The photochemical study for oxidation of ascorbic acid with methylene green was studied in aqueous and aqueous methanol, ethanol, and isopropanol at temperature ranges from 303-318 K in acidic medium. The quantum yield of the photochemical oxidation of ascorbic acid with methylene green was determined by irradiation of reaction mixture by monochromatic light of 657 nm and transmitted light was recorded by photocell response as electrical signals on galvanometer. The rate of reaction mixture in terms of quantum yield was determined and found independent of variation in methylene green concentration and had increased with the concentration of ascorbic acid. The effect of dielectric constant was studied by varying the compositions of alcohol and the results obtained revealed that the quantum yield decreased with the decrease of the polarity of system in acidic medium. On the basis of kinetics the reaction follow pseudo first order and mechanism has been proposed. Thermodynamic parameter such as energy of activation (Ea*), change in enthalpy of activation (ΔH*), change in free energy of activation (ΔG*), and change in entropy of activation (ΔS*) were also calculated.

Sameera Razi Khan, Rehana Saeed, Maria Ashfaq, Summyia Masood