Tollens' reagent is a chemical reagent used to determine the presence of an aldehyde, aromatic aldehyde and alpha-hydroxy ketone functional groups. The reagent consists of a solution of silver nitrate and ammonia. 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.
This reagent is not commercially available due to its short shelf life, as such 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 silver nitrate. The OH− ions convert the silver aquo complex form into silver oxide, Ag2O, which precipitate from the solution as a brown solid:
- 2 AgNO3 + 2 NaOH → Ag2O (s) + 2 NaNO3 + H2O
In the next step, sufficient aqueous ammonia is added to dissolve the brown silver(I) oxide. The resulting solution contains the [Ag(NH3)2]+ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed:
- Ag2O (s) + 4 NH3 + 2 NaNO3 + H2O → 2 [Ag(NH3)2]NO3 + 2 NaOH
Alternatively, aqueous ammonia can be added directly to silver nitrate solution. At first, ammonia will induce formation of solid silver oxide, but with additional ammonia, this solid precipitate dissolves to give a clear solution of diamminesilver(I) complex ( [Ag(NH3)2]+). Filtering the reagent before use helps to prevent false-positive results.
Qualitative organic analysis
Once the presence of carbonyl group has been identified using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish ketone vs aldehyde. Tollens' reagent gives a negative test for most ketones, with alpha-hydroxy ketones being one exception.
The test rests on the premise that aldehydes are more readily oxidised compared with ketones; this is due to the carbonyl-containing carbon in aldehydes having an attached hydrogen. The diamminesilver(I) complex in the mixture is an oxidizing agent and is the essential reactant in Tollens' reagent. The test is generally carried out in a test tube in a warm water bath.
In a positive test, the diamminesilver(I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution, occasionally onto the inner surface of the reaction vessel, giving a characteristic "silver mirror". The carboxylate ion on acidification will give its corresponding carboxylic acid. The carboxylic acid is not directly formed in the first place as the reaction takes place under alkaline conditions. The ionic equations for the overall reaction are shown below; R refers to an alkyl group.
- 2 [Ag(NH3)2]+ + RCHO + H2O → 2 Ag(s) + 4 NH3 + RCO2H + 2 H+
Tollens' reagent can also be used to test for terminal alkynes (RC2H). A white precipitate of the acetylide (AgC2R) is formed in this case. Another test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity.
In anatomic pathology, ammoniacal 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 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.  Methods to produce telescope mirrors include additional additives to increase adhesion and film resilience, such as in Martin's Method, which includes tartaric acid and ethanol.
- Brady's reagent
- Benedict's reagent
- Fehling's solution
- Walden reductor (opposite use involving metallic silver)
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