Thermochemistry of aqueous pyridine-3-carboxylic acid (nicotinic acid)

The molar enthalpy of solution of solid nicotinic acid (NA) at T = 298.15 K, to give an aqueous solution of molality m = 3.748 · 10−3 mol · kg−1, was determined as ΔsolHmΔsolHm = (19,927 ± 48) J · mol−1, by solution calorimetry. Enthalpies of dilution, ΔdilHmΔdilHm, of 0.1005 mol · kg−1 aqueous nicotinic acid to yield final solutions with molality in the approximate range (0.03 to 0.09) mol · kg−1 were also measured by flow calorimetry. Combining the two sets of data and the results of pH measurements, with values of proton dissociation enthalpies and ΔfHm∘(NA, cr) selected from the literature, it was possible to derive the standard molar enthalpies of formation of the three nicotinic acid species involved in protonation/deprotonation equilibria, at infinite dilution: ΔfHm∘(HN+C5H4COOH·∞H2O,aq) = (328.2 ± 1.2) kJ · mol−1, ΔfHm∘(HN+C5H4COO-·∞H2O,aq) = (325.0 ± 1.2) kJ · mol−1, and ΔfHm∘(NC5H4COO-·∞H2O,aq) = (313.7 ± 1.2) kJ · mol−1. Finally, the enthalpy of solution of nicotinic acid at T = 298.15 K, under saturation conditions (m = 0.138 mol · kg−1), and the standard molar enthalpy of formation of the corresponding solution could also be obtained as ΔsolHmΔsolHm= (19,773 ± 48) J · mol−1 and ΔfHm∘(NA · 402H2O, aq) = (324.9 ± 1.2) kJ · mol−1, respectively.

Research highlights
► We determined the ΔsolHmΔsolHm of solid nicotinic acid (NA) in water by solution calorimetry.
► We determined ΔdilHmΔdilHm of an aqueous nicotinic acid solution by flow calorimetry.
► We determined (aq, ∞) for the 3 NA species involved in acid/base equilibria.
► We determined the enthalpy of formation of NA(aq) under saturation conditions..