When the activities () can be considered as equal to the molar, or the molal, concentrations () at sufficiently diluted concentrations when the activity coefficients () tend to one, the term regrouping all the activity coefficients is equal to one, and the Nernst equation can be written simply with the concentrations () denoted here with square braces [ ]:
There are three types of line boundaries in a Pourbaix diagram: Vertical, horizontal, and sloped.