Determination of stoichiometric dissociation constants of lactic acid in aqueous salt solutions at 291.15 and at 298.15 K

Abstract

Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant, K m, of lactic acid in aqueous salt solutions at 291.15 and 298.15 K from the thermodynamic dissociation constant, K a, of this acid and from the ionic strength, I m, of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study, and the equations for K m were based on the single-ion activity coefficient equations of the Hückel type. New data measured by potentiometric titrations in a glass electrode cell at 298.15 K and the literature data obtained by Larsson and Adell with quinhydrone (qh) electrode cells at 291.15 K were used in the estimation of the parameters for the Hückel equations (HE) of lactate ions in NaCl and KCl solutions. The Harned cell data measured at 298.15 K by Nims and Smith were used to obtain the activity parameters for lactate ions in dilute LiCl, BaCl 2 and SrCl 2 solutions. The conductance data measured at 298.15 K by Martin and Tartar were used to determine the thermodynamic value of the dissociation constant of lactic acid. By means of the calculation method suggested in this study, K m can be obtained almost within experimental error at least up to I m of about 0.5 mol kg −1 for lactic acid in NaCl and KCl solutions at the two temperatures considered. The K m values obtained by this method were compared to those obtained by the calculation methods presented recently in the literature for a general treatment of thermodynamics of weak acids in NaCl and KCl solutions.