Objective
The OLI software offers two methods to display species: the "True Species" approach, which shows the actual species in solution, and the "Apparent Species" approach (or molecular representation), which converts ions back into neutral species. This article explains the differences between these two methods.
Examples With the Different Representations
We will show two examples of the differences between True and Apparent Species.
Example 1: True vs. Apparent Species Representation
In OLI software, an aqueous solution can be represented using True Species or Apparent Species. Consider this input composition:
- Temperature: 25°C
- Pressure: 1 Atmosphere
- H2O: 0.7 moles
- KHCO3: 0.3 moles
Using the OLI electrolyte engine, the equilibrium solution composition (based on the AQ thermodynamic framework) is as follows:
True Species Representation
- pH: 8.02324
- Phases: Aqueous, Solid, Vapor
This is the “True” species, or “Ionic,” representation. The solution pH is 7.99597, and there are three phases present. Some of the input material is present as a solid phase, some present as a vapor phase, and the remainder is in the aqueous phase.
However, some users prefer a molecular (Apparent) representation. This is often preferable when the user needs to translate OLI's output to other computer programs that do not accept ions.
Apparent Species Representation
- pH: 8.02324
- Phases: Aqueous, Solid, Vapor
In both representations, the pH, the amount of KHCO₃ in the solid phase, and the vapor components (H₂O and CO₂) remain the same. However, in the Apparent Species view, the aqueous components include KOH and CO₂, which were not present in the True Species view.
Let’s consider all the equilibria equations that make up this calculation, using OLI TAG names where the suffix -AQ denotes neutral aqueous molecules, -ION denotes an ionic species, -VAP denotes a vapor species, and -PPT denotes a solid species:
- CO2(aq) + H2O = H+ + HCO3-
- CO2(vap) = CO2(aq)
- H2O = H+ + OH-
- H2O(vap) = H2O
- HCO3- = H+ + CO22-
- KHCO3(s)=K+ + HCO3-
- KOH(s) = K+ + OH-
The OLI software automatically adds input components for every vapor and neutral aqueous species detected in the equation set. This explains the presence of CO₂ and KOH in the Apparent Species view.
New Input Components:
- H2O: 0.7 moles
- KHCO3: 0.3 moles
- CO2: 0 moles
- KOH: 0 moles
Example 2: Different Compositions, Same Speciation
Solution 1 has 55.5082 moles of H₂O, 1 mol of NaOH, and 1 mol of HCl, and the fully speciated solution (or True composition) is given in the True column in the table below. Notice that the total moles apparent (57.5082 moles) are different from the total true moles (58.5082).
Solution 2 has 56.5082 moles of H₂O and 1 mol of NaCl. The fully speciated solution (or True composition) is given in the True column in the table below.
Both solutions have the same True speciation, but Solution 2, having fewer species, is preferred by the software for reporting the Apparent composition.
By understanding these differences, users can choose the appropriate representation for their needs, whether for detailed analysis or compatibility with other software.