Thermodynamic Consistency
The internal thermodynamic consistency of each
result is tested in the following ways, and warnings issued to the user
where problems are detected:
-
The final assemblage of solids, liquid phase components, and gases should
obey Gibbs' phase rule. This test is applied at the end of each calculation.
(This is error flag "P" in the column output of results from 'parametric' or 'batch'
calculations.)
-
If an aqueous and/or hydrophobic liquid phase exist, the relative
humidity and partial pressures
of any trace gases partitioned between the condensed and vapour phases
should agree with equilibrium values calculated from the liquid phase
activities. This test is applied, using an internally specified tolerance
(typically < 1%). (This is error flag "G" in the column output of results from 'parametric' or 'batch'
calculations.)
-
If both an aqueous and a hydrophobic liquid phase exist, solute species
that are present in both phases should be at equilibrium with one another.
This test is applied, using an internally specified tolerance
(typically < 1%). (This is error flag "R" in the column output of results from 'parametric' or 'batch'
calculations.)
-
In a system containing an aqueous phase and dissolved organic compounds the activities
of both the water and organic compounds should be <1.0, when expressed relative to
a pure liquid reference state. This is tested in order to detect cases where the user
has allowed a hydrophobic organic compound to be present in the aqueous phase, or
has constrained it to exist there. (If the activity coefficients are calculated by UNIFAC
then very large values may be generated for this physically unrealistic case, and it is
this that causes activities to be > unity.) This test is applied, using an internally specified tolerance
(typically < 1%). (This is error flag "A" in the column output of results from 'parametric' or 'batch'
calculations.)
The test will not detect all such errors, and the calculated phase partitioning may
be a local rather than a global minimum of the Gibbs energy. Users should therefore
ensure that they understand the physical and thermodynamic properties of the
organic compounds that they include in their system.
-
If an aqueous and/or hydrophobic liquid phase exists, the saturation ratios of
any solids present should be equal to unity. A warning is printed if there is
disagreement beyond an internally specified
tolerance (typically < 1%). (This is error flag "S" in the column output of results from 'parametric' or 'batch'
calculations.)
-
If volatile solids such as HNO3 · 3H2O exist,
then the equilibrium partial
pressure product of the constituent gases is defined (in this example
pHNO3 x pH2O3).
If both gases that can exist in equilibrium with a solid that
has formed are present in the vapour phase, then the actual pressure product
can be compared with the calculated equilibrium value. This test is carried
out, and a warning is printed if there is disagreement beyond an internally specified
tolerance (typically < 1%). (This is error flag "G" in the column output of results from 'parametric' or 'batch'
calculations.)
-
Some of the versions of the models present on the web site do not include the
dissociation of liquid
water (H2O = H+ +
OH−) as a reaction. This can affect model
predictions of equilibria involving aqueous H+
(principally partial pressures of NH3 and the acid gases) for pHs approaching
neutral where the equilibrium has a significant effect. For problems involving
such equilibria the aqueous H+ molality is therefore
compared with the square
root of the ion product of water at the relevant temperature. If it exceeds
this by less than a given factor (internally set, typically x100) then a
warning is output. (This is error flag "G" in the column output of results from 'parametric' or 'batch'
calculations.)
Note: the molal ion product of water is equal to
(mH+ x mOH−),
and is 10-14 at 298.15 K.