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Demonstration of Microscale Projection Experiments - Chemistry en miniature

Formation of Quinhydrone

Peter Keusch





German version



Chemicals:
0.1 M aqueous hydroquinone solution  (220 mg / 20 mL H2O)
0.1 M aqueous p-benzoquinone solution  (216 mg / 20 mL H2O)

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


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.

Safety glasses, protective gloves, effective ventilation. Both the solution of hydroquinone and the solution of p-benzoquinone should be prepared in a fume hood!


Experimental procedure:

3-5 drops of freshly prepared p-benzoquinone solution are added to 1 mL of the hydroquinone solution.


Test tube 1 hydroquinone solution
Test tube 2   hydroquinone solution    some drops of benzoquinone solution  


Result:

Dark, needle-shaped crystals are precipitated  (see Photo below).


Foto1


Discussion:

 ·   The formed quinhydrone represents a complex, which can be isolated. The complex is obtained by mixing equivalent quantities of p-benzoquinone and hydroquinone. In the presence of light, an electron is transfered from hydroquinone (donor) to p-benzoquinone (acceptor) giving the quinhydrone the very characteristic purple color. 1:1-adducts with this property are called "charge transfer complexes".

equation

·   The redox system hydroquinone / benzoquinone in the molar ratio 1:1 is used in the pH-measurement. The electrode-potential E of this organic half cell can described in accordance with the equation  E = 0,699 + 0,059 logs [H+] (simplified method). The equation allows to determine the hydrogen ion concentration of any solution.

Reference:
  Demonstration Experiment on Video   Redoxsystem Hydroquinone / Benzoquinone (2)


General experimental instructions and index of experiments






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