Dissociation constant of protonated tris(hydroxymethyl)aminomethane in N-methylpropionamide and related thermodynamic quantities from 10 to 55°C

The dissociation constant of protonated tris(hydroxymethyl)aminomethane (tris·H⁺) in the solvent N-methylpropionamide (NMP) has been determined at intervals of 5°C from 10 to 55°C by measurement of the emf of cells without liquid junction using hydrogen and silver-silver chloride electrodes. At 25°C, pK_a was found to be 8.831, as compared with 8.075 in water. The standard changes in Gibbs energy, enthalpy, and entropy for the dissociation process have been evaluated from the dissociation constant and its change with temperature. By comparison with similar data for the dissociation of tris·H⁺ in water, thermodynamic functions for the transfer from water to NMP have been derived. The dissociation process is isoelectric, and the solvent dielectric constant is high (=176 at 25°C). Consequently, electrostatic charging effects are expected to be minimal, and the change in dissociation constant depends primarily on solute-solvent interactions. The results, combined with transfer energies for HCl, tris, and tris·HCl from emf and solubility measurements, demonstrate that the decreased acidic strength of tri·H⁺ in NMP is attributable in large part to the fact that NMP is less effective than water in stabilizing tris and its salts.