A study of some pyridylazo dyestuffs as chromogenic reagents and the elucidation of the nature of their metal complex spectra

A series of dyestuffs having the same basic structure as 2-(2-pyridylazo)-5-diethyl-aminophenol (PADAP) but varying in their substituents on the 5-positions of both rings has been prepared. Dissociation constants for three such dialkylaminophenol derivatives have been determined, the pK(OH) values being of the same order as that reported for 4-(2-pyridylazo)resorcinol (PAR), while the pK(NH) values are lower. Two different 5-bromo-PADAP complexes are formed with the uranyl ion, one between pH 2 and 5 which is a neutral complex and probably a dimer, and the other an unstable neutral complex which forms between pH 5.5 and 8 and is a 1:1:1 ternary hydroxo-complex. A stable 1:1:1 ternary fluoro-complex is also formed between pH and 6 and 8. Double maxima occur in the spectra of all the metal complexes of these dyes. Evidence given shows the double peaks to originate from the one moiety and to be directly related to the presence of a substituent on the benzene ring para to the azo linkage. A tautomeric equilibrium with two imine tautomers in addition to the azo form of the complex is postulated and it is suggested that they correspond to the three composite bands given by computer analysis of the spectrum.