Metal chelates of cerium(III), thorium(IV) and dioxouranium(VI) with some heterocyclic azo dyes; Potentiometric and spectrophotometric studies

The stepwise formation constants of Ce3+, Th4+ and UO22+ complexes with four azo compounds based on 1-phenyl-2,3-dimethylpyrazoline-5-one nucleus namely, 4-phenylazo- (2-hydroxy,5-x) 1-phenyl-2,3-dimethyl-pyrazoline-5-one, where x = H (I), OH (II), COOH (III) and NH2 (IV) have been determined potentiometrically at different temperatures and ionic strengths in 30% (v/v) ethanol-water solutions, then the thermodynamic parameters are calculated. Negative values of both Delta H and Delta G are obtained indicating the exothermic and spontaneous nature of complexation reactions, whereas positive values of Delta S show that entropy consideration favor complex formation. The study at different ionic strengths shows that an increase in the latter causes a decrease in the pK values. The azo compounds are also tested as new reagents for the spectrophotometric determination of Ce3+, Th4+ and UO22+ ions in synthetic and natural solutions by extensive investigation of the optimum conditions favoring the formation of colored complexes.