Schiff's Reagent

Demonstration Experiment on Video

Schiff's Reagent

Objective: Test for Aldehydes

Peter Keusch

German version

Chemicals:
acetaldehyde
glucose
ethanol 96 %
0.01 % aqueous solution of fuchsin
sodium disulfite
1 N HCl

Glass wares:
3 graduated cylinders with stopper 500 mL
3 glass stirring rods
2 beakers 100 mL
pipette 1 mL, graduated in 0.1 mL
2 volumetric pipettes 10 mL
volumetric pipette 50 mL
4 pipette bulbs
2 snap-cap vials 20 mL

Hazards and safety precautions:

Acetaldehyde is harmful by inhalation, ingestion and through skin absorption. Some experiments with animals suggest that this substance may be anticipated to be a carcinogen.

Ethanol is highly flammable.

Sodium disulfite may cause irritation and may be harmful if swallowed. May cause allergic reaction, skin and eye irritant.

Safety glasses, protective gloves, effective ventilation!

The following solutions are prepared in advance:

Acetaldehyde solution: 0.5 mL of acetaldehyde in 10 mL ethanol
Glucose solution: 1.6 g of glucose in 10 ml of dist. water
Sodium disulfite solution: 6 g of sodium disulfite in 60 mL of dist. water

Experimental procedure:

Three cylinders are each filled with 300 mL of aqueous fuchsin solution. 30 mL of aqueous disulfite are added to the solutions in the second and third cylinder, causing the fuchsin solutions to be decolorized. An ethanolic solution of acetaldehyde is added to the solution in cylinder 2. The decolorized fuchsin solution in cylinder 3 is mixed with the aqeous glucose solution.

Results:

The decolorized fuchsin solution in cylinder 2 turns red-purple immediately. The solution in cylinder 3 remains colorless.

Video clip (Download RealPlayer .rm file)

Discussion:

Aldehydes are characterized by the red-purple color which they give with a solution of fuchsin that has been decolorized by sulfurous acid.

· Bisulfite reacts with fuchsin via an electrophilic addition to generate a colorless reagent. The bisulfite ion is attached to the central C atom of the triphenylmethane compound and thus the branched conjugated system is disrupted (1).

· SCHIFF'S REAGENT reacts with aldehydes, regenerating the chromophoric system. Via a carbinolamine a diimine is formed, which reacts with sulfurous acid to give a resonance stabilized cation (2). The reaction is kinetic controlled.

The addition of bisulfite to aldehyde (3) is a competing reaction. Maybe the reaction of glucose with Schiff's reagent proceeds in such a way.

References:
Demonstration Experiment on Video Addition of Bisulfite to Aldehydes
Demonstration Experiment on Video Addition of 2,4-Dinitrophenyl hydrazine to Aldehydes and Ketones
Microscale Projection Experiments Test for Aldehydes - Schiff's Reagent

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