The Influence of Transition Metal Ions on the Kinetics of Ascorbic Acid Oxidation by Methylene Blue in Strongly Acidic Media

The oxidation of ascorbic acid (Vitamin C) is a very important redox reaction, as it has interesting biological properties and is also a powerful reductant. Ascorbic acid is a lactone with a 2,3-endiol group. The enolic group imparts acidity to the molecule; the enol group at position 3 has a pKa of 4.1 and the enol at position 2 has a pKa of 11.6. It is very effective as a reducing agent and is quantitatively reversibly oxidized in aqueous solution by different oxidizing agents. Ascorbic acid is susceptible to oxidation in both acidic and basic media. The products of the oxidation depend largely on the pH of the reaction; however, its oxidation by various oxidizing agents in acid solution produces dehydroascorbic acid, a lactone whose ring can be easily hydrolyzed to give the free carboxylic group. A number of papers have been devoted to the oxidation of ascorbic acid in acidic and basic media with various inorganic(2−7) and organic substrates. However, the reduction of organic dyestuff with ascorbic acid is very seldom reported. Recently, Strizhak studied the kinetics of the catalytic and uncatalytic oxidation of ascorbic acid by methylene blue, a cationic organic dyestuff, and reported the effects of the concentration of the reactants and pH of the medium on the reaction. However, his reports were somewhat limited in kinetic and mechanistic details. We have therefore extended the study to involve a new pH range, ascorbic acid concentration, salt effects, ionic strength and temperature. Most interesting,