A MODIFIED SELF-CONSISTENT LOCAL COMPOSITION MODEL FOR THE THERMODYNAMIC PROPERTIES OF SINGLE AND MIXED ELECTROLYTES

In the recently developed self-consistent-local composition (SCLC) the ory (Ananth and Ramachandran, 1990) the osmotic and activity coefficie nts in single electrolyte solutions are expressed as sums of several i ndependent contributions. In the present work, the approximate proport ionality between certain pairs of these contributions is identified an d the model is reformulated with fewer and more orthoganal parameters. The numerical difficulties experienced with the original model in the evaluation of parameters and in the extension of the calculations to mixed electrolyte systems are thus eliminated. The simplified model is shown to correlate the activity and osmotic coefficient data of large number of single and those of mixed electrolyte systems with good acc uracy. The osmotic and activity coefficient of single electrolyte solu tions are expressed in terms of two binary adjustable parameters and m ixed electrolyte systems containing two salts and one solvent are expr essed in terms of only one adjustable parameter in addition to the bin ary parameters determined from the component single electrolyte system s. The theory contains no additional adjustable parameters in the case of multielectrolyte (more than two salts) systems. The model is found to represent satisfactorily the behavior of electrolyte solutions at different temperatures with a single set of temperature-independent pa rameters.