Abstract
The electrochemical behaviour of Geshoidin was investigated at a glassy carbon electrode in mixtures of citric acid and di-sodium hydrogen orthophosphate aqueous buffer system over a wide pH range (pH 2-11) using cyclic voltammetry. Chemically irreversible single oxidation and reduction peaks were obtained in the potential and pH range investigated. Variations in the peak potential and peak current of the oxidation peak have been observed as function of pH. The wave characteristics, the reversibility of the reactions, the diffusion coefficient and the number of electrons transferred have been studied. Linear sweep voltammetry was applied for the voltammetric determination of Geshoidin and a linear calibration curve over the range 1.00 x 10-6 - 1.00 x 10-4 M Geshoidin was achieved. The detection limit was found to be 5.00 x 10-7 M Geshoidin. For eight successive determinations of 1 x 10-5 M Geshoidin, a relative standard deviation (RSD) of 3.2 % was obtained. The voltammetric method was applied to the direct determination of Geshoidin in Gesho. The absorption spectra of Geshoidin are interpreted in terms of structural changes caused by protonation and deprotonation of the molecule as a result of changes in pH. The pKa values of the compound have been determined from the voltammetry and spectrophotometry measurements. The superoxide anion scavenging ability of Geshoidin was examined by differential pulse voltammetry and its antioxidant activity has been compared with natural antioxidants. KEY WORDS: Gesho, Geshoidin, Rhamnus prionoides, Electrochemical behaviour, Voltammetric determination, Spectrophotometry, Antioxidant activity Bull. Chem. Soc. Ethiop. 2007, 21(2), 189-204.
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