Wurster's Blue

Demonstration Experiment on Video

Wurster's Blue - Oxidation of N,N,N',N'-Tetramethyl-p-phenylendiamine

Peter Keusch

German version

Chemicals:
N,N,N',N'-tetramethyl-p-phenylendiamine (TMPD)
bromine

Apparatus and glass wares:
3 gas washing bottles, 150 mL, 29/32
volumetric flask 100 mL
volumetric flask 150 mL
measuring cylinder 100 mL
water jet pump
rubber tubings

Hazards and safety precautions:

Bromine is highly toxic if inhaled, ingested or comes in contact with the skin.

Bromine water is harmful if ingested or inhaled. Prolonged skin contact can cause burns. Eye irritant - lengthy contact will lead to eye damage.
TMPD is harmful by ingestion, inhalation or through skin contact.

Safety goggles and protective gloves must be worn when handling bromine. The experiment should be performed under a portable fume cupboard giving all-round visibility!

Experimental procedure:

The following solutions are prepared:

Bromine water: 2 g of bromine are dissolved in 100 ml of dist. water.
Tetramethyl-p-phenylendiamine solution: Approx. 2 mg of tetramethyl-p-phenylendiamine are dissolved in 1 mL of ethanol in a 150 mL volumetric flask. The solution is made up to the mark with water saturated with nitrogen.

Three gas washing bottles are connected in series by means of two short rubber tubings. The third bottle is connected to a water jet pump. The first bottle is filled with 75 mL of bomine water. The second bottle remains empty and the third bottle contains 75 mL of a TMPD solution. The suction of the water jet pump is adjusted in such a manner that the bromine vapour released from bromine water bubbles slowly and continously through the TMPD solution.

Video clip (Download RealPlayer .rm file)

Results:

The colorless TMPD solution turns blue violet and becomes finally colorless again.

Discussion:

· Reacting with bromine the diamine loses an electron. The blue solution contains a radical, which is resonance stablized and proves to be stable. When more bromine gas is introduced into the solution a colorless dication is formed. The chromophore resembles that of the p-benzoquinone. Due to the low concentration the solution is colorless.

Reference:
Wurster's original paper of 1808 (in German!)
C. Wurster und R. Sendtner Zur Kenntmis des Dimethylparaphenylendiamins - I. Einwirkung von Brom au Dimethylparaphenylendiamin

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