Tollen’s reagent is an alkaline solution containing a silver–ammonia complex, which is used in organic chemistry to test for the presence of aldehydes. In this reaction, the aldehyde is oxidized to the corresponding carboxylic acid and the silver ion is reduced to metallic silver, which deposits as a thin film on the inner surface of the glass and is commonly known as a silver mirror. There are a number of possible causes for the Tollen’s Test activity not working as expected. Not having a scrupulously clean glass surface is one of them, but also impurities in the mix that would act as nuclei for very fine particles to precipitate onto other than the surface of the glass. Metallic silver deposits appear as dark grey to black if they are rapidly formed and are very fine grained—for example, silver on photographic film looks black rather than metallic shiny silver. The darkening could also be due to silver oxide (a stage of the process), which has not been chemically reduced to metallic silver. The feasibility of this reaction also depends on the freshness of the Tollen’s reagent.
The general equation for the silver mirror test is:
R–CHO + 2 [Ag(NH3)2]+ + 3 OH- → R–COO- + 2Ag + 2H2O + 4NH3
- Tollen’s reagent should never be prepared in advance as ammoniacal silver solutions are hazardous and can explode on standing. Any unused solution should be deactivated as soon as possible and within two hours.
- There are a number of chemical hazards to be aware of when preparing the diluted solutions for this activity. Many of the concentrated solutions used are corrosive, may cause respiratory irritation and can cause severe skin burns.
- Silver ions are harmful for aquatic organisms and release to the environment should be avoided.
Science ASSIST recommends the following:
- A site-specific risk assessment should be conducted.
- Prepare the diluted solutions in an operating fume cupboard.
- Conduct this activity in a well-ventilated room.
- Suitable PPE such as a laboratory coat, closed-in shoes, gloves and safety glasses should be worn at all times.
- Use scrupulously clean glassware.
- Immediately dispose of wastes generated as described below.
- Thoroughly scrub the glassware with soapy water and then rinse well with distilled water.
- Wear appropriate PPE such as closed-in shoes, rubber or neoprene gloves, safety glasses and laboratory coat.
- Using a dropper, rinse the glassware with 2–3 mL of concentrated nitric acid under an operating fume cupboard.
- Discard the nitric acid used in a beaker and rinse the glassware thoroughly with distilled water.
- Dilute the concentrated nitric acid by carefully adding it to a large volume of water. Neutralize the solution with sodium bicarbonate to a pH of between 6–8 and flush down the sink.
Residual reaction mixture
Residues from the reaction should not be stored as they can form explosive compounds on standing.
Small quantities: the residues should be diluted with a large volume of water and flushed down the sink.
Larger quantities: treat as for surplus Tollen’s reagent.
Deactivation of Tollen's reagent
Surplus Tollen’s reagent should be deactivated within two hours of preparation as follows:
- Dilute any unused Tollen’s reagent 20:1 with water.
- While stirring frequently, slowly add 1M hydrochloric acid solution until the pH reaches 2.
- The addition of hydrochloric acid is exothermic; cool in a water bath if necessary.
- Silver chloride will precipitate out at pH 2. Collect the silver chloride by filtration and allow to dry. Place in a suitable labelled container and store for collection by a licenced chemical waste disposal contractor.
- Neutralise the filtrate with sodium bicarbonate to a pH within 6–8 and flush down the sink.
Removal of silver deposit from glassware:
The thin layer of silver formed can be removed from the silvered glassware using concentrated nitric acid in an operating fume cupboard and wearing suitable PPE.
- Nitrogen dioxide, a toxic gas, is evolved during the process.
- Add the concentrated nitric acid (2–3 mL for one test tube) drop wise to the glass vessel. A Pasteur pipette can be used to ensure that the acid covers the inside surface so that all of the precipitated silver is dissolved.
- To the resulting solution, slowly add saturated sodium chloride solution and white silver chloride will precipitate out. Add the sodium chloride solution slowly until there is no further precipitation of silver chloride.
- Collect the silver chloride precipitate by filtration and allow to dry. Place in a suitable labelled container and store for collection by a licenced chemical waste disposal contractor.
- Neutralise the filtrate with sodium bicarbonate to a pH of between 6–8 and flush down the sink.
Recipes for Tollen's Test:
Tollen's Test using ammonium nitrate:
The purchase of ammonium nitrate is regulated according to state and territory legislation.
For ammonium nitrate regulations, please see the previous question Ammonium Nitrate
We have tested the following concentrations and procedure and can confirm that it works:
- The recommended concentration for the reagents are: 0.5 M glucose solution, 0.5 M aqueous silver nitrate solution, 1.5 M aqueous ammonium nitrate solution and 2.5 M aqueous sodium hydroxide solution.
- Mix the aqueous silver nitrate and ammonium nitrate solutions just before conducting the experiment.
- Reduce the volume of each reagent used to 2 mL and use a test tube for the experiment.
- Carefully shake the test tube containing the glucose mixture to ensure maximum coverage.
Here is a link to another method using ammonium nitrate:
- 'Tollens' Test – Silver Mirror' The Ohio State University website, https://u.osu.edu/cbcdemolab/files/2016/02/Tollens-Test-14tw3a0.pdf (Accessed August 2016)
Tollen's Test using ammonia
There are alternative methods, which do not involve the use of ammonium nitrate. We have tested the following procedure and can confirm that it works:
- To 50 mL of 0.1 M aqueous silver nitrate solution add concentrated ammonium hydroxide solution drop wise. A brown precipitate of silver (I) oxide is formed.
- Continue adding the ammonium hydroxide solution until a clear solution is formed.
- To the resulting clear solution add 25 mL of 0.8 M potassium hydroxide solution followed by the addition of ammonium hydroxide solution drop wise until the solution becomes clear.
- To a clean, dry test tube add 2.5 mL 0.5 M of glucose solution. Carefully shake the test tube to ensure that the entire surface is covered with the glucose solution.
- Add 12.5 mL of the freshly prepared Tollen’s reagent to the glucose solution. The solution turns yellow, brown, then cloudy and dark and silver will begin to form on the inner surface of the test tube.
Here are some links to other alternative methods:
- 'A giant silver mirror experiment' Royal Society of Chemistry website. http://www.rsc.org/learn-chemistry/resource/res00000822/a-giant-silver-mirror-experiment?cmpid=CMP00004158 October 2015.
- Flinn Scientific, Inc. 2010. The Mirrored Flask, Flinn Scientific website, http://www.flinnsci.com/media/395452/cf10145.pdf
- 'Tollen's Test (Silver Mirror)', North Carolina State University website, https:/ncsu.edu/project/chemistrydemos/Organic/TollensTest.pdf (Accessed August 2016)
- It may be helpful to place the glucose solution in the glassware first, stopper and shake it to ensure that the entire inside surface is covered. The other ingredients are then added.
- Sometimes it is helpful to warm the mixture to get a silver mirror. This can be achieved by running the test tube/vessel under warm water from a tap, or using a warm water bath.
Monosaccharides are the simplest carbohydrates and are the building blocks from which all bigger carbohydrates are made. Glucose is a monosaccharide and a reducing sugar. The characteristic property of a reducing sugar is that it generates one or more compounds containing an aldehyde group in aqueous solution. Glucose reacts with water to give an open-chain form containing an aldehyde group.
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‘Ammonium nitrate’ Safety Data Sheet, Merck Millipore website, http://www.merckmillipore.com/INTERSHOP/web/WFS/Merck-AU-Site/en_US/-/US... (Broken link fixed, October 2018).
A giant silver mirror experiment', Royal Society of Chemistry website, http://www.rsc.org/learn-chemistry/resource/res00000822/a-giant-silver-mirror-experiment?cmpid=CMP00004158 (October 2015)
Flinn Scientific, Inc. 2010. The Mirrored Flask, Flinn Scientific website, http://www.flinnsci.com/media/395452/cf10145.pdf
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'Tollens' Test – Silver Mirror' The Ohio State University website, https://u.osu.edu/cbcdemolab/files/2016/02/Tollens-Test-14tw3a0.pdf (Accessed August 2016)
'Tollen's Test (Silver Mirror)', North Carolina State University website, https:/ncsu.edu/project/chemistrydemos/Organic/TollensTest.pdf (Accessed August 2016)
Waddell, Dave. 2015. Laboratory Waste Management Guide, Local Hazardous Waste Management Program in King County, http://www.labwasteguide.org/