Investigate saturation index value

[vineetbansal]

The following currently passing test gives a value of 5.408 while the hand-calculated value is closer to 7.2

    solution = Solution({"Fe+3": "0.01 mol/L", "OH-": "10**-7 mol/L"}, volume="1 L", engine="phreeqc")
    solution.equilibrate()
    Fe_3 = solution.get_amount("Fe+3", "mol/L").magnitude
    OH_ = solution.get_amount("OH-", "mol/L").magnitude
    assert np.isclose(Fe_3, 3.093e-11)
    assert np.isclose(OH_, 1.067e-07)
    # We need to explain why phreeqc gives a value of ~5.408 instead of the
    # expected ~7.2
    # SI_FeO3H3 = np.log10((Fe_3) * (OH_) ** 3 / 10**-38.8)
    assert solution.engine.ppsol.si("Fe(OH)3(a)") > 0

[rkingsbury]

Just to confirm - the value of 10**-38.8 comes directly from the PHREQC database?