Protolytic Equilibrium Constants of Nicotinic Acid Solutions in Water–Dimethylsulfoxide Mixtures

Potentiometric titration and spectrophotometry are used to determine the stepwise protonation constants of nicotinate ions at 298.2 K and the ionic strength of the medium of 0.25 (NaClO4) in water-dimethylsulfoxide solutions. It is established that the dependence of the protonation constants of nicotinate ion on the composition of the solvent is characterized by a complex S-shaped curve, while that of nicotinic acid depends only slightly on the concentration of the solvent. The results are considered from the viewpoint of the solvation-thermodynamic approach. It is shown that the change in the Gibbs energy of the stepwise protonation of nicotinate ions is mainly due to a change in the solvate state of nicotinic acid.