2 + > K+. The dissociation constants of iron(I1I) phosphate have been determined from potentiometric titration data in the temperature range 303-323 K. In addition, the exchange of Cu 2 +, Zn 2 + and C0 2+ on iron(I1I) phosphate and the effect of these ions on dissociation were also studied as a function of concentration, temperature and solution pH value. The thermodynamic parameters and for the dissociation of iron(III) phosphate are also presented. INTRODUCTION Metal phosphates exist in various amorphous and crystalline forms and in an intermediate state of crystallinity (Kullberg and Clearfield 1981), all having different surface properties. These metal phosphates possess pH-dependent exchange capacities which are responsible for adsorption processes in soil and water systems. Because of the good ion-exchange properties of metal(IV) phosphates, a great variety of ion- exchange studies and ion separations have been carried out with such exchangers (Clearfield and Kalnins 1978; Albertsson 1966). Various applications have been made of zirconium phosphate materials in the purification of reactor coolants, the decontamination of radioactive wastewater, in ion-exchange membranes, in H/O fuel cells and electrodialysis (Alberti 1978; Vesely and Pekarek 1972). Most of the recent work concerning the ion-exchange properties of metal(IV) phosphates has been reported in various articles (Clearfield and Kalnins 1978; Alberti 1978). In contrast, very little is known about the ion-exchange properties of metal(III) phosphates and although both metal(III) phosphate and metal(IV) phosphate exchangers are expected to be of the weak acid type, to date no data about their dissociation are available in the literature. Thus, as a part of our continuing study of the mechanism of exchange in metal(III) phosphate exchangers, we present here the dissociation and ion-exchange studies of alkali, alkaline earth and transition metal ions on an iron(III) phosphate cation exchanger. *Author to whom all correspondence should be addressed.