Kinetics of Methylene Green and Ascorbic Acid Reaction in Aqueous 2-propanol at Different Temperatures

The kinetics of reaction of methylene green with ascorbic acid was carried out spectrophotometrically in mixed solvent system by using aqueous 2- propanol ratios as a solvent in the presence of hydrochloric acid. The pseudo-first order rate constants were proposed on the basis of the experimental kinetic data.

The reaction was carried out in 10%, 20%, 30%, 40%, 50% aqueous 2-propanol ratios at different ionic strength. The effects of these solvent compositions and ionic strength on rate of reaction were studied. The effect of concentration of ascorbic acid on the rate of reduction was also determined. The values of rate constants at all initial concentrations of methylene green revealed that reaction follows 1st order kinetics with respect to ascorbic acid and relatively higher value of the rate of the reaction was observed in the solvent of high dielectric constant. The reaction was found to be zero order dependence on the concentration of methylene green, and the variation of hydrochloric acid concentration did not bring about significant changes in the values of rate constants in various aqueous propanol mixtures.

The influence of temperature on the rate constant as a function of ascorbic acid concentration was also studied. The Arrhenius relationship was used to determine the various activation parameters such as activation energy (Ea), change in enthalpy (DH#), change of entropy (DS#), and the change of free energy of activation (DG#). The activation parameters determined were found to be dependent on concentration of ascorbic acid and solvent composition. The radii of the activated complex was calculated. It was found that activated complex exists as a single sphere model. The iso-kinetic temperature was also calculated.