Projection Experiment: Test for Aldehydes

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Test for Aldehydes - Schiff's Reagent

Objective: Schiffs Reaction

Peter Keusch

German version

Chemicals:
acetaldehyde
formaldehyde solution 37 %
ethanol 96 %
aqueous solution of fuchsin 0.01 %
sulfurous acid
Schiff's Reagent (Merck)

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.

Formaldehyde 37% solution causes burns. Very toxic by inhalation, ingestion and through skin absorption. Readily absorbed through skin. Probable human carcinogen. Mutagen. May cause damage to kidneys, allergic reactions, sensitisation and heritable genetic damage.
Sulfurous acidis harmful if swallowed. Corrosive - causes burns. Skin or eye contact will lead to severe irritation or burns.

Ethanol is highly flammable.

Safety glasses, protective gloves, good ventilation. The experiments should be performed under a portable fume hood!

Experimental procedure:

First the formation of fuchsin sulfurous acid (SCHIFF'S REAGENT) is demonstrated. Some drops of sulfurous acid are pipetted to 1 mL of the fuchsin solution until the solution is decolorized.

Three test tubes are set up as described in the following table.

Test tube 1 1 mL of fuchsin solution (reference solution)
Test tube 2 0.3 mL of acetaldehyde 1 mL of ethanol
Test tube 3 0.3 mL of formaldehyde 1 mL of ethanol

10 drops of SCHIFF'S REAGENT are added to the ethanolic aldehyde solutions in test tube 2 and test tube 3.

Results:

The ethanolic aldehyde solutions turn reddish purple immediately (see foto below).

Discussion:

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

· Sulfurous acid discolors fuchsin. The bisulfite ion is attached to the central C-atom of the triphenylmethane compound disrupting the conjugated system (1).

· SCHIFF'S REAGENT reacts with aldehydes, regenerating the chromophore system. The reaction starts with the formation of a carbinolamine, which is dehydrated to a diimine. The diimine reacts with sulfurous acid to produce a resonance stabilized cation (2). The reaction is kinetic controlled.

The addition of bisulfite to aldehyde (3) is a competing reaction. The reaction is thermodynamic controlled.

The different colors of the solutions in T1, T2 and T3 are due to different "kinds of fuchsin". In T1 fuchsin is present. In T2 and T3 are formed N-substituted fuchsin derivates (Photo above).

References:
Demonstration Experiment on Video Addition of Bisulfite to Aldehydes
Demonstration Experiment on Video Schiff's Reagent

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