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Tollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver(I) complex is present. It was named after its discoverer, the German chemist Bernhard Tollens.
A positive test with Tollens' reagent results in elemental silver precipitating out of solution, occasionally onto the inner surface of the reaction vessel, producing a characteristic and memorable "silver mirror" on the inner vessel surface.
Aldehydes will be positive in Tollens' test and a mirror-like material will be formed.
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. In this solution, the Ag+ ions from the aqueous silver nitrate exist in a hydrated form as [Ag(H2O)4]+ complexes, i.e. tetraaquasilver(I) ion. The OH- ions from the sodium hydroxide react with the Ag+ ions to give silver oxide, Ag2O. This is insoluble, and precipitates out of the solution as a brown solid. Aqueous sodium nitrate is also produced in the mixture as a by-product. This then creates:
In the next step, aqueous ammonia is added until all of the brown silver(I) oxide is dissolved. At this point the mixture will be clear, and there are now aqueous silver ions existing as [Ag(NH3)2]+ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed at the end of the preparation.
Alternatively, aqueous ammonia can be added in a continuous fashion directly to silver nitrate solution. At first, silver oxide will be formed and precipitate out, but as more ammonia solution is added the precipitate dissolves and the solution becomes clear as diamminesilver(I) is formed. At this point the addition of the ammonia should be stopped. This may be a preferable method as fewer reagents are involved. Filtering the reagent before use helps to prevent false-positive results.
Once it has been ascertained that there is a carbonyl group on an organic molecule using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH), Tollens' reagent can be used to determine whether the compound is a ketone or an aldehyde. Importantly, there is a special case in which Tollens' reagent will give a positive for a ketone; if the ketone is an alpha-hydroxy ketone, then the Tollens' reagent will react.
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.
The negative result for the test is no precipitate of silver formed when the carbonyl to be tested is added. A ketone will give a negative result because it cannot be oxidized easily. A ketone has no available hydrogen atom attached to the carbonyl carbon, meaning it cannot be so easily oxidized – unlike an aldehyde, which has this hydrogen atom.
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.
Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask. Around 500 mL of solution is prepared, much more than would be made for analytical use. This is then introduced to the clean glass surface which is to be mirrored and the solution reduced using a glucose solution. For a high quality finish the glass surface is cleaned using an oxidizing acid to remove all traces of organic compounds and the glass surface is pretreated with aqueous tin(II) chloride.
The reagent should be freshly prepared and stored refrigerated in a dark glass container. It has an approximate shelf-life of 24 hours when stored in this way. After the test has been performed, the resulting mixture should be acidified with dilute acid before disposal. These precautions are to prevent the formation of the highly explosive silver nitride.