Thermodynamic modeling of quaternary (CaCl2 + Ca(NO3)2 + L-alanine + water) system based on potentiometric measurements at T = (298.2, 303.2 and 308.2) K: Experimental measurements and thermodynamic modeling

Abstract In this paper, the thermodynamic properties of a quaternary system including CaCl 2 and Ca(NO 3 ) 2 as mixed electrolytes in the aqueous L-alanine mixture, were reported using the potentiometric method. The potentiometric measurements were performed on the galvanic cells of the type: (a) Ag-AgCl|CaCl 2 (m C ), H 2 O|Ca–ISE, (b) NO 3 -ISE|Ca(NO 3 ) 2 ( m B ), L-alanine ( m A  = 0.2 mol kg −1 ), H 2 O|Ca–ISE, (c) Ag-AgCl|Ca(NO 3 ) 2 ( m B ), L-alanine ( m A  = 0.2 mol kg −1 ), H 2 O|Ca–ISE, and (d) Ag-AgCl|CaCl 2 ( m 1 ), Ca(NO 3 ) 2 ( m 2 ), L-alanine (0.2 mol kg −1 ), H 2 O|Ca–ISE, over total ionic strengths from 0.0100 to 4.0000 mol kg −1 at T  = (298.2, 303.2 and 308.2) K and P  = 0.1 MPa. Different series of the salt molal ratios r ( r = m C a C l 2 / m C a ( N O 3 ) 2 = 0.1, 1.0, 2.5, 5.0, 7.5, and 10) and pure Ca(NO 3 ) 2 in mixed solvent system containing 0.2 mol kg −1 of L-alanine were used for the potentiometric measurements. The Ca 2+ and NO 3 − ion selective electrodes were prepared in our laboratory using related ionophores. Data analyze was performed based on Pitzer ion interaction model. At the beginning, the mixed ionic interaction parameters ( θ ClNO 3 , ψ CaClNO 3 ) were investigated according to Pitzer graphical method. After that, thermodynamic properties such as the mean activity coefficients, the osmotic coefficients, the solvent activity, the excess Gibbs free energy, the excess enthalpy and the excess entropy for electrolyte solutions were calculated. Moreover, the ratio of L-alanine activity coefficients in presence of electrolyte ( γ A ) to values of this property in absence of electrolyte ( γ A 0 ) was obtained by Scatchard equation.