Development of the Hydronium Ion for MSE

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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The hydrogen ion (H+) has been replaced by the hydronium ion (H3O+) in MSEPUB.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-


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 Delta image.jpg of this reaction to be equal to 0. Since Delta image.jpg for H+ is equal to 0 at all temperatures and pressures this means that Delta image.jpg for H3O+ has to equal Delta image.jpg for H2O at all temperatures and pressures. In OLI’s thermodynamic framework this requires the development of Helgeson parameters for H3O+ by matching Delta image.jpg for H2O as closely as possible. This was done for a temperature range of 0 to 300 Degree symbol.jpgC 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.

Hydronium image.jpg