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. measure ments with glass and silver-silver chloride electrodes in a cell without liquid junctions. Both methods give pKh values which increase with the applied ion-size parameter a and which are identical (pKii = 2.038 + 0.008) when 1010 a/m = 5 is used. The isotope effect ApK = pKn —pKn decreases slightly with a but for all reasonable ion-size parameters it has an abnormally high value (e.g. ApK = 0.561 ± 0.012 when IO10 a/m = 5) which may be due to internal hydrogen bonding in its anion. For 3 < 1010 a/m<7 the pKn-values are significantly higher than that generally accepted, and the ApK values support the lower of the two previous ones.