thumb|upright=1.0|Tollens' test for aldehyde: left side positive (silver mirror), right side negative

thumb|right|200px|[[Ball-and-stick model of the diamminesilver(I) complex]]

Tollens' reagent (chemical formula ) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide (to maintain a basic pH of the reagent solution). It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.

Laboratory preparation

This reagent is not commercially available due to its short shelf life, so it must be freshly prepared in the laboratory. One common preparation involves two steps. First a few drops of dilute sodium hydroxide are added to some aqueous 0.1 M silver nitrate. The ions convert the silver aquo complex form into silver(I) oxide, , which precipitates from the solution as a brown solid:

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In the next step, sufficient aqueous ammonia is added to dissolve the brown silver(I) oxide. The resulting solution contains the [Ag(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed:

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Alternatively, aqueous ammonia can be added directly to silver nitrate solution.

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Tollens' reagent can also be used to test for terminal alkynes (). A white precipitate of the acetylide () is formed in this case. Another test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity. It also gives a positive test with hydrazines, hydrazones, α-hydroxy ketones and 1,2-dicarbonyls.

Both Tollens' reagent and Fehling's reagent give positive results with formic acid.

Staining

In anatomic pathology, ammonical silver nitrate is used in the Fontana–Masson stain, which is a silver stain technique used to detect melanin, argentaffin, and lipofuscin in tissue sections. Melanin and the other chromaffins reduce the silver nitrate to metallic silver.

In silver mirroring

Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask. The underlying chemical process is called silver mirror reaction. The reducing agent is glucose (an aldehyde) for such applications. Clean glassware is required for a high quality mirror. To increase the speed of deposition, the glass surface may be pre-treated with tin(II) chloride stabilised in hydrochloric acid solution.

For applications requiring the highest optical quality, such as in telescope mirrors, the use of tin(II) chloride is problematic, since it creates nanoscale roughness and reduces the reflectivity.

Safety

Aged reagent can be destroyed with dilute acid to prevent the formation of the highly explosive silver nitride.

See also

  • Benedict's reagent
  • Walden reductor (opposite use involving metallic silver)

References

  • Video of experimental process involving Tollens' reagent
  • Tollens' reagent on www.wiu.edu
  • Univ. of Minnesota Organic Chemistry Class Demo Result

fr:Réaction de Tollens

pl: Próba Tollensa