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

The photodegradation of ascorbic acid (AH ₂ ) 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 (k _obs ) 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 (k ₀ ) 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 k _obs for the photodegradation of AH ₂ in the presence of NA are about 2 fold compared to those of the AH ₂ alone. The second-order rate constants (k ^' ) for the photochemical interaction of AH ₂ 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 AH ₂ to give ascorbyl anions (AH ^- ) which are more susceptible to photodegradation compared to the neutral molecule (AH ₂ ). NA appears to undergo photochemical interaction with AH ₂ during the reaction by acting as an electron acceptor to enhance its rate of photodegradation. The concentrations of AH ₂ 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.
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