Ion Exchange Equilibria

When an ion exchange solid is allowed to reach equilibrium (checked by a prior kinetic experiment) with a solution containing two counterions, generally one ion will be taken up preferentially into the solid. The solid is then said to be exhibiting selectivity for the preferred ion. Selectivity can be quantified by the experimental construction of an ion exchange iso therm. At a fixed temperature solutions containing counterions A and B in varying proportions are allowed to equilibrate with known, equal, weights of exchanger in, say, the MA form. The total ionic concentration of the ions A and B in the respective solutions is kept constant, i.e. each solution has the same normality (N) but, as the concentration of B increases it is compensated by a decrease in concentration of A. At equilibrium the solids and liquids are separated and both phases analysed for A and B.

This enables an isotherm to be plotted that records the equilibrium distributions of one of the ions between the two phases. Examples of typical isotherms are shown in Figure 1. The selectivity shown by an isotherm can be quantified; a general example of cation exchange will be used to illustrate this. First eqn [1] will be rewritten for an exchange involving cations (A, B) of any charge, as in eqn [3]:

where ZA B are the valences of the ions and the bar represents the ions inside the solid phase.

The axes of the isotherm record the equivalent fraction of the ingoing cation (A) in solution (AS) against its equivalent fraction in the exchanger (AC). These quantities are defined in eqns [4] and [5] below:


Solar Panel Basics

Solar Panel Basics

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