Activity coefficients of glycine, DL-serine and DL-valine in aqueous solutions containing nitrates at 298.15K

Both an electrochemical and an isopiestic methods were used to measure the mean activity coefficients of amino acids in water-electrolyte-amino acid systems. The electrochemical method used an electrochemical cell with an anion and a cation ion selective electrodes, each measured versus a double junction reference electrode. The activity coefficients of the electrolyte were obtained from the e.m.f measurements for four systems at 298.15K. The amino acids were glycine, DL-serine and DL-valine and the electrolyte were KNO3 and NaNO3. The isopiestic method was applied to measure the activity of water for the system water + DL-serine + KNO3 at 298.15K and the results were compared with those from electrochemical method. The activity coefficients of the amino acids in the ternary systems were obtained from those of the electrolyte measured by the electrochemical method or from those of the water measured by isopiestic method, according to the cross differential relation.Two models were used to correlate the measured data. First the excess Gibbs free energy model proposed by Khoshkbarchi and Vera (1996a) with the NRTL equation was used to correlate the activity coefficient data. A simple model derived from a modification of a more complex model based on the perturbation theory (Khoshkbarchi and Vera, 1996c) was proposed. The model was applied to correlate the activity coefficients and solubilities of amino acids in binary aqueous solutions and the activity coefficients in ternary systems with satisfactory results