Beyond pH

Abstract

pH is one of the few chemical quantities known also to the lay public. However, when we use acid solution more concentrated than 1 M, very few chemists are aware of how to define the activity of the proton under such conditions. In this review, we will describe the scientific efforts to understand in a comprehensible way the behavior of organic compounds in moderately concentrated aqueous solutions. In the second part of the past century, it became clear that, since increasing the aqueous acid concentration, we must decrease the concentration of water, protonation of weak bases is always accompanied by a change in solvating ability of the medium. As a consequence, the description of the protonation equilibria must be made either by the pK in water and by the solvent effect on the equilibrium. We will show that treatments by Bunnett and Olsen, Marziano, and Cox and Yates all give the same results, although on a slightly different scale. It is possible to classify organic ions solvation abilities in the scale oxonium > ammonium > sulphonium > carbenium. The same scale is obtained by evaluating the energetics of transfer of the same ions from gas phase to water. More recently, it has become possible to calculate the energetic of ion solvation, obtaining the same scale described above for the experimental data. Copyright © 2013 John Wiley & Sons, Ltd.