APPLICABILITY OF THE MODIFIED PITZER EQUATIONS TO THE FITTING OF ACTIVITY-COEFFICIENTS IN ELECTROLYTE-AMINO-ACID AQUEOUS MIXTURES - THE CASE OF NACL-ALANINE AT 298.15 K(BETA)

To prove the applicability of the Fitter equations, modified by us aft er taking into account the dependence of the solution concentration on the mixing parameters related with the binary solute-solute interacti ons, we are presenting the analysis of accurate experimental activity coefficient values here determined for the mixed system NaCl + beta-al anine + H2O up to medium concentrations of both NaCl and the aminoacid . These activity coefficient values were obtained from the emfs of the galvanic cell: Na-glass \ beta-Alanine (m(A)), NaCl (m(B) = I), AgCl \ Ag. The fitting was done by considering only the binary interaction mixing parameters, i.e. by neglecting the ternary ones (ion-zwitterion -zwitterion and ion-ion-zwitterion). The standard deviation thus found for the fitting (+/-0.16 mV, i.e. +/-0.003 in ln gamma(+/-)) was of t he same order as the accepted experimental uncertainty. Activity coeff icients for beta-alanine in these aqueous solutions, calculated from t he use of these modified Fitter equations, are also presented. (C) 199 8 Elsevier Science S.A. All rights reserved.