Moisture induced solid phase degradation of l-ascorbic acid: Part 1: a kinetic study using tristimulus colorimetry and a quantitative HPLC assay

Abstract

l-Ascorbic acid was found to degrade in the solid phase with discoloration under the influence of moisture in proportion to the moisture content. This degradation pattern was different to that in solution and followed zero order kinetics. The exclusion of air reduced the rate of reaction suggesting the degradation may proceed via an oxidative route but no evidence was found for the presence of dehydroascorbic acid. A method was developed for the determination of dehydroascorbic acid using an automated precolumn reduction reaction with dl-dithiothreitol. The degradation was found to be zero order and activation energy was been measured at 37.57 kJ mol−1 by high performance liquid chromotography (HPLC) assay and 33.30 kJ mol−1 by tristimulus colorimetry, resulting in a 12.8% difference between the two methods. Tristimulus colorimetry was more sensitive to the onset of degradation than HPLC assay, but it is non-specific. The purpose of this study was to obtain kinetic data on the rate of degradation of l-ascorbic acid alone under the influence of moisture and air and to identify whether tristimulus colorimetry could be used as a rapid and non-destructive means of monitoring for the degradation of l-ascorbic acid in the solid phase. Further studies to determine the degradation pathway and to identify the degradation products are to be reported in subsequent papers.