The kinetics of photo‐induced chemical transformation reaction of methylene blue and titanium trichloride in different solvents

The kinetics of photo‐induced chemical transformation reaction of methylene blue (MB) and titanium trichloride (TiCl 3 ) was investigated in water and aqueous ethyl acetate by determining the quantum yield of the reaction using optical spectrometry as the analytical technique. The reaction is pseudo‐first order, dependent only on the concentration of titanium trichloride at a fixed concentration of MB. The effect of the solvent on the quantum yield was studied in the acidic range from 4.0 to 7.0. The quantum yield was maximum at acidity (pH/ H o ) 5.0, while it decreased on either increase or decrease in acidity (pH/ H o ). The quantum yield of the reaction was high in water and decreased in 10% aqueous ethyl acetate. The absence of any reaction intermediate was confirmed by spectral analysis of the partially reacted mixture. A reaction mechanism has been suggested in accordance with the existence of different excited states of MB. The quantum yield increased with increase in temperature. The effect of temperature on the quantum yield of the reaction was used to determine the activation energy ( E a ) using Arrhenius relation, and other thermodynamic parameters, such as enthalpy change (Δ H *), free energy change (Δ G *), and entropy change (Δ S *), were also evaluated.