CrO42− and Cr2O72− are interconvertible in aqueous solution and they are in equilibrium at pH = 4. The colour of the solution at pH = 3 and pH = 8 are respectively.

Cr2O72−+2OH⟶2CrO42−+2H2O Yellow 2CrO42−+2H⊕⟶Cr2O72−+H2O orange red In acidic medium (pH = 3) colour changes to orange red and in basic medium (pH = 8) colour changes to yellow.