In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.

2 CrO²⁻
₄ + 2 H⁺ ⇌ Cr
₂O²⁻
₇ + H₂O
The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium.
The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions.

Further condensation reactions can occur in strongly acidic solution with the formation of trichromates, Cr
₃O²⁻
₁₀, and tetrachromates, Cr
₄O²⁻
₁₃. All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO₄ units sharing corners.

The hydrogen chromate ion, HCrO₄⁻, is a weak acid:
HCrO⁻
₄ ⇌ CrO²⁻
₄ + H⁺;      pKₐ ≈ 5.9
It is also in equilibrium with the dichromate ion:
2 HCrO⁻
₄ ⇌ Cr
₂O²⁻
₇ + H₂O
This equilibrium does not involve a change in hydrogen ion concentration, which would predict that the equilibrium is independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion.
The hydrogen chromate ion may be protonated, with the formation of molecular chromic acid, H₂CrO₄, but the pKₐ for the equilibrium 
H₂CrO₄ ⇌ HCrO⁻
₄ + H⁺
is not well characterized.
Reported values vary between about −0.8 and 1.6.

The dichromate ion is a somewhat weaker base than the chromate ion:
HCr
₂O⁻
₇ ⇌ Cr
₂O²⁻
₇ + H⁺,      pK = 1.8
The pK value for this reaction shows that it can be ignored at pH > 4.
In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.
2 CrO²⁻
₄ + 2 H⁺ ⇌ Cr
₂O²⁻
₇ + H₂O
The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium. The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions.
Further condensation reactions can occur in strongly acidic solution with the formation of trichromates, Cr
₃O²⁻
₁₀, and tetrachromates, Cr
₄O²⁻
₁₃. All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO₄ units sharing corners.
The hydrogen chromate ion, HCrO₄⁻, is a weak acid:
HCrO⁻
₄ ⇌ CrO²⁻
₄ + H⁺;      pKₐ ≈ 5.9
It is also in equilibrium with the dichromate ion:
2 HCrO⁻
₄ ⇌ Cr
₂O²⁻
₇ + H₂O
This equilibrium does not involve a change in hydrogen ion concentration, which would predict that the equilibrium is independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogen chromate ion may be protonated, with the formation of molecular chromic acid, H₂CrO₄, but the pKₐ for the equilibrium 
H₂CrO₄ ⇌ HCrO⁻
₄ + H⁺
is not well characterized. Reported values vary between about −0.8 and 1.6.
The dichromate ion is a somewhat weaker base than the chromate ion:
HCr
₂O⁻
₇ ⇌ Cr
₂O²⁻
₇ + H⁺,      pK = 1.8
The pK value for this reaction shows that it can be ignored at pH > 4.
Chromate and dichromate