Contribution to the understanding of oxicam ionization constants.

Determinations of the pKa-values by the spectrophotometric method indicate significant differences in the acidic behaviour between piroxicam and tenoxicam on the one hand and isoxicam on the other. In the case of the first named 2-pyridyl-substituted substances the two measurable and due to the UV-shift individually assignable pKa-values might be explained by the existence of zwitterionic species. In contrast to this isoxicam exhibits only one pKa-value for the enole-enolate system in the pH-range 1-14.