An equilibrium exists between ${\mathrm{Cr}}_2{\mathrm{O}}_7^{-2} \mathrm{and} {\mathrm{CrO}}_4^{-2}$ at ${\mathrm{p}}^{\mathrm{H}}=\mathrm{x} ;$ At ${\mathrm{p}}^{\mathrm{H}}<\mathrm{x}$, the predominant species is

$$\begin{gathered} {\mathrm{Cr}}_2{\mathrm{O}}_7^{-2}+2{\mathrm{OH}}^{-}\rightleftharpoons 2{\mathrm{CrO}}_4^{-2}+{\mathrm{H}}_2\mathrm{O}; \\ \mathrm{In} \mathrm{basic} \mathrm{medium} \mathrm{dichromate} \mathrm{ion}\left(\mathrm{orange}\right) \mathrm{converts} \mathrm{to} \mathrm{chromate} \mathrm{ion}\left(\mathrm{yellow}\right); \\ 2{\mathrm{CrO}}_4^{-2}+2{\mathrm{H}}^{+}\rightleftharpoons {\mathrm{Cr}}_2{\mathrm{O}}_7^{-2}+{\mathrm{H}}_2\mathrm{O}; \\ \mathrm{In} \mathrm{acidic} \mathrm{medium} \mathrm{chromate} \mathrm{ion}\left(\mathrm{yellow}\right) \mathrm{converts} \mathrm{to} \mathrm{dichromate} \mathrm{ion}\left(\mathrm{orange}\right) \end{gathered}$$