Voltammetric behaviour of ascorbic acid at a graphite—epoxy composite electrode chemically modified with cobalt phthalocyanine and its amperometric determination in multivitamin preparations

Cyclic voltammetry was used to investigate the electrochemical behaviour of ascorbic acid at a carbon—epoxy composite electrode modified with the electron mediator cobalt phthalocyanine. The modified electrode reduced the overpotential necessary for the oxidation of the vitamin by approximately 150 mV to 0.21 V vs. The saturated calomel electrode; the process was dependent on the pH of the supporting electrolyte, but independent of ionic strength over the range studied. The relative standard deviation (r.s.d.) of the peak heights of the cyclic voltammograms was 0.81% for a 1 × 10^?4 M ascorbic acid solution (n = 7). The optimum supporting electrolyte was found to be 0.05 M phosphate buffer (pH 5).Amperometry in stirred solutions was then done at an applied potential of +0.25 V. The limit of detection was 0.65 ng ml^?1 and the calibration graph was linear in the range 175 ng ml^?1?50 μg ml^?1. The method was used to determine ascorbic acid concentrations in single- and multivitamin preparations; the recovery was 97.86% for the vitamin added to one preparation. The r.s.d. for the analyses in these samples was about 5%. For comparison, the vitamin was also determined in these tablets using LC with UV detection at 254 nm; the correlation coefficient for the levels determined was 0.9989 (p = 0.0007).