${\mathrm{CrO}}_4^{2-}\text{ and }{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$ 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.

${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+2\mathrm{OH}⟶\underset{\text{ Yellow }}{2{\mathrm{CrO}}_4^{2-}+2{\mathrm{H}}_2\mathrm{O}}$

$2{\mathrm{CrO}}_4^{2-}+2{\mathrm{H}}^{\oplus }⟶\underset{\text{ orange red }}{{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}_2\mathrm{O}}$

In acidic medium (pH = 3) colour changes to orange red and in basic medium (pH = 8) colour changes to yellow.