Dissociation constant of 2,6-dihydroxybenzoic acid in water and in deuterium oxide at 25°C

Abstract

The dissociation constant of 2,6-dihydroxybenzoic acid has been determined at 25°C in ordinary water by conductance measurements and in both ordinary water and deuterium oxide by means of a cell with glass and silver-silver chloride electrodes and without liquid junctions. Both methods give pK_H values which increase with the ion-size parameter a and which are identical (pK_H= 1.229 ± 0.045) when 10¹⁰a/m = 6. The isotope effect ΔpK = pK_D–pK_H was also found to depend on a and abnormally high values (e.g. ΔpK = 0.474 when 10¹⁰a/m = 6) consistent with internal hydrogen bonding in the 2,6-dihydroxybenzoate anion were observed.

The pK values were extremely sensitive to the precision of the e.m.f. measurements and a technique with which a precision of 0.05 mV could be achieved with high resistance glass electrodes in H₂O and D₂O was developed. Activity coefficients obtained for HCl and DCl confirmed that the method was accurate and precise.