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In this study, the spectrophotometric studies of aqueous ethanol solution of Congo red in various buffers solutions at room temperature 25oC and at different ionic strength of 0.05, 0.1, 0.3, and 0.5 KCl mol L-1 were investigated. The acidity constants of Congo red were estimated by three different methods. The two protonation stages were observed in pH range 2 - 4 and 4 - 6 as result as protonation of amino group and azo groups. The dependence of absorption spectra of Congo red on the ionic strength of solutions which exhibited blue shift with increases ionic strength in addition the shape and intensity were varied. Therefore, the acid-base equilibria shifted toward the higher pHs with the rise in ionic strength. Also, the acidity constants were affected by the ionic strength which the acidity constants decrease with increasing ionic strength of solution because the electrostatic interaction force increased. [ABSTRACT FROM AUTHOR]

In this study, the effect of polarity and hydrogen bonding ability of solvents on the absorption spectrum of Congo red was investigated. UV–Visible absorption spectra were recorded in eight neat solvents and four aqueous binary solutions of methanol, ethanol (as HBD solvents), and dimethylsulfoxide and dioxane (as HBA solvents). Congo red has two absorption maxima in the range (250–700 nm) that have been assigned to the azo and hydrazo systems. Molar transition energy (ET) values of Congo red in different solvents were correlated with the solvent parameters using linear solvation energy relationships. Multiparameter analysis shows the dependence of the electronic spectra of Congo red on the non-specific and specific interactions. Preferential solvation was observed in all binary mixtures when ET plotted as a function of mole fraction of organic solvents. The results showed that the Congo red was solvated by water–organic solvent complex species in all binary mixtures except in case of aqueous-ethanol mixtures, whose preferred solvation by ethanol in all mole fractions with f12/2 and f12/1 values less than unity. The dual-parameters model is a powerful tool for explaining binary mixture data with Kamlet–Taft parameters. The findings indicated that the HBD and HBA were responsible for the solvatochromism of the binary mixtures. [ABSTRACT FROM AUTHOR]