Spectrophotometric and thermodynamic study on the dimerization equilibrium of ionic dyes in water by chemometrics method

The monomer–dimer equilibrium and thermodynamic of several ionic dyes (Neutral Red, Nile Blue A, Safranine T and Thionine) has been investigated by means of spectrophotometric and chemometrics methods. The dimerization constants of these ionic dyes have been determined by studying the dependence of their absorption spectra on the temperature in the range 20–75 °C at concentrations of Neutral Red (1.73 × 10−5 M), Nile Blue A (3.94 × 10−5 M), Safranine (6.59 × 10−5 M) and Thionine (6.60 × 10−5 M). The monomer–dimer equilibrium of these dyes has been determined by chemometrics refinement of the absorption spectra obtained by thermometric titrations performed. The processing of the data carried out for quantitative analysis of undefined mixtures, based on simultaneous resolution of the overlapping bands in the whole set of absorption spectra. The enthalpy and entropy of the dimerization reactions were determined from the dependence of the equilibrium constants to the temperature (van’t Hoff equation).