Electroreduction and spectrophotometric studies of some pyrazolyl-azo dyes derived from 3-acetylamino-1-phenyl-5-pyrazolone in buffered solutions

Four pyrazolyl-azo dyes derived from 3-acetylamino-1-phenyl-5-pyrazolone were prepared and characterized. The electroreduction of the prepared azo dyes was studied at a mercury electrode in the universal buffer at pH 2–12.2 containing 10% DMF using dc-polarography, cyclic voltammetry and controlled-potential coulometry. The dc-polarograms of the examined pyrazolyl-azo dyes (I–IV) exhibited a single 4-electron irreversible cathodic wave in solutions of pH values: <7, <9.4, <10.5, and <9.4, respectively, which can be attributed to reduction of the NN– group of the dye to the corresponding amine via the consumption of four electrons. Increase of pH resulted in the reduction wave splitting into two, the limiting current of the 2nd wave increasing at the expense of the first until the latter disappeared completely in solutions of pH ≥9.5 (I), ≥11.2 (II) and ≥12.2 (III and IV). This behavior indicated that both the acidic and basic forms of the azo dye molecule were electro-active at the mercury electrode. The ionization constants (pKa) of the dyes were determined by means of dc-polarographic and spectrophotometric measurements. The electrode reaction pathway of the studied compounds was elucidated and discussed.