Acid-base equilibria of some organic acids and indicator dyes in water – 1-butanol binary solvent system

Abstract

This work is devoted to the acid-base equilibria in water–1-butanol mixtures near the region of stratification. The $\text{p}{a}_{{\text{H}}^{+}}^{*}$ scale is estimated basing on the so-called NBS USA method for 82 mass % butanol, a solvent with low relative permittivity, ${\epsilon }_{\text{r}}$ = 20.8, high molar fraction of water, $x_{{\text{H}}_{\text{2}}\text{O}}$ = 0.475, and a polarity characterized by $E_{\text{T}}^{\text{N}}$ = 0.657. The results are confirmed by measuring compensating effects of Volta cells. Glass electrode exhibits a good response in the $\text{p}{a}_{{\text{H}}^{+}}^{*}$ region from 2.49 to 12.15. Dissociation constants of different organic acids and indicator dyes are determined in 5 and 82 mass % of 1-butanol. The analysis of medium effects for pK_a values in 82 mass % 1-butanol, $\Delta \text{p}{K}_{\text{a}}$ = $\text{p}{K}_{\text{a}}$ − $\text{p}{K}_{\text{a}}^{\text{w}}$, of 32 acid–base couples of various charge types and functional groups, shows that this mixed solvent displays differentiating action: the $\Delta \text{p}{K}_{\text{a}}$ vary within 6.7 units. The Hammett’s $\rho$ constant increases by 1.6–1.8 times compared to water. The data are compared with those reported for other solvents. The $\log {}^{\text{w}}{\gamma }_{{\text{H}}^{+}}^{\text{s}}$ = −0.30 value of the logarithm of the activity coefficient of transfer from water to 82 mass % 1-butanol is deduced. The last solvent can be recommended as an easily available medium that is suitable for studying of acid-base equilibria, e.g., for poorly soluble in water compounds. A set of pK_a values in 100 % 1-butanol is presented and compared with the literature data.