Whimsical precipitate of barium chromate

The initial orange solution reversibly turns yellow with the formation of precipitate.

Scientific name: Barium nitrate precipitates only in alkaline solutions. The equilibrium between chromate and dichromate solution is shifted by addition of base and acid.

YouTube

Barium chromate precipitate

by ChemToddler

Reaction of Barium solution with Potassium chromate solution

by Akram Amir El Ali

13 Chromate - Dichromate.mpg

by sciencedemo

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

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

Ba2+ + CrO42- → BaCrO4

Ba2+ + Cr2O72- → BaCr2O7

Step-by-step instruction

  1. Dissolve some potassium dichromate in a large beaker.
  2. Divide the obtained solution into two parts.
  3. Add some sodium hydroxide to the first beaker. The color should change from orange to yellow.
  4. Add some hydrochloric acid to the second beaker. No change takes place.
  5. Prepare barium nitrate solution.
  6. Add some amount of barium nitrate solution to both prepared mixtures.
  7. The precipitate is formed in the beaker with alkaline solution.
  8. Pour sodium hydroxide to the acidic solution to shift the equilibrium. Stop adding when the solution turns yellow.
  9. Try to add barium nitrate solution again.
  10. The formation of precipitate is observed.

Scientific background

The occurring process involves the equilibrium between chromate(VI), dichromate(VI) and hydrogen ions. 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. Barium nitrate interacts with chromate ions producing insoluble barium chromate while while BaCr2O7 does not precipitate.

Danger:
Coolness:
Difficulty:

Published on 15 June 2015

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