Solid-Phase Extraction Cleanup for Determining Ascorbic Acid and Dehydroascorbic Acid by Titration with 2,6-Dichlorophenolindophenol

Abstract

Ascorbic acid is frequently determined by titration with 2,6-dichlorophenolindophenol. The determination is rapid, but the method is neither specific for ascorbic acid nor very sensitive. The coloring matter in the assay solution interferes with the visual endpoint, and iron(ll), copper(l), sulfite, and sulfhydryl substances such as cysteine and glutathione interfere with the color reaction. Sample cleanup by solid-phase extraction with C18 bonded silica was developed to remove the coloring matter. Extraction sorbent impregnated with 2,2′-bipyridyl, 2,9-dimethyl-1,10-phenanthroline (neocuproine) and Jv-ethylmaleimide removes Fe(ll), Cu(l), and sulfhydryl compounds, respectively. The procedure was applied to highly colored multivitamin pharmaceuticals, soft drinks, and fruit and vegetable juices. In contrast to the results from the original method, which is not applicable to such samples, the results obtained by the method incorporating cleanup were accurate and selective for ascorbic acid. The sample cleanup also permitted determination of dehydroascorbic acid by reducing it to ascorbic acid with cysteine and titrating the ascorbic acid formed with indophenol. As little as 3 μg ascorbic acid was determined by the method incorporating cleanup.

Extrusion of cassava starch with either variations in ascorbic acid concentration or pH

“…There are advantages in using indophenol as it reacts almost instantaneously in a fixed stoichiometry with ascorbic acid at all concentration levels; it acts as a self‐indicator; and because of the deep blue colour of its solutions, a very dilute titrant can be used without hampering visual detection of the end point. The last advantage accounts for the sensitivity of indophenol titration, which is much greater than those of other titrations ( Verma et al ., 1996 ).…”

Vitamin C: Sources, Functions, Sensing and Analysis

“…Many analytical techniques are used for the determination of vitamin C in different matrices, such as titrimetric [39], fluorimetric [40], spectrophotometric [41], high-performance liquid chromatography [42], enzymatic [43], kinetic [44,45] and electrochemical, etc.…”

Métodos Para a Determinação De Vitamina C Em Diferentes Amostras

“…Outra modificação para amostras com coloração e também para a análise de vitamina C total, consiste na prévia utilização de extração em fase sólida para remoção de compostos coloridos e interferentes. A redução do ácido dehidroascórbico é realizada com cloridrado de cisteína (VERMA et al, 1996).…”