DISSOCIATION QUOTIENT OF BENZOIC-ACID IN AQUEOUS SODIUM-CHLORIDE MEDIA TO 25-DEGREES-C

The dissociation quotient of benzoic acid was determined potentiometrically in a concentration cell fitted with hydrogen electrodes. The hydrogen ion molality of benzoic acid/benzoate solutions was measured relative to a standard aqueous HCl solution at seven temperatures from 5 to 250 degrees C and at seven ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The molal dissociation quotients and selected literature data were fated in the isocoulombic (all anionic) form by a six-term equation. This treatment yielded the following thermodynamic quantifies for the acid dissociation equilibrium at 25 degrees C and 1 bar: logK(a) = -4.206+/-0.006, Delta H-a(o)=0.3+/-0.3 kJ-mol(-1), Delta S-a(o)=-79.6+/-1.0 J-mol(-1)-K-1, and Delta C-p;a(o) - -207+/-5 J-mol(-1)-K-1. A five-term equation derived to describe the dependence of the dissociation constant on solvent density is accurate to 250 degrees C and 200 MPa.