Solvent effect of aqueous ethanol on complex formation and protolytic equilibria in nicotinic acid solutions

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

“…Finally, eq and the appropriate values of the a and b parameters for reactions 2 and 3 lead to p K a1 = 2.19 ± 0.06 and p K a2 = 4.86 ± 0.03, respectively, at 298.15 K. The indicated uncertainties represent standard deviations and were calculated from the differences between the p K a values given by eq and their experimental equivalents . The p K a1 and p K a2 values determined in this work at 298.15 K are compared in Table with corresponding published data. ,,,,− ,,, When necessary the stoichiometric values taken from the literature were first corrected to I m = 0, based on Davies equation, and then converted to molality scale by using eq and the mass density of water at 298.15 K, ρ = 0.997048 kg·dm –3 . As shown in Table , the thermodynamic acidity constants of nicotinic acid here reported are in the range of previous determinations: 1.98 to 2.86 for p K a1 and 4.67 to 5.12 for p K a2 .…”

“… a This work. b Recommended in the database of reference . c Reference . d Reference . e Reference . f Reference . g Reference . h Mean of five values from reference . i Reference . j Reference . k Reference . l Reference . …”

“…Some early views favored the predominance of the AH o form . However, a consensus seems now to exist that the AH o ↔ AH ± equilibrium is strongly shifted toward the zwitterionic species, − with the contribution of AH o to the equilibrium mixture at approximately 293 to 298 K estimated in ∼ 3 %, ∼ 6 %, 7 % to 8 %, 10 %, , and 22 %, according to different authors. This conclusion was supported by a variety of information, namely: (i) the use of Hammett relationships to predict the equilibrium constant of the AH o ↔ AH ± process; , (ii) the comparison of the ultraviolet (UV) spectrum of isoelectric nicotinic acid with that of the corresponding methyl ester; (iii) the dependence of the infrared spectrum (IR) and the 1 H and 13 C nuclear magnetic resonance ( 1 H and 13 C NMR) spectroscopy chemical shifts of aqueous nicotinic acid on the pH of the solution and its content in dimethyl sulfoxide; (iv) the failure to produce copper complexes containing nicotinic acid with an nonionized −COOH group in acidic media; (v) entropic and enthalpic considerations based on equilibrium and solution calorimetry measurements; , and (vi) computational chemistry results. , Although for some applications, such as the interpretation of the pH dependence of partition coefficients of drugs, the equilibrium constants (normally dubbed microconstants) relating all four species in Scheme may be required, , their determination is not aimed in this work.…”

“…There have been several determinations of the stoichiometric (molarity or molality scale, p K a1 ′ and p K a2 ′ ) and thermodynamic (p K a1 and p K a2 ) acidity constants of nicotinic acid in aqueous solution by using UV spectroscopy, ,,,,− potentiometry, ,,,− ,,, conductivity, and 13 C NMR measurements, but in most cases, they were based on experiments carried out at a single temperature or ionic strength, none involving a systematic study of both the influence of temperature and ionic strength on the results. At temperatures close to ambient (293 to 298 K) the reported p K a ′ or p K a values (the last often calculated from stoichiometric counterparts by using Debye–Hückel type approaches) for the first and second proton dissociation equilibriums vary in the ranges 1.87 to 3.60 and 4.67 to 5.12, respectively. ,,,− ,,− To the best of our knowledge, the temperature dependence of p K a1 or pK a2 has only been investigated twice. , These studies further afforded the corresponding enthalpies and entropies of ionization through van’t Hoff plots (second law method) . Direct measurements of the enthalpies of ionization of nicotinic acid have also been performed by calorimetry. ,,− …”

Solution thermodynamics of aqueous nicotinic acid solutions in presence of tetrabutylammonium hydrogen sulphate

“…According to Sharnin et al39 nicotinic acid exists in the zwitterionic form in aqueous solution and within the pH range of the experimental aqueous solutions, nicotinic acid remains in equi-Plots B-coefficients of transfer (ΔB) from water to aqueous solutions of nicotinic acid for Bu 4 NHSO 4 at different temperatures: A, 298.15 K; B, 308.15 K; C, 318.15 K; -··· □ ··· , ΔB. Assuming the values of φ vw and φ void are of same magnitude in water and in aqueous nicotinic acid solutions of for the same solute, can be attributed to decreases in the shrinkage volumes (φ S ) of water by Bu 4 NHSO 4 in presence of nicotinic acid.…”