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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 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]

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 influence solvents from different polarity and hydrogen bonding ability on electronic absorption spectrum of Methylene blue was investigated. The visible absorption spectra were recorded in eleven neat solvents in the range 400–700 nm. Methylene blue has two absorption maxima, the first band assigned to n-π* from amino groups and the second band assigned to weakly forbidden n-π* transition with charge transfer. The charge transfer band of Methylene blue showed red shift with increasing the relative permittivity of neat solvents. The red shift in wavelength(λmax) for the charge transfer band of Methylene blue was observed when proceeding from dioxane (λmax = 650 nm) into methanol (λmax = 655 nm) into cyclohexanone (λmax = 660 nm) into dimethylsulfoxide (λmax = 665 nm) as well as water (λmax = 665 nm), this shift not agree with the polarity of solvents but due to combination of several parameters. The absorption of charge transfer band in methanol and ethanol as hydrogen bonding donating solvents (HBD) showed the highest intensity than the absorption band in dimethylsulfoxide and dimethylformamide as hydrogen bonding accepting solvents (HBA) due to non-electrostatic interaction between the amino groups and solvents. The charge transfer band in neat solvents were correlated with several parameters using linear solvation energy relationships. The results showed that electrostatic interactions of the solvents play an important role in the shift of absorption maxima of Methylene blue in neat solvents. The acidity constants (pKa) of Methylene blue were estimated by using absorbance measurements in different media. The acidity constants (pKa) values of Methylene blue were affected by cosolvent, which the pKa values increasing in the order propanol < methanol < dioxane, this order not agreement with increasing the relative permittivity of the medium. [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]

The solvatochromic characteristics of methyl red were examined in several aqueous solutions from pure water, with methanol, ethanol, propanol, acetonitrile, and dioxane. In order to explain the preferred solvation of the probe azo dye in the binary mixed solvents, the solvent exchange model of Bosch and Roses was used to evaluate the association between the empirical solvent polarity scale (ET) values of MR and solvent composition. Non-linear solvatochromism of methyl red was noted in all aqueous mixtures containing methanol, ethanol, propanol, acetonitrile, and dioxane. In addition to calculating the local mole fraction of each solvent composition in the cybotactic area of the probe, the impact of the solvating shell composition on the preferential solvation of the solute dye was examined in terms of both solvent-solvent and solute-solvent interactions. The local mole fraction of each solvent composition in the cybotactic region of the probe was also calculated. The results indicated that the MR solvation shell was thoroughly saturated with the solvent complex S12 in the following order: dioxane > ethanol > methanol > acetonitrile > propanol. Data from the binary systems were analyzed with KAT parameters using a multi-model; in aqueous methanol and ethanol solutions, the hydrogen acidity was more responsible for the spectral shift, whereas in aqueous acetonitrile and dioxane solutions, the basicity has a greater influence. [ABSTRACT FROM AUTHOR]