Ascorbic acid assisted photodegradation of methylcobalamin using corrective irrelevant absorption spectrophotometric...

Photodegradation of drug substances leads to the formation of known and unknown degradation products. These unknown degradation products interfere and give erroneous results because of absorption on analytical wavelengths. This interference could be eliminated using the correction of irrelevant absorbancies. This study is based on the application of linear and non-linear correction of irrelevant absorption for the determination of methylcobalamin (MC) and hydroxocobalamin in the photolytic degradation assisted by ascorbic acid (AH₂). MC follows first-order degradation kinetics and the rate of degradation (k_obs) ranges from 1.99-2.34 × 10^-2, min^-1 at pH 2.0-12.0. The second-order rate constants (k₂) for the photochemical interaction of MC and AH₂ are in the range of 17.9-60.3 × 10^-2 M^-1, min^-1 (acidic region) and 10.3-24.6 × 10^-2 M^-1, min^-1 (alkaline region). The k₂-pH profile was found to be bell-shaped and the maximum rate of degradation in the presence of AH₂ is at pH 5.0 (60.3 × 10^-2 M^-1, min^-1) due to the protonation of MC. However, in alkaline pH, the rate of photodegradation decreases due to the ionization form of AH₂ which is AH^- species.