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This work aimed to the study effect of ethanol ratios on the acidity constants of bromocresol purple. The acidity constants of BCP were determined in binary mixtures of water with ethanol containing 0, 30, 40, 50, 60, and 70% (v/v) using spectrophotometric methods. The results indicate the dependence of acidity constants on the ratio of ethanol. The values of log10Ka1 and log10Ka2 were correlated with the macroscopic (relative permittivity, ε) and microscopic Kamlet–Taft parameters (α, β, and π*) of binary mixtures. The influence of solvent on the acidity constants was analyzed in terms of the KAT parameters in order to study the solvent–solute interaction. Multiple linear regression was used to find the contribution of the microscopic parameters α (acidity), π* (dipolarity/polarizability), and β (basicity). It was found that hydrogen bond donating and hydrogen bond accepting abilities were the most predominant to change the acidity constants. Also, a relationship with the reciprocal relative permittivity was obtained based on Born's model. The hydrogen bonding interactions between solute and solvent components are mainly responsible for the change in acidity constants of bromocresol purple of binary mixtures. [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]

In this study, the solvatochromic behavior of Methyl orange was studied visible spectrophotometrically in seven aqueous binary systems from water with methanol, ethanol, propanol, DMF, DMSO, acetone and dioxane solvents. The spectral data was interpreted in terms of solute-solvent and solvent-solvent interactions. The deviation from linearity in the plots of νmax versus x2 due to preferential solvation of the Methyl orange by one component of the mixed solvent and due to solvent microheterogeneity. The preferential solvation parameters local mole fraction X2L, solvation index δs2 and exchange constant K12 were evaluated. The preference of solute to be solvated by one of the solvating species relative to others was explained. All values of K12 were less than unity that indicates the preferential solvation of Methyl orange by water, except in case the water-propanol mixture where K12 was higher than unity. The preferential solvation index δs2 values were calculated and interpreted for each binary mixture. The magnitude of preferential solvation index was highest in water-DMSO mixtures than in the all other solvent mixtures. The energy of electronic transition in maximum absorption (ET) was calculated in each binary mixture. The extent and importance of each solute-solvent interactions to ET were analyzed by the linear solvation energy relationships using the Kamlet-Taft strategy. [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]