Ion Exchange

Ion exchange is an exchange of ions between a liquid and a solid phase (matrix). The exchange takes place because the chemical energy at equilibrium after the exchange is lower than before. If a pure ion exchange process takes place, the number of ions released is equivalent to the number of ions taken up by the process.

Ion exchange is known to occur with a number of natural solids, such as soil, humus, metallic minerals and clay. Clay and, in some instances, other natural materials can even be used for ion exchange demineralization of drinking water.

The exchange reaction between ions in solution and ions attached to the matrix is generally reversible. The exchange can be treated as a simple stoichiometric reaction.

For cation exchange the equation is:

The ion exchange reaction is selective, so that the ions attached to the matrix will have a preference for one counter ion over another. Therefore the concentrations of different counter ions in the ion exchange will be different from the corresponding concentration ratio in the solution.

According to the law of mass action, the equilibrium relationship for reaction (3.38) will give for diluted solutions:

The selectivity coefficient, is not actually constant, but is dependent upon experimental conditions. The plot in Fig. 3.33 is often used to illustrate the preference of an ion exchanger for a particular ion. As can be seen, the percentage in the matrix is plotted against the percentage in solution.

% Solution

Fig. 3.33. Equilibrium plot between rr of ions in solution and rr in matrix. The dotted line indicates the case when the matrix has the same preference for the two competing ions.

% Solution

Fig. 3.33. Equilibrium plot between rr of ions in solution and rr in matrix. The dotted line indicates the case when the matrix has the same preference for the two competing ions.

A selectivity coefficient of 50% in solution is often used, and called aw; when activities are considered. If we use concentration, when n = 1 in reaction (3.38)

and for low concentration of solute

The plot in Fig. 3.33 can be used to read (i,liv

The selectivity of the ion exchange material for the exchange of ions is dependent upon the ionic charge and the ionic size. An ion exchanger generally prefers counter ions of high valence. Thus, for a series of typical anions of interest, one would expect the following order of selectivity:

Similarly for a series of cations:

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