What is Potential–pH diagram, working concentration?

The potential–pH diagram of an element is the plane $(\mathrm{pH}, E)$ divided into regions, each the domain of predominance (dissolved species) or of existence (solids) of one form of the element. It is drawn for a chosen working concentration $c$, the total concentration of the element in solution, with these conventions:

• on a boundary between two dissolved species, each is at $c$ (counted in atoms of the element);
• on a boundary between a dissolved species and a solid, the dissolved species is at $c$ and the solid is just present;
• gases are at $1\,\mathrm{bar}$.

In this chapter $c = 0.010\,\mathrm{mol}/\mathrm{L}$.