Comment on ‘Thermodynamic cycles and the calculation of pKa’ [Chem. Phys. Lett. 367 (2003) 145]

In a recent Letter, Pliego [Chem. Phys. Lett. 367 (2003) 145] has raised some questions about the methodology that we have employed for calculating pKa values in aqueous solutions. In this comment we show that the problem with Pliego’s analysis is the fact that he used Ben-Naim’s definition of ΔGsol for both the solute and the solvent, which implies that the concentration, for both components, should be equal to 1 M. For the solute, this is a reference state fully compatible with the quantum description, but for the solvent this choice is unphysical, as discussed in the Letter.