EQUILIBRIUM STUDIES WITH
A
2093
CHELATING ION-EXCHANGE RESIN
Equilibrium Studies with the Chelating Ion-Exchange Resin Dowex A.1 1-3
by Donald E. Leyden and A. L. Underwood Department of c h e m i s t r y , Emorti University, Atlanta, Georgia
(Receiced Januaru 9 , 1964)
The apparent, ionization constants of the iminodiacetic acid groups of the chelating ionexchange resin Dowex A-1 have been measured in 0.1 M and in 1 M sodium chloride solution. Despite the fact that the ionogenic groups are situated in a styrene-diviiiylbeiizeiie copolymer matrix, the measured values are of the same magnitude as those of the simple monomeric analog, benzylimiiiodiacetic acid. However, the pH dependence of metal ion distribution coefficients is not predictable from the pK, values using the custoniary treatment applied to monomeric chelons. Experimental distribution coefficients as functions of pH are reported. From the temperature dependence of the D values, “formal” values of AH and of A S have been calculated for the exchange process.
Introduction Attempts to “tailor-make” ion-exchange resins for specificity have achieved only limited success. Clearly a more realistic goal is a resin exhibiting a more modest selectivity ; separations will then be accoiiiplished through ingenuity based upon a detailed knowledge of its properties and of the coordination chemistry of ions in solution. Dowex A-1, a chelating resin with iminodiacetic acid groups attached to a styrene-divinylbenzene copolymer matrix, although it interacts with many metal ions, is inherently much more selective than conventional cation exchangers. Numerous analytical applications of Dowex A-1 have been reported, involving mainly the removal of di- and trivalent cations from solutions of the alkalies and alkaline earths, based upon the marked selectivity of the resin toward ions of greater coniplexing ability. The resin first Became generally available in 1959; in 1962 a considerable change in the manufacturing process led to an iinproved p r ~ d u c t . ~ I n view of the widespread interest in the resin, the literature is noticeably scanty with regard to the fundamental data needed to place the use ol Dowex A-l on a sound basis. The first dissociation constant of the acid form of Dowex A-1 was measured by Marinsky and I
