Tollens' Test/Reagent

Tollens' reagent, used in what is known as the Tollens' test, or ‘silver mirror test’, is a special test chemical developed by Bernhard Tollens. It is used in organic chemistry as a distinguishing procedure between organic ketones (RCOR’) and organic aldehydes (RCHO). With aldehydes the reagent will react and deposit a silver coating on the glass surface of the reaction apparatus, hence coining the ‘silver mirror’ name for this test. However, with ketones, no reaction occurs and no silver is deposited.

The reaction behind the Tollens' test is generally an alkali-catalysed redox reaction. To create Tollens' reagent, one first adds concentrated ammonia to aqueous silver nitrate (AgNO₃) in slight excess to create the diamminesilver complex ion, [Ag(NH₃)₂]⁺ (aq).

During this procedure, insoluble silver oxide is first precipitated, and with the addition of excess concentrated ammonia, the silver oxide is 'ammoniated' to form the soluble silver complex ion, which is indicated by the re-dissolving of the precipitate. After this, a little aqueous sodium/potassium hydroxide is added to the reagent as a catalyst, and ammonia added again to dissolve any precipitate.

Thus, in the reagent we now have an oxidizing agent, which is the diamminesilver complex ion, and a catalyst, i.e. the hydroxide ion. However, after a short period of time (roughly a day) the reagent will decompose to form a very hazardous and extremely shock-sensitive explosive, silver nitride (Ag₃N). Thus, Tollens' reagent should be used immediately after preparation and disposed of properly after use.

To run the test, one just has to add the organic compound that is to be tested into the reagent and observe any changes.

For aldehydes, the –CHO group acts as a reducing agent, reducing the diamminesilver ion to aqueous ammonia, and also elemental silver, which deposits on the walls of the reaction vessel to form a silver mirror. In the process, the aldehyde is oxidized to the corresponding carboxylic acid.

Half-reaction 1 (reduction)

Half-reaction 2 (oxidation)

Overall redox reaction

For ketones, however, the C=O group cannot reduce the diamminesilver ion, and no reaction occurs.

Besides analytical purposes, Tollens' reagent was also used before the advent of electroplating as a means of manufacturing mirrors. However, due to its hazardous properties this method of production has fallen out of use.

Do refer to the video below for the procedure.