Re-evaluation of the second thermodynamic dissociation constants of α-alanine, valine, and leucine using potentiometric data measured for aqueous potassium chloride solutions at 298.15 K

Equations were developed for the calculation of the second stoichiometric (molality scale) dissociation constants (Km2) of α-alanine, valine, and leucine in aq. KCl solutions at 298.15 K from the revised thermodynamic dissociation constants (Ka2) of these acids and the ionic strength (Im) of the solutions. The ionic strength of the solutions considered in this study is determined mostly by KCl alone, and the equations for Km2 were based on the single-ion activity coefficient equations of the Hückel type. The existing literature data obtained from Harned cell measurements and new potentiometric titration data were used to revise the Ka2 values of the three amino acids. The values (1.295 ± 0.013) × 10–10, (1.894 ± 0.009) × 10–10, and (1.685 ± 0.011) × 10–10 were obtained from the new potentiometric titration data for α-alanine, valine, and leucine, respectively. For alanine and valine, the new Ka2 values are also supported by the Harned cell data used, but the value (1.80 ± 0.02) × 10–10 obtained for leucine from these data is significantly different. The potentiometric values are recommended here. The activity coefficient equations for the calculation of Km2 values were also determined from the new potentiometric data. By means of the activity coefficient equations obtained for these three amino acids for KCl solutions, Km2 can be evaluated almost within experimental error up to an Im of about 1.0 mol kg–1. The Km2 values calculated by this method are also compared with the values suggested in the literature.Key words: ionic strength dependence, stoichiometric dissociation constant, Debye-Hückel equation, potentiometry, α-alanine, valine, leucine.