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 ( I m ) 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 < I m < 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 p K a1 = 2.19 ± 0.06 and p K a2 = 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, Δ r H m,l ° = (4.5 ± 3.5) kJ·mol –1, Δ r S m,l ° = −(26.8 ± 11.8) J·K –1 ·mol –1, Δ r H m,2 ° = (12.5 ± 2.1) kJ·mol –1, and Δ r S m,2 ° = −(51.2 ± 7.0) J·K –1 ·mol –1 . These values were compared with previously reported data.