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. In this solution, the Ag+ ions from the aqueous silver nitrate exist in a hydrated form as + 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:
- 2 AgNO3 (aq) + 2 NaOH (aq) → Ag2O (s) + 2 NaNO3 (aq) + H2O (l)
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 + complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed at the end of the preparation.
- Ag2O (s) + 4 NH3 (aq) + 2 NaNO3 (aq) + H2O (l) → 2 Ag(NH3)2NO3 (aq) + 2 NaOH (aq)
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 less reagents are involved. Filtering the reagent before use helps to prevent false-positive results.
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