The literature on various aspects of ascorbic acid has been extensively reviewed to develop an understanding of its structural characteristics, the techniques of analysis and modes of degradation.
The present investigation involves the study of the photolysis of ascorbic acid at pH 1-11 and in selected rule solvents using a UV (254 nm) radiation source. The role of riboflavin as a sensitizer in the photolysis of ascorbic acid has been elaborated using a visible (tungsten lamp) radiation source. The intensity of the radiation sources has been determined by ferrioxalate actinometry.
Ascorbic acid in photolysed solutions has been assayed by a direct UV method (243 nm) without any interference from its photoproducts. In the presence of riboflavin, ascorbic acid has been assayed by a two-component spectrophotometer method (243 and 266 nm) which is specific and has reproducibility of the order of ±3%. A multicomponent spectrophotometric method has been used to assess the magnitude of interference from riboflavin photoproducts in the assay of degraded solutions. It has been estimated to amount 10 a maximum of about 10% in the analytical range for kinetic data and has been found to depend upon the reaction pH and irradiation time.
Ascorbic acid alone and in the presence of riboflavin is photolysed to dehydroascorbic acid (pH 1-11) and 2,3-dietogulonic acid (pH 9-11) by an apparent first-order kinetics. The first-order rate constants for the photolysis of ascorbic acid vary from 0.057x10-2 min-1(pH1.0) to 3.948x10-2 min-1 (pH11.0) indicating an increase in the rates of photolysis, due to ionisation of the molecule (pKa 4.2). The second-order rate constants for the bimolecular interaction between ascorbic acid and riboflavin range from 0.126x10-2 M-1min-1 (pH 1.0) to 11.440x10-2 M-1 min-1(pH 8.0) indicating greater susceptibility of ascorbate monoanion to interaction with riboflavin in the presence of visible light. Thus riboflavin appears to play the role of sensitizer in the photo degradation of ascorbic acid. This is indicated by its own relatively slow rate of photolysis (in equimolar concentrations with ascorbic acid) as compared to that when subjected to photolysis alone.
A pH-rate profile for ascorbic acid may be represented by a sigmoid curve suggesting the participation of ascorbic acid (unionised), ascorbate monoanion and ascorbate radical in the photolysis reaction and the susceptibility of these forms to UV radiation in the pH range studied. The pH at the midpoint of the curve for ascorbic acid photolysis is around 9.4 which may indicate the pH corresponding to the pKa of dehydroascorbic acid . The conjugate base of dehydroascorbic acid is not known. The pH-rate profile for the bimolecular interaction of ascorbic acid and riboflavin in the presence of light shows pH max at 8.0.
The influence of buffer salt concentration on the photolysis of ascorbic acid in the presence of riboflavin has been studied using divalent phosphate ions (HPO4 2-) An increase in phosphate ion concentration at constant riboflavin concentration leads to a decrease in the rate of photolysis due to complexation with riboflavin (decrease in fluorescence intensity) which then largely undergoes cyclophotoaddition instead of the normal intermolecular photo reduction.
Ascorbic acid photolysis is influenced by solvent polarity due to a change in the ionisation of the molecule as indicated by the linearity of a plot of log k versus dielectric constant (€). An increase in the dielectric constant of the medium results in an increase in the rate of photolysis. Similarly the solvent viscosity also influences the rate of photolysis which is a linear function of reciprocal viscosity (cp-1) showing an increase in the rate with a decrease in viscosity of the medium. Thus solvent dielectric constant and viscosity both are important factors in the stabilisation of ascorbic acid.
Reaction schemes for the photo degradation pathways of ascorbic acid alone and in the presence of riboflavin have been presented. The kinetic data support the view that riboflavin acts as a sensitizer in the photo degradation of ascorbic acid.