| 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 Delta H-sol(m) = (19,927 +/- 48) J.mol(-1), by solution calorimetry. Enthalpies of dilution, Delta H-dil(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 Delta H-f(m)degrees (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: Delta H-f(m)degrees(H N C5H4COOH center dot infinity H2O, aq) = (328.2 +/- 1.2) kJ.mol(-1), Delta H-f(m)degrees (H N C5H4COO-: infinity H2O, aq) = (325.0 +/- 1.2) kJ.mol(-1), and Delta H-f(m)degrees(NC5H4COO-center dot infinity 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 Delta H-sol(m) = (19,773 +/- 48) J.mol(-1) and Delta fH(m)degrees(NA center dot 402H(2)O, aq) = (324.9 +/- 1.2) kJ.mol(-1), respectively. (C) 2011 Elsevier Ltd. All rights reserved. |
