Solvent effects on the dissociation of weak acids of three charge types in water/N-methylacetamide media

Abstract

In order to elecidate the role of charge type on the change of dissociation constant with increasing polarity of the solvent medium, the pK values and associated thermodynamic quantities for an uncharged acid (acetic acid), a cationic acid (protonated tris(hydroxymethyl)aminomethane), and two ampholytes (MOPS and Tricine) were determined in mixtures of water andN-methylacetamide (NMA). Electromotive-force measurements of cells without liquid junction containing hydrogen electrodes and silver-silver bromide electrodes were used to determine the pK at nine temperatures from 5 to 45°C. The solvent compositions varied from pure water to a mixture containing a mole fraction of NMA of 0.25 (57.5 mass % NMA). The solvent effects are compared with similar data for water/methanol solvents of decreasing polarity. They reflect both the reduction in interionic energy accompanying the elevation in dielectric constant and also the greater protophilic character of NMA as compared with methanol.