Development of the Hydronium Ion for MSE

Hydronium Ion (H3O+)

The hydrogen ion (H⁺) has been replaced by the hydronium ion (H₃O+) in MSEPUB. The major reason for doing this is to improve the prediction of properties for strong acids. As a result of doing this, the ionization of water is changed from:

H₂O → H⁺ + OH^-

To:

2H₂O → H₃O⁺ + OH^-

The net result of these two equations is:

H₃O⁺ = H₂O + H⁺

In order to keep predictions the same for dilute solutions it is necessary for of this reaction to be equal to 0. Since for H+ is equal to 0 at all temperatures and pressures this means that for H₃O⁺ has to equal for H₂O at all temperatures and pressures. In OLI’s thermodynamic framework this requires the development of Helgeson parameters for H₃O⁺ by matching for H₂O as closely as possible. This was done for a temperature range of 0 to 300 C and for pressures up to over 1000 atmospheres.

The attached figure compares the predicted pH for water using the H⁺ and H₃O⁺ ions. The largest difference is about 0.001 pH units.