How to choose a subsystem

The common question faced by users while generating a stability diagram is: How to choose a subsystem and how to determine the stability of metal containing species in certain subsystems?

The logical approach to take for this is the following algorithm:

Please note:

What is a redox subsystem?

• A set of species that contain a given element in all possible oxidation

states.

• Example: The iron subsystem consists of all species that contain Fe0, Fe2+ and Fe3+.

Now let us study an example:

If we have Iron, Nickel and Chromium , all three present in the same system, then the following will happen.

When we highlight the Iron subsystem, both FeCr2O4 and NiFe2O4 will be shown to be present.

The iron subsystem will look at follows:

When we select the Nickel subsystem from the specs options, you will see that NiFe2O4 is present.

But when you highlight Chromium, only Cr2O3 will be shown as stable:

The reason being that, certain species(like FeCr2O4) will be most stable at a certain E & pH among all Fe-containing species. But this species is not the most stable among all Cr-containing species. ( In other words , it is metastable with respect to a species that does not contain Fe).