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

Mamdouh S. Masoud, Rehab M. I. Elsamra, Sokaina S. Hemdan


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.


azo dyes; solvatochromism; Kamlet–Taft parameters; acid dissociation constant


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