Publication Details
Abstract
Acid–base equilibria in mixed solvents are important to understand in order to explore proton-transfer reactions and solvation dynamics in industrial and analytical chemistry. Mixture systems of ethanol and glycerol give dielectric and viscosity tunability and therefore are interesting models to study solvent effects on the dissociation of ions. Weak acids—acetic and benzoic acids were studied conductometrically and potentiometrically at 25 °C in glycerol to ethanol ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 (v/v). Dielectric constant (ε) ranged from 24.3 to 61.5, and viscosity (η) ranged from 1.07 mPa·s to 9.68 mPa·s with glycerol content. Dissociation constants (Ka) were obtained using Ostwald's dilution law and Gran's method, while the thermodynamic parameters were determined from Van't Hoff plots between 25–45 °C. Conductometric data showed sequential reduction Λ₀ from 385.2 S·cm²·mol⁻¹ (100% ethanol) to 191.2 S·cm²·mol⁻¹ (100% glycerol). Ka of acetic acid decreased from 1.67 × 10⁻⁵ mol·L⁻¹ to 0.36 × 10⁻⁵ mol·L⁻¹ going down the series of solvents, and potentiometric titrations observed the same increase in pKa to 5.41 from 4.75. Strong correlations between log Ka and 1/ε (r = 0.982) and log Ka and log η (r = 0.963) were obtained. Thermodynamic calculations of ΔH° = 10.2 kJ·mol⁻¹ and ΔS° = +62 J·mol⁻¹·K⁻¹ indicate an endothermic, entropy-favored dissociation reaction.The results confirm that solvent polarity and viscosity cooperatively control acid dissociation equilibria. The glycerol-ethanol mixtures are highly inhibitory ionization compared to conventional aqueous solvents and form a safe model for the study of solvation-dependent proton-transfer reactions in chemical, pharmaceutical, and biochemical systems.