Debye-Huckel equation associated electrolytes

The BMREP and SDM currently use the Davies technique for activity coefficient prediction. The Davies technique is a combination of the extended Debye-Huckel equation (6) and the Davies equation (7). The Davies technique (and hence both equilibrium models) is accurate up to ionic strengths of 0.2 molal and may be used for practical calculations up to ionic strengths of 1 molal (8). Ion-pair equilibria are incorporated for species that associate (e.g., 1-2 and 2-2 electrolytes). The activity coefficients (y ) are calculated as a simple function of ionic strength (I) and are represented as ... 

The distinction between "pure" electrostatic effects and ion association is somewhat arbitrary for example many workers have used an extended Debye-Huckel equation with fixed ion-size parameters to compute the activity coefficients, and assigned all other nonideality to ion-pairing equilibria (3, 13, 14). Recent reviews have surveyed more elaborate solvation-based thermodynamic theories of electrolytes (15, 16). 

It is found that in many cases experimental values of conductances do not agree with theoretical values predicted by the Onsager equation (see Equation (4.18)) and that mean ion activity coefficients cannot always be properly predicted by the Debye-Huckel theory. It was suggested by Bjerrum that, under certain conditions, oppositely charged ions of an electrolyte can associate to form ion pairs. In some circumstances, even association to the extent of forming triple or quadruple ions may occur. The most favourable situation for association is for smaller ions with high charges in solvents of low dielectric constant. Hence such phenomena occur to a usually small extent in water. 

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