Vitamin methods

Abstract

SO2 in acid solution reduces 2,6-dichlorophenolindophenol instantaneously and causes serious error. Two methods are described for estimating ascorbic acid in such extracts: (a) prior removal of the SO2 from a 5% metaphosphoric acid extract of the food, by a current of N2 and exhaustion in vacuo; (b) addition of acetone to a conc. of 20% to the metaphosphoric acid prior to titration, to form the acetone-bisulphite complex which does not reduce indophenol dye. The titration must be completed in 40-60 sec. By both methods, the limits of error average less than 1%. The acetone method gives results as reliable as those obtained by the usual method, even when cysteine or glutathione are present in as high conc. as ascorbic acid.