Projection Experiment: Reduction of p-Benzoquinone by Disulfite

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Reduction of p-Benzoquinone by Disulfite

Peter Keusch

German version

Chemicals:
0.2 M aqueous benzoquinone solution (216 mg / 10 mL H₂O)

0.2 M aqueous Na₂S₂O₅ solution (380 mg / 10 mL H₂O)

The solution of benzoquinone should be prepared immediately before the demonstration, because it decomposes rapidly.

Hazards and safety precautions:

p-Benzoquinone is toxic. May be fatal if swallowed, inhaled or absorbed through skin. Severe irritant.

Sodium disulfite may cause irritation and may be harmful if swallowed. May cause allergic reaction, skin and eye irritant.

Safety glasses, protective gloves, suitable ventilation. The benzoquinone solution should be prepared in a fume hood!

Experimental procedure:

A sodium disulfite solution (approx. 5 drops) is added to 1 mL of 0.2 M benzoquinone, until the precipitation of quinhydrone occurs. Afterwards 20 drops of disulfite solution are added dropwise, until the solution becomes colorless.

Test tube 1 Na₂S₂O₅ solution
Test tube 2 benzoquinone solution Na₂S₂O₅ solution

Result:

A needle-shaped precipitate is formed (Photo 1). The addition of disulfite is continued until the precipitate is again dissolved (Photo 2).

Photo 1

Photo 2

Discussion:

· The reduction of p-benzoquinone to hydroquinone by disulfite runs via the charge-transfer-complex quinhydrone.

General experimental instructions and index of experiments