Determination of the Glass Electrode Parameters by Means of Potentiometric Titration of Acetic Acid in Aqueous Sodium or Potassium Chloride Solutions at 25°C

Abstract

Single-ion activity coefficient equations are presented for the calculation of stoichiometric (molality scale) dissociation constants Km for acetic acid in aqueous NaCl or KCl solutions at 25°C. These equations are of the Pitzer or Huckel type and apply to the case where the inert electrolyte alone determines the ionic strength of the acetic acid solution considered. Km for a certain ionic strength can be calculated from the thermodynamic dissociation constant Ka by means of the equations for ionic activity coefficients. The data used in the estimation of the parameters for the activity coefficient equations were taken from the literature. In these data were included results of measurements on galvanic cells without a liquid junction (i.e., on cells of the Harned type). Despite the theoretical difficulties associated with the single-ion activity coefficients, Km can be calculated for acetic acid in NaCl or KCl solutions by the Pitzer or Huckel method (the two methods give practically identical Km values) almost within experimental error at least up to ionic strengths of about 1 mol-kg−1. Potentiometric acetic acid titrations with base solutions (NaOH or KOH) were performed in a glass electrode cell at constant ionic strengths adjusted by NaCl or KCl. These titrations were analyzed by equation E = Eo + k(RT/F) ln[m(H+)], where m(H+) is the molality of protons, and E is the electromotive force measured. m(H+) was calculated for each titration point from the volume of the base solution added by using the stoichiometric dissociation constant Km obtained by the Pitzer or Huckel method. During each base titration at a constant ionic strength, Eo and k in this equation were observed to be constants and were determined by linear regression analysis. The use of this equation in the analysis of potentiometric glass electrode data represents an improvement when compared to the common methods in use for two reasons. No activity coefficients are needed and problems associated with liquid junction potentials have been eliminated.