Electrochemical behaviour of azo dyes used as excipients in pharmaceutical preparations

Electrochemical behaviour of five azo dyes, of which three with one (Allura Red AC, Red 2G, and Sunset Yellow FCF) and two with two azo groups (Brilliant Black BN and Chocolate Brown HT), has been exhaustively studied at a glassy carbon electrode in a buffered aqueous solution of physiological environment using cyclic voltammetry to describe their both electrochemical pathways and predict the most probable reaction products. This study has confirmed and deepened recently discovered insights into the electrochemical properties of azo dyes, which indicate an electron-transfer–electron-transfer–chemical reaction (EEC) mechanism in their cathodic reduction pathways and an electron-transfer–chemical reaction–chemical reaction (ECC) mechanism in their anodic oxidation pathways. Therefore, for diazo dyes, both reaction pathways proceed via two-step mechanisms: EEC–EEC and ECC–ECC. Typical redox couples, reflecting quasi-reversible behaviour between formed variable substituted (sulphonated) 1-amino-2-naphthol with the corresponding quinoneimine derivatives, were observed in the subsequent cyclic voltammograms. Here it is striking that in addition to these highly reactive products, other toxic sulphonated aminobenzenes (reduction) or even carcinogenic sulphonated nitrosobenzenes (oxidation products) can also be formed. In addition, it can be assumed with high probability that the formation of quinones can occur by hydrolysis of the above-mentioned quinoneimine derivatives, as evidenced by the shift of cyclic voltammograms for individual redox couples.