Dissociation constants for citric acid in NaCl and KCl solutions and their mixtures at 25 degrees C
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abstract
The constants for the dissociation of citric acid (H3C) have been determined from potentiometric titrations in aqueous NaCl and KCl solutions and their mixtures as a function of ionic strength (0.05-4.5 mol-dm -3) at 25C. The stoichiometric dissociation constants (K i*) H3C H+ + H2C -, K1* = K1(H3C)/{ (H+)(H2C-)} H2C- H+ + HC2-, K 2* = K2(H2C-)/{ (H+)(HC2-)} HC 2- H+ + C3-, K3* = K3(HC2-)/{(H+) (C3-)} were used to determine Pitzer parameters for citric acid (H3C), and the anions, H2C-, HC2-, and C3-. The thermodynamic constants (Ki) needed for these calculations were taken from the work of R. G. Bates and G. D. Pinching (J. Amer. Chem. Soc. 71, 1274; 1949) to fit to the equations (T/K): ln K1 = -646.52280 + 14264.1855/T + 115.34510 ln T - 0.2204T, = 0.0032 ln K2 = -52.19970 + 1842.97387/T + 11.19421 ln T - 0.05487T, = 0.0023 ln K3 = -129.89305 + 394.04129/T + 25.36088 ln T - 0.09394T, = 0.0021 The values of Pitzer interaction parameters for Na+ and K+ with H3C, H2C -, HC2-, and C3- have been determined from the measured pK values. These parameters represent the values of pK 1*, pK2*, and pK3*, respectively, with standard errors of = 0.003-0.006, 0.015-0.016, and 0.019-0.023 for the first, second, and third dissociation constants. A simple mixing of the pK* values for the pure salts in dilute solutions yield values for the mixtures that are in good agreement with the measured values. The full Pitzer equations are necessary to estimate the values of pK i* in the mixtures at high ionic strengths. The interaction parameters found for the mixtures are Na-K-H2C = -0.00823 0.0009; Na-K-HC = -0.0233 0.0009, and Na-K-C = 0.0299 0.0055 with standard errors of (PK1) = 0.011, (pK2) = 0.011, and (pK3) = 0.055.
