Equilibrium of orange and yellow solution
name: The equilibrium between two colored chromate and dichromate forms in aqueous solution
Wear eye protection goggles. Be careful with the acid and sodium hydroxide solutions! Don't drink or eat potassium dichromate and chromium-containing solutions, they are very toxic!
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The occurring process involves the equilibrium between chromate(VI), dichromate(VI) and hydrogen ions.
2CrO42-(aq)(yellow) + 2H+(aq) ↔ Cr2O72-(aq)(orange) + H2O.
Chemical equilibrium behaves according to the Le Chatelier’s principle. It can be stated as "When a system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established."
The addition of acid shifts the equilibrium to the right, producing more orange-colored dichromate(VI) ions. The addition of hydroxide ions causes the concentration of hydrogen ions to decrease, and this brings the equilibrium back to the left-hand side, regenerating yellow chromate(VI) ions.