Riboflavin (vitamin B₂) sensitized photooxidation of ascorbic acid (vitamin C): A kinetic study

Ascorbic acid (AH₂) photoxidation sensitized by riboflavin (RF) has been studied between pH 2.0 and 12.0 in ambient air and anaerobic environment using UV and visible irradiation sources. The kinetics of AH₂ degradation in aqueous medium along with RF is found to be first-order for its photodegradation. AH₂ photolysis rate constants in aerobic and anaerobic conditions with RF (1.0-5.0 × 10^-5 M) are 0.14-3.89 × 10^-2 and 0.026-0.740 × 10^-2 min^-1, respectively. The rate constants (k₂) of second-order kinetics for AH₂ and RF photochemical interaction in aerobic and anaerobic conditions are in the range of 0.24-3.70 to 0.05-0.70 × 10^-3 M^-1 min^-1, respectively, which manifests that increasing the RF concentration also increases the rate of photodegradation (photooxidation) of AH₂. The k₂ versus pH graph is bell-shaped which indicates that increasing the pH increases photolytic degradation rate of AH₂ with RF. Increasing the pH results in the increased ionization of AH₂ (ascorbyl anion, AH^-) and redox potential which leads to the higher rates of photodegradation of AH₂. Two-component spectrophotometric (243 and 266 nm, AH₂ and RF, respectively) and high-performance liquid chromatography (HPLC) methods have been used to determine the concentration of AH₂ and RF in pure and degraded solutions. The results obtained from these two methods are compared using a student t-test which showed no noteworthy difference between them.