The Tollen’s Test for an Aldehyde

This mirror was formed by a positive Tollens' test

To carry out the Tollen’s test and form a silver mirror you must rigorously clean your glassware with concentrated KOH (Potassium Hydroxide)

The test involves using Tollens’ reagent, which must be prepared immediately prior to its use as an explosive substrate can be formed if it is allowed to dry.

To form the Tollens’ reagent:

  • Add a fingers depth of silver nitrate Solution 0.05n to a test tube.
  • To this add ammonia solution (which forms a white precipitate.)
  • Keep adding ammonia solution until the white precipitate just disappears.
    (DO NOT STORE THIS. Once Tollens’ solution is prepared it must be used within a few hours or explosive silver fulminate may form)

To test for an aldehyde add a few drops of aldehyde to the solution and place the test tube in a warm water bath for around 5 minutes.

If an aldehyde is present a silver mirror will form on the glass. (if the glassware is not absolutely perfectly clean a dark precipitate may appear in the solution instead)

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