Development of Calibration and Standard Addition Polarographic Determination of Ascorbic Acid

Abstract

Aims: Effect of pH on polarographic waves of ascorbic acid were studied by recording polarograms of ascorbic acid solution, between 0 to 400 mV at different pH (0.065 to 9.6) using 0.008% solution of gelatin and 0.05 M potassium hydrogen phthalate buffer (containing 0.25% oxalic acid) as maxima suppressor and supporting electrolyte respectively. Methods: Ascorbic acid is strong reducing agent and produces an anodic wave which shifts with pH. There is no significant change in height of wave with change in pH from 2.25 to 4.85. For determination of ascorbic acid pH 4.0 is chosen. Ascorbic acid present in synthetic sample is determined by calibration, external standard addition and internal standard addition methods. The results obtained are in good agreement with the quoted values. Result: The number of electrons taking part in the reversible reaction is found to be 2. The half-wave potential is found to be independent of the ascorbic acid concentration. peak current is proportional to the concentration of ascorbic acid in the range 10-6-10-3M and the reproducibility is better than ±1 per cent. Chloride and sulphur compounds, such as sulphides and thiols, do not interfere, and sulphite can be determined at the same time as the ascorbic acid. Some substituted phenols interfere but can often be detected by reversing the direction of polarisation. Reductones interfere but tin(II) and manganese(II) do not. A method has also been developed to determine ascorbic acid in the presence of an excess of iron. Different extraction media are discussed in terms of their influence on the redox potential of iron. Comparative titrimetric determinations of ascorbic acid in some fruits, vegetables and beverages gave higher results than the voltammetric method (6). A polarographic study of the oxidation mechanism of L-ascorbic acid and of the reduction mechanism of dehydro-L-ascorbic acid was carried out in an acid medium. For L-ascorbic acid, the oxidation process involves a two electron transfer. The polarographic curve shows that the limiting current is governed by diffusion. On the rising portion of the wave, the two electron oxidation process consists of two consecutive one electron transfers, the second being the rate determining step. The reaction orders, together