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

Abstract

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 Δ sol H m = (19,927 ± 48) J · mol −1, by solution calorimetry. Enthalpies of dilution, Δ dil H m , 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 Δ f H m ∘ ( 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: Δ f H m ∘ ( H N + C 5 H 4 COOH · ∞ H 2 O,aq ) = (328.2 ± 1.2) kJ · mol −1, Δ f H m ∘ ( H N + C 5 H 4 COO - · ∞ H 2 O,aq ) = (325.0 ± 1.2) kJ · mol −1, and Δ f H m ∘ ( NC 5 H 4 COO - · ∞ H 2 O,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 Δ sol H m = (19,773 ± 48) J · mol −1 and Δ f H m ∘ (NA · 402H 2O, aq) = (324.9 ± 1.2) kJ · mol −1, respectively.