Stepping Calculations in an Aqueous Solution

Performing the stepping calculation overwrites all the previous settings and stepping results on the POLY3-file. To avoid this, use SAVE_WORKSPACES to create a new workspace.

When you have calculated a Pourbaix diagram, you can use the saved results of the calculation (the POLY3 file), and make a property diagram based on stepping calculation.

Since the program normally sets the calculated initial equilibrium point as the starting point, the minimum value for the stepping variable must be specified as smaller or equal to the specific initial condition, and the maximum value as larger or equal to that condition. If pH is set as the stepping variable, the program automatically calculates the equilibrium constant in logarithmic (log10Ka) of the electrolysis reaction H2O (water) = H+ + OH- for the real aqueous solution under the current temperature-pressure-composition conditions, and thus this constant log10Ka value is considered as the highest pH in specifying the maximum value along stepping. Subsequently, confirm or modify an increment factor with a default value; for pressure and mole of a component, specify the step in logarithmic (log10P or log10N). 

You can use any of these state variables as a stepping variable.

State Variable Abbreviation

Temperature (K)

T

Pressure (bar)

P

Acidity

pH

Electronic potential (V)

Eh

Mole number of a dissolving component (such as Fe, Na, Cl, or NaCl) in 1 kg of water.

N(Comp)

(for example, N(Fe))

You must have plotted a Pourbaix diagram before making a stepping calculation on an aqueous solution.

  1. Use goto_module pourbaix to enter the POURBAIX module.
  2. Type 4 to open an old file and make another STEPPING calculation.
  3. Choose which POLY3 file to load.

  4. Specify the conditions of the calculation by answering the questions that follows.

    The system definition is the same as in the previous Pourbaix calculation, but you can modify conditions such as bulk composition, temperature and pressure, and change some component definitions if necessary.

  5. Specify which variable to use as the stepping variable (see the preceding table), its minimum and maximum value, and the length of each incremental step.

    The settings and stepping results are written to the opened POLY3 file, and a first default property diagram is plotted.