Potentiometric Determination of Thermodynamic and Apparent Dissociation Constants by Nonlinear Least Squares Fitting

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Abstract
Potentiometric titration curve models have been developed for mono-, di- and triprotic weak electrolytes. Apparent dissociation constants or thermodynamic dissociation constants, corrected for ionic strengths of up to O.5M, can be determined by nonlinear least squares fitting of potentiometric data to the models. A commercially available program, PCNONLIN (an IBM PC version of NONLlN84), was utilized for the fitting, allowing the models to be portable. The models have been tested with theoretical data containing random or systematic error, and data for several model compounds of various weak electrolyte class. The method in this report has several advantages over traditional 'half neutralization' or graphical methods. First, the models are based on the analytic solution of the charge balance equation, with no simplifying approximations. Second, the method can determine dissociation constants in cases where no inflection point is present, such as for overlaping ionization processes or low concentration of compound. Finally, alterations in ionic strength during the titration were accounted for in the models.