Reduction of Chromium

Purpose:

To demonstrate two oxidation states of chromium, generate heat, and produce a gel by mixing two liquids.

Materials:

• Hot plate
• Tall form 250mL beaker

Reagents:

• 4M Sodium bisulfite (NaHSO₃)
• 0.5M Potassium dichromate (K₂Cr₂O₇)

Hazards and PPE:

• Potassium dichromate is an oxidixer, acute toxin, carcinogen, and reproductive toxin. Handle with extreme care.
• Wear approved safety goggles and gloves.

Protocol:

1. Add 50mL K₂Cr₂O₇ solution to beaker.
2. Add 50mL NaHSO₃ to same beaker.
3. The mixture will immediately turn a very dark green, generating quite a bit of heat, and after about 20-30 seconds it will become a gel and lighten.

Disposal:

• Remaining solution/solid should be placed in a properly labeled waste container.
• A spatula may be necessary to scoop out the gel.

Reactions:

• The most significant reaction involves the reduction of chromium and the subsequent formation of hydrated hydroxide; It is likely the ions of the sulfate and sodium are incorporated in the gel to form an alum [KCr(SO₄)₂].

Citations:

1. Summerlin, L., Ealy, J., and Borgford, Chemical Demonstrations, Volume II, ACS, 1987.

Videos:

• The colour change of acidified Potassium Dichromate - The Real Chemist

  The Real Chemist