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

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: where is molality. Between 0° and 50° C, the dissociation constant ( ) is given as a function of temperature () in degrees Kelvin by The changes of Gibbs free energy (Δ°), of enthalpy (Δ°), of entropy (Δ°), and of heat capacity (Δ °) for the dissociation process in the standard state were calculated from the constants of this equation. At 25° C the following values were found: Thermodynamic constants for the basic dissociation of 4-aminopyridine at 25° C were also computed.