Returns the Gibbs free energy of proton in bulk solution.

    Parameters
    ----------
    pH : pH of bulk solution
    temperature : numeric
        temperature in K
    pressure : numeric
       pressure in mbar

    Returns
    -------
    G_H, G_OH : Gibbs free energy of proton and hydroxide.

def proton_hydroxide_free_energy(temperature, pressure, pH):
    """Returns the Gibbs free energy of proton in bulk solution.

    Parameters
    ----------
    pH : pH of bulk solution
    temperature : numeric
        temperature in K
    pressure : numeric
       pressure in mbar

    Returns
    -------
    G_H, G_OH : Gibbs free energy of proton and hydroxide.
    """
    H2 = GasMolecule('H2')
    H2O = GasMolecule('H2O')
    G_H2 = H2.get_free_energy(temperature = temperature, pressure = pressure)
    G_H2O = H2O.get_free_energy(temperature = temperature)
    G_H = (0.5*G_H2) - ((R*temperature)/(z*F))*ln10*pH
    G_OH = G_H2O - G_H  # Do not need Kw when water equilibrated
    return(G_H, G_OH)