Dissociation constants of substituted aryl azo barbituric and thiobarbituric acids

The acid dissociation constants of 5-(substituted phenylazo) barbituric and thiobarbituric acids were determined in aqueous media and in 25\%-75\% (v/v) ethanol-water and dioxane-water mixtures potentiometrically in the temperature range 25-40 degreesC, in presence of 0.5 M KCl. The values of the enthalpy change DeltaH (kJ mol(-1)), entropy change DeltaS (J K-1 mol(-1)) and the free energy change DeltaG (kJ mol(-1)) of these compounds were evaluated at 25 degreesC. The data were explained from the electronic character of the substituent.