Solvent, substituents and pH effects towards the spectral shifts of some highly colored indicators

The solvatochromic responses of six indicators namely Sudan orange, Alizarin yellow R, Aurin tricarboxylic acid, Alizarin yellow GG, Titan yellow and Eriochrome black-T, dissolved in seven solvents of different polarities, have been measured at room temperature. The UV/Vis absorption spectral shifts were analyzed by the multiple linear regression analysis and Kamlet–Taft equation. The observed solvatochromism was found to depend on the presence of the donor and acceptor substituents in the conjugated systems of the indicator and the physical properties of the solvent molecules. The pH effects on the wavenumbers of the absorption band maxima of some indicators with different constituents at room temperature were discussed and the mechanism of ionization was explained. The dissociation constants (pKa) of the investigated compounds were precisely assessed and the existence of the individual predominant ionic species was assigned by constructing distribution diagrams at different pH ranges