Chromate-dichromate equilibrium

Equilibrium of orange and yellow solution

Scientific name: The equilibrium between two colored chromate and dichromate forms in aqueous solution

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Safety

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!

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

2CrO42-(aq)(yellow) + 2H+(aq) ↔ Cr2O72-(aq)(orange) + H2O

Step-by-step instruction

  1. Take a very small amount of potassium dichromate and dissolve it in 30-40 ml of water in a 250 ml heat resistant flask.
  2. Add about 5 ml of dilute sulfuric acid and mix. You have a orange solution.
  3. Add about 10 ml of dilute sodium hydroxide solution and shake the mixture. Now you have a yellow solution.
  4. Add a small amount of acid to the flask to make the solution orange.
  5. Add a small amount of sodium hydroxide to the flask to make the solution yellow.

Scientific background

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.

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Published on 13 April 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst