Dissociation Constant of 4-Aminopyridinium Ion in Water From 0 to 50 °C and Related Thermodynamic Quantities.

Abstract

The dissociation constant of 4-aminopyridinium ion in water at 11 temperatures from 0° to 50° C has been determined from electromotive force measurements of 19 approximately equimolal aqueous buffer solutions of 4-aminopyridine and 4-aminopyridinium chloride. Cells without liquid junction were used; the cell is represented as follows: Pt ; H 2 ( g ) , H 2 NC 5 H 4 N ⋅ HCl ( m 1 ) , H 2 NC 5 H 4 N ( m 2 ) , AgCl ; Ag where m is molality. Between 0° and 50° C, the dissociation constant (Kbh ) is given as a function of temperature (T) in degrees Kelvin by - log K b h = 2575.8 T + 0.08277 + 0.0013093 T The changes of Gibbs free energy (ΔG°), of enthalpy (ΔH°), of entropy (ΔS°), and of heat capacity (ΔCp °) for the dissociation process in the standard state were calculated from the constants of this equation. At 25° C the following values were found: - log K b h = 9.114 , Δ G ° = 52 , 013 j mole - 1 , Δ H ° = 47 , 090 j mole - 1 , Δ S ° = - 16.5 j deg - 1 mole - 1 , Δ C p ° = - 15 j deg - 1 mole - 1 . Thermodynamic constants for the basic dissociation of 4-aminopyridine at 25° C were also computed.