Tollens test
The Tollens test , or Tollens test, is used to demonstrate the presence of reducing substances in a sample. The test is based on the reduction of complex bound silver ions to metallic silver.
Preparation of Tollens reagent[edit | edit source]
Tollens' reagent, chemically diamine silver nitrate – [Ag(NH 3 ) 2 ]NO 3 , is prepared in two steps.
- Mixing sodium hydroxide (NaOH) with silver nitrate (AgNO 3 ) produces a brown-gray precipitate of silver oxide (Ag 2 O).
- By slowly adding ammonium hydroxide (NH 4 OH), the precipitate gradually disappears, forming a complex [Ag(NH 3 ) 2 ] + . When the solution becomes completely transparent, the Tollens reagent is ready.
2 AgNO3 + 2 NaOH → Ag2O + 2 NaNO3 + H2O
Ag2O + 2 NaNO3 + 4 NH4OH → [Ag(NH3)2]NO3 + 2 NaOH + 3 H2O
Tollens' reagent is always prepared fresh. It is not advisable to store it for a long time, as explosive silver nitride forms in the solution over time.
Proof of reducing substances[edit | edit source]
If the reducing agent meets diamine silver nitrate, it will reduce the silver from the diamine complex. Evidence of the reaction is the formation of silver (Ag 0 ), which can be observed either as a mirror deposited on the walls of the test tube, or as mushroom silver (a macroscopic cluster of molecules floating in a solution resembling a sea sponge, which gradually settles at the bottom of the test tube).
An example can be the detection of an aldehyde group in reducing carbohydrates:
- R-CHO + [Ag(NH3)2]NO3 → Ag + 2 NH3 + R-NO3
Practical use[edit | edit source]
The Tollens test is used not only in the detection of reducing compounds, but also in the production of thermoses - a glass container is coated with a continuous layer of silver using a large amount of Tollens reagent and, for example, glucose.
Links[edit | edit source]
Related articles[edit | edit source]
References[edit | edit source]
- Tollens reagent Wikipedia article (cited 2014-10-25)
