Predominance Diagrams

A predominance diagram answers “which species dominates here?” over a field of two variables — classically pe against pH, giving an Eh–pH or Pourbaix diagram, but the two axes can be anything the model depends on: temperature, a total concentration, a partial pressure.

The Predominance module draws them, and does so without simulating the whole field. It is a licensed module, and works over a PHREEQC study or a function model.

Two ways to get one

From a grid. Run a study over a lattice of the two parameters — a grid study, or a sweep — and plot the result. Every point is simulated. Simple, and expensive in proportion to the resolution you want.

By tracking boundaries. A predominance study follows the lines where one expression gives way to another, rather than evaluating everything either side of them. The interesting part of a predominance diagram is its boundaries, so tracking them gets a sharp diagram for a fraction of the simulations a lattice of the same sharpness would need.

Tracking is the reason the module exists. Use a grid when you want the field itself — a surface of some quantity — and tracking when you want the map.

Levels

A species need not be the dominant one to be worth drawing. Each expression is reported at a level: predominant, sub-predominant, sub-sub-predominant. Drawing the second rank shows what is waiting behind the first, which is often where the chemistry of interest is — a species that never dominates anywhere can still matter.

Areas, masks and labels

The plot draws, for each area expression, where it holds at the level you asked for.

Constraint areas (masks) restrict the diagram to where it means something. The water stability limits are the usual case: outside them the diagram is arithmetic rather than chemistry, and masking says so.

Labels name the fields. A predominance diagram without labels is decoration.

The trace works on any study’s data, not only a tracking study’s — including a sweep plotted against a computed axis such as pe. A quick result builds the plot straight from a tracking study when you do not need to assemble it yourself.

Reading one

The lines are where two expressions are equal, so a boundary’s position depends on the total concentrations assumed, not only on the thermodynamics. Two diagrams of the same system at different totals are different diagrams. Say which you used.

The fields are thermodynamic predominance, not what you will measure: they say what is stable, not what forms, and kinetically persistent species sit happily in the wrong field. Within the mask, at equilibrium, the diagram is what the database says — which is also why the same system drawn from two databases can differ. Check the log K values before concluding that a published diagram is wrong.

Examples