Medium Effects on the Protonation Equilibria of l-Norvaline†.

The protonation constant pKa2of l-norvaline has been determined in aqueous solution at different ionic strengths and different temperatures, using the pH-metric technique. The thermodynamic quantities (ΔG°, ΔH°, and ΔS°) have been studied and discussed. Evaluation of the effect of organic solvent of the medium on the protonation processes was also reported and discussed. The organic solvents used were methanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetone, and dioxane. The pKa2values for the ionization in a water (1) + dioxane (2) mixture [100 w2= 10 %, 20 %, 30 %, 40 %, and 50 %] have been determined at five different temperatures from T= (288.15 to 308.15) K. The thermodynamic quantities were calculated. [ABSTRACT FROM AUTHOR]