Primary acidity constant of maleic acid in ordinary water and in deuterium oxide

Abstract

The primary acidity constant of maleic acid has been determined at 298.2 K in ordinary water by conductance measurements and in both ordinary water and deuterium oxide by precise e.m.f. measurements with glass and silver–silver chloride electrodes in a cell without liquid junctions. Both methods give pK_H values which increase with the applied ion-size parameter a and which are identical (pK_H= 2.038 ± 0.008) when 10¹⁰a/m = 5 is used. The isotope effect ΔpK = pK_D–pK_H decreases slightly with a but for all reasonable ion-size parameters it has an abnormally high value (e.g. ΔpK = 0.561 ± 0.012 when 10¹⁰a/m = 5) which may be due to internal hydrogen bonding in its anion. For 3⩽10¹⁰a/m⩽7 the pK_H-values are significantly higher than that generally accepted, and the ΔpK values support the lower of the two previous ones.