Redox kinetics of methylene green: Titanium trichloride as a novel photo redox agent

Abstract The kinetics of the redox reaction of methylene green (MG) and the novel photo‐redox agent titanium trichloride (TiCl₃) in water were investigated by real‐time spectrophotometric analysis of the change in absorbance of the reaction mixture at 655 nm. The novelty of this redox reaction is that the TiCl₃ is an inorganic compound in contrast to the previously studied organic redox couples. The redox reaction was analysed in depth and the effects of the concentration of the TiCl₃, MG, pH, dielectric constant, ionic strength, metal ions, and temperature on the reaction kinetics were observed. The rate of reaction had a direct relationship with the pH, dielectric constant, and ionic strength of the reaction medium, while an increase in temperature decreased the rate of reaction. The activation energy (E_a) was calculated by the Arrhenius relation, and the other thermodynamic parameters, such as enthalpy change of activation (∆H*), free energy change of activation (∆G*), and entropy change of activation (∆S*) were also evaluated to support the findings of the kinetic measurements. The presence of any reaction intermediate other than the species in the reaction mechanism was ruled out by the spectroscopic analysis of the reaction mixture and Michaelis–Menten plot while the absence of any free radicals was confirmed by the negative polyacrylamide test. A reaction mechanism for the redox reaction of MG⁺ and TiCl₃ was proposed based on the different excited states of the MG⁺ involved in the reaction and the results of the product analysis. The decolourisation of MG by the TiCl₃ is overall a second‐order reaction and is significantly affected by the pH of the reaction medium. The overall rate law for the redox reaction of MG and TiCl₃ is as follows.