where the first three terms on the right hand side of the equation together equals to the negative of the proton solvtion free energy -∆ solv G H + . ∆ zp E H(OH 2 ) + is the zero-point energy of H 2 O + -H bond, which was calculated to be about 0.35 eV. Here χ is the surface dipole potential present at the liquid-air interface, which is an non-measurable quantity but estimated to be around 170 meV. It is worth noting that the surface dipole potential χ is much smaller than the Poisson potential of liquid phase itself. We assume here χ is a constant that is independent of salt concentration. Therefore, to determine W H + , one needs to know how the Poisson potential of liquid phase shifts with the salt concentration in addition to the deprotonation free energy ∆ dp A H 3 O + .