DISSOCIATION QUOTIENTS FOR CITRIC ACID IN AQUEOUS SODIUM CHLORIDE MEDIA TO 150-DEGREES-C

摘要

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150 degrees 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-Huckel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantities for the first dissociation equilibrium at 25 degrees C: log K-1a = -3.127+/-0.002, Delta H(1a)degrees = 4.1+/-0.2 kJ-mol(-1), Delta S(1a)degrees = -46.3+/-0.7 J-K-1-mol(-1), and Delta Cp(1a)degrees = -162+/- 7 J-K-1-mol(-1); for the second acid dissociation equilibrium at 25 degrees C: log K-2a = -4.759+/-0.001, Delta H(2a)degrees = 2.2+/-0.1, Delta S(2a)degrees = -83.8+/-0.4, and Delta Cp(2a)degrees = -192+/- 15, and for the third dissociation equilibrium at 25 degrees C: log K-3a = -6.397+/-0.002, Delta H(3a)degrees = -3.6=/-0.2, Delta S(3a)degrees = -134.5+/-0.7, and Delta Cp(3a)degrees = -231+/-7. [References: 39]