The Tollens’ test is a classical laboratory test used to distinguish aldehydes from ketones.
Definition
Tollens’ test makes use of Tollens’ reagent, which is an ammoniacal silver nitrate solution (silver ions in ammonia solution).
Principle
- Aldehydes are easily oxidized to carboxylic acids.
- In this process, the silver ions in Tollens’ reagent are reduced to metallic silver.
- Ketones generally do not react (except a few like α-hydroxy ketones).
Observation
- If an aldehyde is present, a shiny silver mirror or a black precipitate of silver forms on the walls of the test tube.
- No such change is seen with ordinary ketones.
Uses
- To detect the presence of an aldehyde group.
- To distinguish aldehydes from ketones.
- Sometimes used for identifying reducing sugars (like glucose), which give a positive Tollens’ test.