Potentiometric Titration Study of the Temperature and Ionic Strength Dependence of the Acidity Constants of Nicotinic Acid (Niacin)

Abstract

The influence of temperature (T) and ionic strength (Im) on the stoichiometric (molality scale) acidity constants of nicotinic acid in aqueous solution was investigated by potentiometry (H+-glass electrode). The background salt used was potassium chloride, and the temperature and ionic strength ranges covered were 283.15 K < T < 318.15 K and 0.05 mol·kg–1 < Im < 0.52 mol·kg–1, respectively. Acidity constants at zero ionic strength were derived by means of a Debye–Hückel type formalism, and their temperature dependence was obtained through a van't Hoff analysis. This led to pKa1 = 2.19 ± 0.06 and pKa2 = 4.86 ± 0.03 at 298.15 K and to the corresponding standard molar enthalpies and entropies of proton dissociation in the temperature range of the experiments, ΔrHm,l° = (4.5 ± 3.5) kJ·mol–1, ΔrSm,l° = −(26.8 ± 11.8) J·K–1·mol–1, ΔrHm,2° = (12.5 ± 2.1) kJ·mol–1, and ΔrSm,2° = −(51.2 ± 7.0) J·K–1·mol–1. These values were compared with previously reported data.