Effect of temperature on the ionisation constants of 2-, 3- and 4-nitrobenzoic, phthalic and nicotinic acids in aqueous solution

Abstract

The association constants (equivalent to pK_a) on the molar scale for the equilibria between hydrogen ion and the 2-, 3- and 4-nitrobenzoate, phthalate, hydrogen phthalate and nicotinate anions and the nicotinic acid zwitterion have been determined in aqueous solution at constant ionic strength from spectrophotometric data at various temperatures between 288 and, in some cases, 473 K. 3- and 4-nitrobenzoic acids were stable to 473 K, 2-nitrobenzoic acid decomposed at 423 K and nicotinic acid at 408 K, and the absorption spectrum for the phthalic acid system was unsuitable for use above 448 K. Only one reaction, namely the protonation of the nicotinate anion [C₆H₄NO₂]^–, was isoelectric and the association constant describing the formation of the zwitterion decreased with increasing temperature. The plot of pK_a against reciprocal temperature was linear within experimental error leading to temperature-invariant values of ΔH(exothermic) and ΔS with ΔC_p= 0 for the association reaction. To 408 K, the association constant for the protonation of the zwitterion changed little in terms of the experimental errors. For 2-nitrobenzoic acid the plot of pK_a against reciprocal temperature was also approximately linear. In this case the abnormally high acid strength at room temperature is caused largely by the highly endothermic nature of the association reaction and the acid becomes appreciably weaker at high temperature. For the rest the association constants (pK_a) were greater at the highest temperatures employed than at 298 K which was largely caused by the high, positive entropy change for the association reaction resulting from a decrease in the dielectric constant of the bulk solvent as the temperature increased. Plots of pK_a against reciprocal temperature were markedly non-linear with minima observed in some cases. These plots were analysed in terms of continually varying values of ΔH, ΔS and ΔC_p over the temperature range although in some cases the changes in ΔC_p were hardly significant in terms of the likely experimental errors.