DETERMINATION OF THE THERMODYNAMIC VALUES OF THE FIRST AND 2ND DISSOCIATION-CONSTANTS OF ASPARTIC-ACID IN AQUEOUS-SOLUTIONS AT 298.15 K BY MEANS OF FORMULAS FOR IONIC ACTIVITY-COEFFICIENTS

A simple equation originating from the Debye-Huckel theory was used for activity coefficients of ionic species in thermodynamical studies of acidic solutions of aspartic acid al 298.15 K. The thermodynamic dissociation constants of this acid and the two ionic parameters for this equation were determined from experimental results of galvanic cells with and without a liquid junction. Of the cells of the latter type, those containing a hydrogen and a silver-silver chloride electrode in solutions of this acid were used. The experimental data for these cells were taken from the literature. Of the cells with a liquid junction, those containing a glass electrode were used, and for these cells the experimental data were measured by potentiometric titrations of aspartic acid solutions at several constant ionic strengths. The estimated dissociation constants and parameter values were tested with all electrochemical data found in the literature for this purpose. It is shown that almost all data used in the tests can be interpreted within experimental error by means of the new dissociation constants and ionic activity coefficient formulas. These data include ionic strengths up to a molality of 0.4 mol kg(-1). The thermodynamic values for the dissociation constants determined here (i.e. the values of K-a.1 = 0.0102 and K-a.2 = 0.000130) also agree quite well with the values suggested previously in the literature. For the potentiometric titrations, a new method was used to estimate the correction of the imperfections in the glass electrode cells including the unknown liquid junction potentials.