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 tempera ture range 25-40 degreesC, in presence of 0.5 M KCl. The values of the enth alpy 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 evaluate d at 25 degreesC. The data were explained from the electronic character of the substituent.