Thermodynamics of cation exchange. Part 4.—Uni-, bi- and tervalent ions on Dowex 50

Abstract

The heats, volumes and entropies of exchange of potassium by a series of cations of various valencies on Dowex 50 crosslinked with 4, 8 and 16% divinyl benzene have been determined. The exchange of potassium by bi- and tervalent ions is endothermic. The heats are 1.5-2.5 kcal/equiv. Much of this heat can be accounted for by the change in electrostatic energy on transferring ions between the solution and the resin. Heat effects arising from changes in ionic hydration may also be important.

The net volumes of exchange may be of either sign. They reflect the changes in hydration of the ions. Ions of small [graphic ommitted] electrostrict less water in the resin than in the solution. This causes an expansion when these ions enter the resin. Ions of large [graphic ommitted] create less icelike structure in the resin than in solution. This causes a contraction when the ions enter the resin.

The preference shown by resins for ions of high valence results from an increase in entropy on exchange. When calculated with respect to an appropriate set of standard states the experimental entropies of exchange are almost entirely a consequence of changes in the configurational entropy of the ions. Although the changes in ionic hydration produce a relatively small net contribution to the entropy of exchange, this is influential in deciding the relative selectivities towards ions of equal valence and also in determining the effect of crosslinking on selectivity.