Ion Exchange, Solvent Extraction and Adsorption Equilibria

The partition of molecules between two phases can be based on different sorts of equilibrium. Meaningful are equilibria concerning the processes of ion exchange, partition of substances between immiscible solvents (solvent extraction), and accumulation of substances at solid surfaces (adsorption), hi some cases, real chemical bonds are formed, but sometimes only weak forces control the process. These equilibria generally are reversible, and they are mobile, i.e. they tend to react fast to concentration changes. This is a valuable property for sensor applications. Furthermore, they contribute to the accumulation of traces at surfaces, and they are important in manufacturing ordered structures at surfaces. The following discussions are dedicated to equihbria of particular interest. 

Weakly acidic resins commonly contain COOH groups instead of SO3H groups. In weakly basic resins are found groups like -NHR-NH3OH. In such polyelectrolytes, the interaction with ions is no longer purely electrostatic but partially covalent. Thus ion exchange is more selective. 

The exchange equilibrium can be written as follows H+ + Na H + Na+ with H+ and Na+ ions that are fixed at the resin. To characterize the equiUbrium, an exchange constant jj+ is defined, which is a thermodynamic constant according to the following expression  

An alternative description is given by the selectivity coefficient lCNa+,H+ defined by concentrations rather than activities  

A common graphical representation of the equilibrium is the exchange isotherm. An example of the equilibrium A + B X+Bis given in Fig. 2.43. 

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