Stability of metal uncharged ligand complexes in ion exchangers. Part 3.—Complex ion selectivity and stepwise stability constants

Abstract

A new method is presented for obtaining the stepwise stability constants of ion-exchanged metal–uncharged ligand complexes: it is based upon the measurement of the effect of increasing ligand concentration on complex-ion selectivity and provides a more sensitive means of characterizing adsorbed complexes than the usual method, relying on the measurement of ligand numbers in the exchanger.

In special cases where a more favourable coordination of the first ligand in the ion exchanger (as compared with the bulk solution) is accompanied by a less favourable coordination of the second ligand, a maximum occurs in the complex ion selectivity coefficient. This maximum occurs at a critical ligand concentration which equals the reciprocal of the average stability constant [graphic omitted] of the complex in solution. The maximum selectivity coefficient of the complex ion depends mainly on the ratio of stepwise stability constants in solution, K1/K2, and its upper limit equals the ratio of stepwise constants, K1/K2 in the exchanger and the solution.

The method is illustrated by the case of the silver–thiourea complex in a macroreticular sulphonic acid resin in which the stability constant for the one-complex exceeds the value in solution by more than four orders of magnitude.