Electrochemical behaviour of dipyrone (metamizole) and others pyrazolones

Abstract The electrochemical oxidation of dipyrone (MTM) in aqueous medium was characterized using antipyrine (AA), 4-aminoantipyrine (4AA), 4-methyl-aminoantipyrine (MAA) and 4-dimethyl-aminoantipyrine (DMAA) as model molecules for the elucidation of all MTM voltammetric signals. The MTM and the other pyrazolones show up to four oxidation electrochemical processes. The voltammograms obtained in AA solutions presented an irreversible electrochemical oxidation process involving one electron at E ap 3, which is common to all pyrazolone derivatives, while the amino pyrazolones present electrochemical oxidation processes at E ap 0 or E ap 1 . The stabilization of the oxidation products depends on different effects: the proton release added to the thermodynamic stability, in the case of the imine formation at E ap 0 (4AA and MAA) and the hyperconjugation (σ-stabilization) in the case of iminium formation (DMAA and MTM) at E ap 1 . The process observed at E pa 0 corresponds to the pH-dependent oxidation of the primary and secondary enamines, while the process observed at E ap 1 occurs in the tertiary enamines, is pH independent. The oxidation peak potential follows the order: MAA −1 to 100 μmol L −1 with a LOD of 1.94 and 2.97 μM for DMAA and MTM, respectively, LOQ of 6.48 and 9.91 μM ( n  = 10) and, sensitivity of 0.96 μA/μM for DMAA and 0.92 μA/μM; with a recoveries of 95–105% for MTM.