Thermodynamics of protolytic equilibrium of nicotinic acid in water-ethanol solutions

The enthalpies of stepwise protonation of nicotinate ion (L⁻) in water-ethanol solvents were determined calorimetrically at 298.15 K. It was found that the reduced exothermicity of the reaction of nicotinate ion protonation upon an increase of the organic component content in solution is due mainly to increased solvation of the proton. The minor influence of solvent on the enthalpy of protonation of nicotinic acid (HL) is characterized by compensation from solvation of the proton, the ligand, and its protonated form (H₂L⁺).