Voltammetric assay of rifampicin and isoniazid drugs, separately and combined in bulk, pharmaceutical formulations and human serum at a carbon paste electrode

The electrochemical oxidation of both the rifampicin (RIF) and isoniazid (INH) drugs has been investigated by cyclic and square-wave voltammetry in Britton-Robinson buffers (pH 2-11) at a carbon paste electrode. The oxidation of rifampicin generated a well-defined pH-dependent quasi-reversible anodic-cathodic peak couple corresponding to a mechanism involving the transfer of two electrons/two protons, typical to that of hydroquinones, in addition to an irreversible anodic peak at a more positive potential which may be due to the oxidation of phenolic hydroxyl group. For the isoniazid, an irreversible anodic peak was observed, which may be attributed to the irreversible oxidation of the amide moiety of the drug molecule. A validated square-wave adsorptive anodic stripping voltammetric procedure was described to assay the two drugs separately or combined in pharmaceutical formulations and human serum. The recoveries of RIF in rimactane^® capsules (300 mg RIF) and INH in isocid^® tablets (200 mg INH) were found to be 98.57±0.81% and 100.57±0.74%, respectively. The proposed procedure was also successfully applied to simultaneous assay of rifampicin and isoniazid drugs combined in rimactazid tablets (150 mg INH+300 mg RIF) with recoveries of 98.79±0.97% and 99.54±0.74%, respectively, without the necessity for sample pretreatment or time-consuming extraction steps prior to the analysis. The results were favorably compared to those obtained by the reported USP method. Moreover, the proposed procedure was successfully applied to simultaneous assay of both drugs in human serum samples with limits of detection and quantitation of 5×10⁻⁸ and 1.7×10⁻⁷ M for RIF and 6.1×10⁻⁸ and 2×10⁻⁷ M for INH.