Electrochemical behaviour of dipyrone (metamizole) and others pyrazolones

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 Eap3, which is common to all pyrazolone derivatives, while the amino pyrazolones present electrochemical oxidation processes at Eap0 or Eap1. 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 Eap0 (4AA and MAA) and the hyperconjugation (σ-stabilization) in the case of iminium formation (DMAA and MTM) at Eap1. The process observed at Epa0 corresponds to the pH-dependent oxidation of the primary and secondary enamines, while the process observed at Eap1 occurs in the tertiary enamines, is pH independent. The oxidation peak potential follows the order: MAA < 4AA < DMAA < MTM and it was demonstrated that DMAA in an aqueous medium can simulate the MTM in an aprotic medium; therefore, the analytical MTM determination can be performed using the DMAA aqueous analytical curve. DMAA and MTM analytical curves, presented a linear range from 10 μmol L−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.