Reduction of Viologen Dication

Demonstration Experiment on Video

Reduction of Viologen Dication

Peter Keusch

German version

The experiment is performed at microscale level. The chemical effect is projected onto the screen using an experiment projector - see Organic Chemistry Microscale Projection Experiments - Chemistry en miniature.

Chemicals:

methyl viologen dichloride hydrate (Aldrich)
N,N-dimethylformamide
sodium sulfate

Solutions:
0.3 mmol viologen dichloride (77 mg) dissolved in 1 mL N,N-dimethylformamide (DMF) / 9 mL 1 M aqueous sodium sulfate solution
1 M aqueous sodium sulfate solution (2.841 g / 20 mL)

Hazards and safety precautions:

Viologen dichloride is very toxic by inhalation, ingestion and if absorbed through skin. Possible mutagen. Possible carcinogen. May be fatal if swallowed, inhaled or absorbed through the skin.

DMF is harmful by inhalation, ingestion or skin contact. May act as a carcinogen. Ingestion or absorption through skin may be fatal.
A prolonged or repeated exposure should be avoided.

Safety glasses, protective gloves, good ventilation! The viologen dichloride solution should be prepared in a fume hood!

Experimental procedure:

An U-tube is fitted with a semipermeable membrane at the bottom. One leg of the U shaped tube is filled with 2 mL of viologen dichloride solution. The other leg is filled with 2 mL of 1 M Na₂SO₄. A platinum electrode is inserted into each of the arms of the U-tube. The electrode dipping into the viologen dichloride solution is connected to the negative terminal of DC voltage device. The source of DC voltage should have a potential of approx. 10 volts.

Result:

Blue 'schlieren' are formed immediately at the cathode (Photo). Finally, the solution at the cathode side is colored intensely blue. A Pasteur pipette is placed in the blue solution. Air is introduced into the solution by gentle squeezing of the pipette bulb. The blue solution becomes pale. Finally, a complete decolorization occurs.

Photo
Videoclip (Download RealPlayer .rm file)

Discussion:

· The colorless viologen dication is reduced at the cathode to the blue-violet viologen radical cation (1).

· The radical cation is oxidized by oxygen in the air to the colorless dication.

Viologen is intensively investigated and is used as a dye in electrochromic system because its ability to colour upon reduction and bleached upon oxidation.

General experimental instructions and index of experiments