Dissociation quotients for citric acid in aqueous sodium chloride media to 150°C

Abstract

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Hückel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantitites for the first dissociation equilibrium at 25°C: logK _1a=−3.127±0.002, ΔH ^o_1a =4.1±0.2 kJ-mol⁻¹, ΔS ^o_1a =−46.3±0.7 J-K⁻¹-mol⁻¹, and ΔCp ^o_1a =−162±7 J-K⁻¹-mol⁻¹; for the second acid dissociation equilibrium at 25°C: logK _2a =−4.759±0.001, ΔH ^o_2a =2.2±0.1, ΔS ^o_2a =−83.8±0.4, and ΔCp ^o_2a =−192±15, and for the third dissociation equilibrium at 25°C: logK _3a=−6.397±0.002, ΔH ^o_3a =−3.6±0.2, ΔS ^o_3a =−134.5±0.7, and ΔCp ^o_3a =−231±7.