Tollens’ Test: Description, Principle, Procedure And Result Interpretation

What Is Tollens’ Test?

Tollens’ test also referred to as silver-mirror test is a qualitative laboratory test used to distinguish between aldehyde and ketone. The test is named after a German chemist, Bernard Tollens who discovered the test. The test involves use of Tollens’ reagent which must be prepared immediately prior to its use as an explosive substrate can be formed if it is allowed to dry. In Tollens’ reaction, an aldehyde is oxidized to a carboxylic acid while the silver ion is reduced to silver metal, which deposits as a thin film on the inner surface of the glass. In other words, aldehydes give a grey black precipitate or a silver mirror when freshly prepared Tollens’ reagent is added to the solution.

Objective Of Tollens’ Test

To distinguish between aldehydic and ketonic functional groups.
To distinguish between reducing and non-reducing sugars.

Principle Of Tollens’ Test

Tollens’ reagent works on the fact that aldehydes are easily oxidized to the carboxylic acid using any mild oxidizing agents. Unlike aldehyde compounds, ketone-containing compounds cannot be easily oxidized to their respective carboxylic acids using mild oxidizing agents. Strong oxidizing agents are required to oxidize a ketone-containing compound.

Tollens’ reagent acts as a mild oxidizing agent and can easily oxidize an aldehyde. An aldehyde gives the positive test result for the Tollens’ test while most of the ketone give a negative test except ketones with alpha-hydroxy group. The reactants used to prepare Tollens’ reagent are aqueous silver nitrate solution (AGNO₃), concentrated ammonia solution (NH₃) and aqueous sodium hydroxide (NaOH). The role of NaOH is to maintain the basic PH of the solution.

When a compound containing an aldehyde group is reacted with a Tollen’s reagent also referred to as ammoniacal silver nitrate solution, the diamminosilver(I) ion-Ag(NH₃)₂⁺in the reagent, though a very weak oxidizing agent, oxidizes aldehydes to carboxylate anions. On the other hand, silver is reduced from the +1 oxidation state [of Ag(NH₃)₂⁺] to metallic silver. If the rate of reaction is slow and the walls of the vessel are clean, metallic silver deposits on the walls of the test tube as a mirror, if not, it deposits as a gray-to-black precipitate. On the contrary, if a ketone-containing compound without alpha-hydroxyl group is warmed with Tollens’ reagent (ammoniacal silver nitrate solution), no silver mirror or black precipitate is generated; in fact, there is no change in the reaction.

Experiment

Reagent And Material Required

Reagent

Tollens’ Reagent
Test solution/sample

Material Required

Test tubes
Test tube stand
Pipette
Water bath

Tollens’ Test Procedure

Take two clean dry, test tubes and add 1 ml of the test solution/sample in one test tube.
Similarly, take 1 ml of distilled water in another test tube to act as control.
Add 2 ml of Tollens’ reagent to both test tubes.
Keep both the test tubes in a water bath for at least 2 minutes.
Observe the development of color in the test tube.

Tollens’ Test Result Interpretation

Positive Test: A positive Tollens’ test is indicated by formation of dark grey precipitate or silver mirror on the bottom and sides of the test tube. This confirms the analyte or sample contains an aldehydic functional group or reducing sugars/Aldoses present.
Negative Test: A negative Tollens’ test is indicated by absence of dark-grey precipitate or no change in the reaction. This confirms absence of aldehydic functional group in the sample or reducing sugar/Aldoses absent.

Limitation Of Tollens’ Test

Some carbohydrates that do not have an aldehyde group might give a positive result on Tollens test because of the isomerization of such sugars under alkaline conditions.
The test involves use of Tollens’ reagent which must be prepared immediately prior to its use as an explosive substrate can be formed if it is allowed to dry.