Chromate(VI)-dichromate(VI) equilibrium. Chromate(VI) ([CrO4]2-) and dichromate(VI) ([Cr2O7]2-) ions exist in an aqueous solution in equilibrium, according to the equation: [CrO4]2- + [H]+ <--> [Cr2O7]2- + H2O. According to Le Chatelier's Principle this equilibrium can be shifted by changing concentration of [H]+ ions (i.e. the pH of the solution). This is demonstrated in this series of photos. Left frame shows 4 mL of 0.25M solution of potassium chromate(VI) (K2CrO4, yellow). Middle frame was taken after 0.5 mL of 1M sulfuric acid (H2SO4) was added and the color changed to orange, indicating dichromate(VI) ions. Right frame was taken after 1.5 mL of 1M potassium hydroxide (KOH) was added and the color changed back to yellow indicating chromate(VI) ions.