Tollens’ reagent, also known as silver mirror test, is a chemical test used to detect the presence of aldehydes in a given compound. It is named after Bernhard Tollens, the German chemist who developed this test.
The reagent is made up of silver nitrate (AgNO3) dissolved in aqueous ammonia (NH3). The Tollens’ reagent is usually prepared freshly before use because it is unstable and can decompose over time.
When Tollens’ reagent is added to a solution containing an aldehyde, such as formaldehyde or glucose, a redox reaction occurs. The aldehyde is oxidized to a carboxylic acid, while silver ions from the Tollens’ reagent are reduced to metallic silver. The metallic silver forms a silver mirror or a silver precipitate on the inner surface of the reaction vessel.
The reaction can be summarized as follows:
2 Ag(NH3)2+ + RCHO + 3 OH- → 2 Ag + RCOO- + 4 NH3 + 2 H2O
The appearance of a silver mirror or a silver precipitate indicates the presence of an aldehyde. If no aldehyde is present, the Tollens’ reagent remains unchanged and no silver mirror is formed.
The Tollens’ reagent test is commonly used as a qualitative test for aldehydes and can be used as an alternative to the Fehling’s test or Benedict’s test. It is particularly useful when dealing with compounds that do not react well with other tests or when the presence of aldehydes needs to be confirmed.