Thermochemical Properties of Dissolution of Nicotinic Acid C₆H₅NO₂(s) in Aqueous Solution

Abstract

Nicotinic acid (also known as niacin) was recrystallized from anhydrous ethanol. X-ray crystallography was applied to characterize its crystal structure. The crystal belongs to the monoclinic system, space group P2₍₁₎/c. The crystal cell parameters are a = 0.71401(4) nm, b = 1.16195(7) nm, c = 0.71974(6) nm, α = 90°, β = 113.514(3)°, γ = 90° and Z = 4. Molar enthalpies of dissolution of the compound, at different molalities m/(mol·kg⁻¹) were measured with an isoperibol solution–reaction calorimeter at T = 298.15 K. The molar enthalpy of solution at infinite dilution was calculated, according to Pitzer’s electrolyte solution model and found to be $\Delta_{\text{sol}} H_{m}^{\infty } = ( 2 7. 3 \pm 0. 2)$ kJ·mol⁻¹ and Pitzer’s parameters ($\beta_{{\text{MX}}}^{{\text{(0)}L}}$, $\beta_{{\text{MX}}}^{{\text{(1)}L}}$ and $C_{{\text{MX}}}^{\phi L}$) were obtained. The values of apparent relative molar enthalpies (${}^{\phi }L$) and relative partial molar enthalpies ($\overline{{L_{2} }}$ and $\overline{{L_{1} }}$) of the solute and the solvent at different molalities were derived from the experimental enthalpy of dissolution values of the compound. Also, the standard molar enthalpy of formation of the anion ${\text{C}}_{ 6} {\text{H}}_{ 4} \text{NO}_{2}^{-}$ in aqueous solution was calculated to be ${\Delta_{\text{f}}^{} H}_{\text{m}}^{\text{o}} ({\text{C}}_{ 6} {\text{H}}_{ 4} {\text{NO}}_{2}^{-} \text{,aq}) = - \left( {603.2 \pm 1.2} \right)\;{\text{kJ}}{\cdot}{\text{mol}}^{-1}$.