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Thermodynamics Study Of The Dimerization Equilibria Of Methylene Green In Different Ionic Strengths By Photometric Titration And Chemometric Method

J.Ghasemi, Sh.Lotfei, M.Mazloum Ardaki, M.Noroozi

The monomer–dimer equilibrium of an asymmetric cyanine dye has been investigated by means of UV-Vis spectroscopy. The dimerization constants of methylene green have been determined by studying the dependence of their absorption spectra on the temperature in the range 20–90 ºC at different total concentrations of methylene green (9.85× 10-5 to 2.98×10-4) and different concentrations of NaCl as supporting electrolyte. The equilibrium parameters of the dimerization of metylene green have been determined by chemometrics refinement of the absorption spectra obtained from thermometric titrations performed at different ionic strengths. The quantitative analysis of the data of undefined mixtures, was carried out by simultaneous resolution of the overlapping spectral bands in the whole set of absorption spectra. The dimerization constants are varied by changing the ionic strength and the degree of dimerization are increased by increasing of the ionic strengths of the medium. The enthalpy and entropy of the dimerization reactions were determined from the dependence of the equilibrium constants on the temperature (van’t Hoff equation).