Exchange equilibria between bicarbonate, carbonate, chloride and bromide on dowex 1 x 8

The exchange reaction 2R(+)HCO(3)(2-) + CO(3)(2-) right harpoon over left harpoon R(2)(+) CO(3)(2-) + 2HCO(3)(2-) has been studied on Dowex 1 x 8 in the presence of bicarbonate solution in equilibrium with atmospheric carbon dioxide (open system). The experiments showed, as theory predicts, that the composition of the resin phase is independent of the concentration of the bicarbonate solution. The mole fraction of carbonate at equilibrium is about 0.4 and the equilibrium constant is 0.15M at 20 degrees . With this value of the constant, the composition of the ion-exchanger for various bicarbonate concentrations has been calculated for a closed system. At [HCO(3)(-)] < 0.01M a substantial part of the resin is in the carbonate form, whereas for [HCO(3)(-)] 0.05M the resin is present almost exclusively in bicarbonate form. The exchange constants of bromide at trace level have been determined for the bicarbonate and mixed carbonate forms of the ion-exchanger. The exchange constant K(Cl)(HCO(3)) has been determined over the whole composition range and the results can be represented by K(Cl)(HCO(3))= 0.428 -0.063x(Cl) -0.115x(Cl)(2), where X(Cl) is the mole fraction of chloride in the resin. The constants are used to discuss the conditions for the Chromatographie enrichment of bromide from fresh water.