TollensReagenz

Tollensreagenz, or Tollens reagent, is a laboratory solution used in qualitative analysis to detect aldehydes. It consists of silver ions in an ammoniacal complex, typically prepared by dissolving silver nitrate in water and adding ammonia solution until the brown silver oxide that initially forms dissolves to give a clear, colorless or pale blue solution containing the diamminesilver(I) complex [Ag(NH3)2]+. The reagent is light-sensitive and should be stored in a dark, cool container.

In alkaline ammonia medium, aldehydes are oxidized to carboxylates while the silver(I) ions are reduced to metallic silver. This reduction deposits a visible silver mirror on the inner surface of the container, which is the characteristic positive result of the Tollens test. Ketones generally do not react under standard conditions, making the test useful for distinguishing aldehydes from ketones. Some exceptions exist, such as certain α-hydroxy ketones or reducing sugars, which can give a positive result due to enediol or aldehyde-forming equilibria.

Applications of Tollensreagenz include identifying aldehydes in organic compounds and in carbohydrate analysis, where reducing sugars can yield a positive mirror test after appropriate treatment. The reagent is widely used in teaching and research to illustrate oxidation–reduction chemistry and the selective oxidation of aldehydes.

Historically, the reagent is named after Bernhard Tollens, a German chemist who described the reaction in the late 19th century. Safety considerations include the corrosive nature of ammonia and the toxic or staining properties of silver salts; proper disposal and protective equipment are advised.