Photochemical interaction of ascorbic acid and nicotinamide in aqueous solution: A kinetic study.

The photodegradation of ascorbic acid (AH2) in the presence of nicotinamide (NA) at pH 2.0-12.0 has been studied using a 30 W UV radiation source. The reaction follows first-order kinetics and the values of apparent first-order rate constants (kobs) at 1 × 10-3 M NA concentration range from 1.17 (pH 2.0) to 3.61 × 10-3 min-1 (pH 12.0). The values of these rate constants (k0) in the absence of NA range from 0.50 (pH 2.0) to 1.75 × 10-3 min-1 (pH 12.0), indicating that the values of kobs for the photodegradation of AH2 in the presence of NA are about 2 fold compared to those of the AH2 alone. The second-order rate constants (k') for the photochemical interaction of AH2 and NA are in the range of 0.67 (pH 2.0) and 1.86 × 10-3 M-1 min-1 (pH 12.0). The k'-pH profile shows a gradual increase in the rate as a function of pH. This is due to the ionization of AH2 to give ascorbyl anions (AH-) which are more susceptible to photodegradation compared to the neutral molecule (AH2). NA appears to undergo photochemical interaction with AH2 during the reaction by acting as an electron acceptor to enhance its rate of photodegradation. The concentrations of AH2 and NA in degraded solutions have been determined by a two-component spectrometric method at 243 and 261 nm (pH 2.0) with a precision of ±2%. The method has been validated and the results are comparable to the HPLC method.