H2O(aq) Does Not Exist: Critique of a Proof-of-Concept Derivation

Abstract

Although the pKa of water has been shown to be 14.0 at 25 °C both experimentally and theoretically, a subset of organic chemists has listed the pKa of water as 15.7 and the pKa of the aqueous proton (H+ or H3O+) as −1.7 since at least the early 1930s. These values were found primarily in research literature until the early 1960s, at which time they began appearing in organic chemistry textbooks. The validity of the 15.7/-1.7 values has been fiercely debated in the literature. At the heart of one argument supporting these values is the proposal that a small concentration of “solute” water molecules, H2O(aq), can exist in solution along with the solvent water molecules, H2O(l). No experimental evidence of H2O(aq) molecules has been reported, but at least one theoretical proof-of-concept for the existence of such molecules has been published in this Journal. Here we show that this theoretical proof is based on flawed assumptions and is therefore incorrect.