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. The lines on a predominance diagram indicate where adjacent species have the same concentration. Either side of such a line one species or the other predominates, that is, has higher concentration relative to the other species.

To illustrate a predominance diagram, part of the one for chromate is shown at the right. pCr stands for minus the logarithm of the chromium concentration and pH stands for minus the logarithm of the hydrogen ion concentration. There are two independent equilibria, with equilibrium constants defined as follows.

${\displaystyle {\ce {{{CrO4^{2-}}+{H+}<=>HCrO4^{-}};\qquad {\mathit {K}}_{1}={\frac {[HCrO4^{-}]}{[CrO4^{2-}][H+]}}}}}$

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