Dissociation constants and related thermodynamic quantities of the protonated acid form of tris-(hydroxymethyl)-aminomethane in mixtures of 2-methoxyethanol and water at different temperatures

Abstract

The emf of the cell: Pt|H2 (g, 1 atm)| Tris·HCl (m1), Tris (m2), water+2-methoxyethanol|AgCl⊢Ag is obtained in a series of water+2-methoxyethanol mixtures at 5°C temperature intervals from 25–45°C. The acidic dissociation constant of the protonated Tris in these media is determined and its variation with the temperature for each solvent composition is calculated by the method of least squares. The standard thermodynamic quantities (ΔG°, ΔH°, ΔS°, ΔCpo) for the acidic dissociation of the protonated form of Tris in the various solvent mixtures are calculated from the usual thermodynamic formulae. The reliability of the method is tested by comparison of the results with previously published data. The free energy change which accompanies the transfer process for the acid-base system [BH+-B]sys, is also computed in order to study the effect of the solvent on the dissociation process. The variation of ΔG°[BH+-B]sys with solvent composition shows a pronounced minimum which shows that solute-solvent interactions play the primary role in the isoelectric dissociation process. It is also concluded from the calculated standard thermodynamic quantities that a better understanding of the behavior of a standard thermodynamic function, ΔXto[BH+-B]sys, entails an understanding of the behavior of the individual species participating in the dissociation equilibrium.