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: Pt ;H 2 ( g ) ,H 2NC 5 H 4 N ⋅ HCl ( m 1 ) ,H 2NC 5 H 4 N ( m 2 ) , AgCl ; Ag where m is molality. Between 0° and 50° C, the dissociation constant ( K bh ) is given as a function of temperature ( T ) in degrees Kelvin by - logK b h = 2575.8 T + 0.08277 + 0.0013093 T The changes of Gibbs free energy (Δ G °), of enthalpy (Δ H °), of entropy (Δ S °), and of heat capacity (Δ C p °) for the dissociation process in the standard state were calculated from the constants of this equation. At 25° C the following values were found:- logK b h = 9.114 , Δ G ° = 52 , 013 jmole - 1 , Δ H ° = 47 , 090 jmole - 1 ,Δ S ° = - 16.5 jdeg - 1mole - 1 , Δ C p ° = - 15 jdeg - 1mole - 1 .Thermodynamic constants for the basic dissociation of 4-aminopyridine at 25° C were also computed.