Projection Experiment: Reduction of p-Benzoquinone by Iodide

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Reduction of p-Benzoquinone by Iodide

Peter Keusch

German version

Chemicals:
0.1 M aqueous p-benzoquinone solution (1.080 g / 100 mL H₂O)
0.01 M aqueous of potassium iodide solution (0.166 g / 100 mL H₂O)
conc. H₂SO₄
0.5 % aqueous starch solution

Hazards and safety precautions:

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

Conc. sulfuric acid is highly toxic. Causes severe burns. May be fatal if swallowed. May cause cancer through inhalation. Very destructive of mucous membranes.

Safety glasses, protective gloves and good ventilation required. The aqueous p-benzoquinone solution should be made under a fume hood!

Experimental procedure:

Three test tubes are set up as shown in the table below.

Test tube 1	1 mL of potassium iodide solution
Test tube 2	1 mL of potassium iodide solution 1 drop of conc. H₂SO₄ 2 drops of freshly prepared p-benzoquinone solution
Test tube 3	1 mL of aqueous starch solution

Two drops of the intensively yellow solution from T2 are added to the starch solution in T3.

Result:

Blue 'schlieren' prove the oxidation of iodide to iodine (see Photo below).

Discussion:

· p-benzoquinone oxidizes iodide to iodine and is reduced to hydroquinone.

equation

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