Acid-Base Behavior in 50-Percent Aqueous Methanol: Thermodynamics of the Dissociation of Protonated Tris(hydroxymethyl)aminomethane and Nature of the Solvent Effect.

Abstract

Electromotive-force methods and cells without liquid junction have been utilized to determine the acidic dissociation constant of protonated tris(hydroxymethyl)aminornethane [2-amino-2-(hydroxymethyl)-1, 3-propanediol] in 50 wt percent methanol solvent at seven temperatures from 10 to 40 °C. The change of the dissociation constant with temperature has been used to calculate the changes of enthalpy, entropy, and heat capacity when the dissociation takes place in the standard state. Comparisons with earlier measurements in the aqueous medium reveal no great differences in the enthalpy and entropy, suggesting that water participates in preference to methanol in the protolytic reaction even in 50-percent methanol. It is shown that electrostatic considerations alone are unable to explain the solvent effect on the dissociation energy, and a substantial "basicity effect" is indicated. The activity-coefficient term for the amine hydrochloride in equimolal amine-salt buffers has been evaluated and compared with similar data in the water solvent.