Adsorptive stripping voltammetry of rifamycins at unmodified and surfactant-modiried carbon paste electrodes

The electrochemical behavior of the antibiotics rifampicin and rifamycin SV is investigated by cyclic voltammetry at carbon paste and in situ surfactant modified carbon paste electrodes. Both antibiotics adsorb on the unmodified electrodes and show a reversible redox process due to the oxidation of the 6,9-dihydroxynaphthalene moiety to the corresponding naphthoquinone. This process is used as analytical signal for developing adsorptive voltammetric methods for the determination of the antibiotics. Experimental parameters, such as pH of the supporting electrolyte, accumulation potential and time are optimized. After accumulation from acidic solutions (0.1 M KCl pH 2 or HCl 0.2 M) at -0.1 or 0 V for 3 min, the differential pulse oxidation peak current changes linearly with the antibiotic concentration in the range 3.5 x 10(-10) M -5.4 x 10(-9) M or 5 x 10(-11) M -1.0 x 10(-9) M for rifampicin and rifamycin SV, respectively. Rifamycin SV is not accumulated on carbon paste electrodes modified in situ with the anionic surfactant sodium dodecyl sulfate, whereas rifampicin is readily accumulated on this modified electrodes resulting in a signal enhancement and allowing rifampicin determinations without interference from rifamycin SV. On the other hand, selective determination of rifamycin SV in the presence of rifampicin is achieved by using carbon paste electrodes in situ modified with the cationic surfactant cetyltrimethylammonium chloride.