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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]

The acidity constants (pKa) of bromocresol purple in various water-dioxane mixtures were determined. The proposed approaches allow prediction of pKa values of bromocresol purple in any mixture of water-dioxane. The results were discussed in terms of the solvent characteristic. The pKa values of the investigated indicator were found to be largely dependent on the ratio of dioxane. This result is confirmed by linear correlation between mol fraction of dioxane and acidity constant due to the solvation of bromocresol purple with both water and dioxane molecules in all binary mixtures. The results showed non-linear relationship between pKa values and reciprocal relative permittivity that indicates both the electrostatic and non-electrostatic which affecting on the acidity constant. A linear solvation free energy relationship was used to study the effects of solvent and solute properties on the acidity constants in some dioxane solvent mixtures. The pKa values were correlated with the Kamlet-Taft solvatochromic parameters of mixtures from water with dioxane. It was mainly affected by non-electrostatic solute-solvent interactions. The KAT multi-parameter correlations represent significant improvements with regard to single-or dual-parameter models with highest F-static. [ABSTRACT FROM AUTHOR]