Tollens' reagent is a chemical reagent used to determine the presence of an
aldehyde or 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. Laboratory preparation
This reagent is not commercially available due to its short shelf life.
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 + 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 + 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)
complex2]+). Filtering the reagent before use helps to prevent
false-positive results. Uses
= Qualitative organic analysis= Once the presence of carbonyl group has
been identified using 2,4-dinitrophenylhydrazine, Tollens'
reagent can be used to distinguish ketone vs aldehyde. Tollens' reagent
gives a negative test for most ketones, except those with alpha-hydroxy ketone.
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. 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.
Both Tollens' reagent and Fehling's reagent also give positive results with
formic acid, which is fully oxidised to water and carbon dioxide.
= Staining= 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 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. Safety
Aged reagent can be destroyed with dilute acid to prevent the formation of
the highly explosive silver nitride. See also
Brady's reagent Benedict's reagent
Fehling's solution Walden reductor
References External links
Video of experimental process involving Tollens' reagent
Tollens' reagent on www.wiu.edu Univ. of Minnesota Organic Chemistry
Class Demo Result