Redox System Benzoquinone / Hydroquinone (1)

Demonstration Experiment on Video

Redox System Benzoquinone / Hydroquinone (1)

Objective: Reduction of p-Benzoquinone with Iodide

Peter Keusch

German version

Chemicals:
0.01 M aqueous potassium iodide solution (664 mg / 400 mL dist. water)
0.5 M aqueous hydroquinone solution (22 g / 400 mL dist. water)
0.02 M aqueous p-benzoquinone solution ( 0.9 g / 120 mL dist water)
0.1 M aqueous AgNO₃ solution (3.4 g / 20 mL dist. water)
conc. sulfuric acid
0.2 % aqueous starch solution

The solutions of benzoquinone and hydroquinone are prepared immediately before the demonstration (the solutions decompose in hours).

Glass wares:
3 conical measures, graduated, 500 mL
2 glass stirring rods
2 measuring cylinders 200 mL
2 beakers 400 mL
volumetric pipette 10 mL
volumetric pipette 20 mL
2 Peleus bulbs
snap-cap vial 20 mL
snap-cap vial 30 mL

Hazards and safety precautions:

Hydroquinone is probable carcinogen. Toxic. Severe skin irritant. Harmful by inhalation and ingestion. May cause sensitization. Eye and respiratory irritant.
Benzoquinone is highly 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 and, protective gloves required! The experiment should be performed in a laboratory fume cupgoard!

Experimental procedure:

Three conical measures are set up as follows:

conical measure 1 400 mL of aqueous KI, 0.5 ml of conc. H₂SO₄
conical measure 2 400 mL of starch solution
conical measure 3 400 mL of aqueous hydroquinone

The acidic KI solution in conical measure 1 is mixed with 30 mL of aqueous p-benzoquinone while stirring. Approx. 20 mL of the solution are added to the starch solution of conical measure 2.

Finally, the hydroquinone solution in conical measure 3 is mixed with 20 mL of aqueous AgNO₃.

Results:

conical measure 1 intensely yellow color
conical measure 2 blue schlieren
conical measure 3 The solution becomes turbid immediately.
The light brown "veil" becomes increasingly darker.

Video clip (Download RealPlayer .rm file)

Discussion:

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

equation

· Hydroquinone is oxidized by the strong oxidizing agent Ag⁺ to p-benzoquinone. In the form of a suspension of fine particle silver is precipitated.

This reaction applies in the black-and-white photography. Alkaline hydroquinone solution is used as a developer. Silver bromide, photo-activated by light exposure, is reduced to metallic silver.

Index of Lecture Experiments

conical measure 1	intensely yellow color
conical measure 2	blue schlieren
conical measure 3	The solution becomes turbid immediately. The light brown "veil" becomes increasingly darker.