The cyclic voltammetric behavior of minoxidil was studied in a buffer with pH 3. Contradictory to that mentioned in a previously published work, the cyclic voltammogram of minoxidil exhibited a single 2-electron irreversible reduction wave in a buffer with pH 3. This wave was attributed to the reduction of the N›O bond. The cathodic differential pulse wave height decreased on the increase of pH till it disappeared in solution with pH 7.2. The quantitative trace determination of minoxidil was studied at a hanging mercury drop electrode by adsorptive cathodic stripping voltammetry. A fully validated sensitive procedure based on controlled adsorptive accumulation of the drug onto a HMDE was developed for its direct determination. Accumulation of minoxidil was found to be optimized in 0.1 M Britton–Robinson buffer with pH 2.0 as supporting electrolyte under the following conditions: accumulation potential –0.2 V, accumulation time 40 s, scan rate 40 mV s–1 and pulse height 50 mV. The proposed procedure was applied successfully for determination of minoxidil in its topical solution and illegal shampoo and cream. The mean recoveries of the minoxidil were 99.8, 97.8 and 96.7% and with RSD of 0.86, 1.24 and 1.89% in pharmaceutical topical solution, shampoo and cream, respectively.

Department of Medicinal Chemistry Faculty of Pharmacy Kermanshah University of Medical Sciences Kermanshah Islamic Republic of Iran

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