Predominance diagram

A predominance diagram purports to show the conditions of concentration and pH where a chemical species has the highest concentration in solutions in which there are multiple acid-base equilibria.

[2] Using these values and the equality conditions, the concentrations of the three species, chromate CrO2−4, hydrogen chromate HCrO−4 and dichromate Cr2O2−7 can be calculated, for various values of pH, by means of the equilibrium expressions.

The chromate ion is the predominant species in the region to the right of the green and blue lines.

Predominance diagrams can become very complicated when many polymeric species can be formed as, for example, with vanadate,[4] molybdate[1] and tungstate.

[1] Another complication is that many of the higher polymers are formed extremely slowly, such that equilibrium may not be attained even in months, leading to possible errors in the equilibrium constants and the predominance diagram.