L-ascorbic acid determination in pharmaceutical formulations using a biosensor based on carbon paste modified with crude extract of zucchini (Cucurbita pepo)

A biosensor based on carbon paste modified with crude extract of zucchini (Cucurbita pepo) as a source of peroxidase is proposed for determining L-ascorbic acid in pharmaceutical formulations. This enzyme in the presence of hydrogen peroxide catalyses the oxidation of hydroquinone to p-quinone whose electrochemical reduction back to hydroquinone was obtained at peak potential of -0.14V. Thus, when L-ascorbic acid is added to the solution, this acid can reduce chemically p-quinone to hydroquinone and/or reduce hydrogen peroxide, decreasing the peak current obtained proportionally to the increase of its concentration. The recovery of L-ascorbic acid from five samples ranged from 98.1 to 102.1% and a rectilinear calibration curve for L-ascorbic acid concentration from 2.0x10(-4) to 5.5x10(-3) mol L-1 (r=0.9992) was obtained. The detection limit was 2.2x10(-5) mol L-1 and relative standard deviation was < 1.3% for a solution containing 4.0x10(-3) mel L-1 L-ascorbic acid, 7.0x10(-3) mol L-1 hydroquinone and 2.0x10(-4) mol L-1 hydrogen peroxide. The results obtained for L-ascorbic acid in pharmaceutical formulations using the proposed biosensor and those obtained using the Pharmacopeia method are in agreement at the 95 % confidence level.