The silver mirror test with Tollens’ reagent

In this experiment, students observe what happens when a solution containing silver nitrate (Tollens’ reagent) and a reducing sugar (glucose) react to form silver. As the silver is deposited, students can see a mirror-like coating forming on the inside of the reaction vessel.

The teacher demonstration should take five minutes to perform. There are also possible extension activities, each of which would take a similar time.

Although the reaction involved is related to the post-16 study of aldehydes and reducing sugars, this dramatic demonstration will fascinate a wider age range.

Equipment

Apparatus

• Eye protection (goggles)
• Round- or flat-bottomed flask, 1 dm³ (see note 9 below)
• Rubber stopper, to fit flask
• Beaker, 250 cm³
• Measuring cylinders, 25 cm³, 100 cm³ and 250 cm³
• Dropping pipette
• Glass rod
• Access to a fume cupboard (for handling ‘880’ ammonia solution)

Chemicals

Note: the quantities listed below are sufficient for three demonstrations.

• Silver nitrate (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT), 8.5 g
• Potassium hydroxide (CORROSIVE), 11.2 g
• Glucose (dextrose), 2.2 g
• ‘880’ ammonia solution (35% w/v) (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT), 30 cm³
• Concentrated nitric(V) acid (OXIDISING, CORROSIVE), 100 cm³
• Purified (distilled or deionised) water, 800 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection (goggles) and disposable nitrile gloves.
• Do NOT prepare Tollens’ Reagent in advance. It is likely to explode on standing.
• Silver nitrate, AgNO₃(s), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC087.
• Potassium hydroxide, KOH(s) (CORROSIVE) – see CLEAPSS Hazcard HC091b.
• Glucose (dextrose), C₆H₁₂O₆(s) – see CLEAPSS Hazcard HC040c.
• Ammonia solution, NH₃(aq), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC006. 
• Concentrated nitric(V) acid, HNO₃(aq), (OXIDISING, CORROSIVE) – see CLEAPSS Hazcard HC067. 
• Thorough cleaning of the 1 dm³ flask is vital if the demonstration is to succeed:
  • First, use detergent and a brush, then rinse with water.
  • Next, rinse with concentrated nitric acid.
  • Finally, wash out several times with purified water.
• Make up the three solutions needed as follows (do NOT mix any of the solutions together before the lesson):
  • Dissolve 8.5 g of silver nitrate in 500 cm³ of purified water to make a 0.1 M solution.
  • Dissolve 11.2 g of potassium hydroxide in 250 cm³ of purified water to make a 0.8 M solution.
  • Dissolve 2.2 g of glucose in 50 cm³ of purified water.
• After the lesson, the silver can be removed from the silvered flask using concentrated nitric acid. Work in a fume cupboard as nitrogen dioxide (TOXIC) is formed.
• After the demonstration, do NOT save the silver solution in a silver residues container. The solution must be disposed of down the sink with plenty of cold water within 30 minutes of mixing at the start of the demonstration. This is to avoid any chance of the formation of a deposit of silver fulminate, a dangerously explosive substance.

Procedure

1. Place 150 cm³ of the silver nitrate solution in a 250 cm³ beaker.
2. Working in a fume cupboard, add ‘880’ ammonia using a dropping pipette until the brown precipitate first formed re-dissolves to give a clear, colourless solution. Less than 5 cm³ of ammonia solution should be needed. The solution then contains the colourless complex ion, [Ag(NH₃)₂]⁺(aq).
3. Add 75 cm³ of the potassium hydroxide solution. A dark brown precipitate of silver(I) oxide will form. Add more ammonia solution dropwise until this re-dissolves to give a clear, colourless solution. About 5 cm³ of ammonia will be needed. This solution is sometimes called Tollens’ reagent.
4. Pour this solution into the 1 dm³ flask and add 12 cm³ of the glucose solution. Stopper the flask and swirl the solution so that the whole of the inner surface of the flask is wetted. The solution will turn brown. Continue swirling until a mirror forms in about 2 minutes.
5. When a satisfactory mirror has formed, wash the solution down the sink with plenty of water. Rinse out the flask well with water and discard the washings down the sink. The flask can now be passed around the class.
6. An alternative to plating the inside of a flask is to silver plate the outside of small glass objects which can be suspended in the plating solution by hanging them on threads. These objects must be thoroughly cleaned beforehand.

Teaching notes

For pre-16 students this demonstration is more likely to be performed for its spectacular nature rather than for the chemical changes that take place. However for post-16 students, these chemical changes may form part of their studies, and you may wish to consider a variant as a student experiment:

1. In a new, clean test tube add one drop of 0.4 M sodium hydroxide solution (IRRITANT) to 1 cm³ of 0.1 M silver nitrate(V). Add 1 M ammonia solution dropwise until the precipitate just re-dissolves.
2. To this solution, add a few drops of glucose solution and warm gently in a beaker of hot water (do NOT heat the tube directly in a flame) with occasional swirling until the silver mirror forms.
3. Discard the solution within half an hour of preparation by washing it down a sink followed by a large amount of water.

The experiment may be repeated as either the large-scale demonstration or as the student version using an aldehydes such as ethanal or propanal. In contrast, a ketone such as propanone does not react, illustrating an important chemical difference between aldehydes and ketones. Qualitative tests for organic functional groups shows how to use this test as part of an investigation to identify unlabelled organic compounds.

Aldehydes reduce Ag⁺(aq) ions to metallic silver. The aldehydes are oxidised to carboxylate ions. The reaction that occurs using propanal (a safer alternative to ethanal) is:

CH₃CH₂CHO(aq) + 2[Ag(NH₃)₂]⁺(aq) + 3OH^–(aq) → 2Ag(s) + CH₃CH₂COO^–(aq) + 4NH₃(aq) + 2H₂O(l)

Further information

Adding the ammonia to the silver nitrate solution makes the silver ion less susceptible to reduction, which produces silver in a more controlled manner. 

Ag+ + e-→ Ag   E° = +0.799 V 

Ag(NH3)2+ + e-→ Ag + 2NH3E° = +0.373 V 

The half-equations indicate that ammonia forms a complex with the silver ion, which is more difficult to reduce than the silver ion. This is because silver ions form more stable complexes with NH3 than with water. 

If silver nitrate is used without ammonia, the silver ion is reduced so quickly that colloidal silver metal would appear. The solution would become a black, cloudy liquid. 

Basic conditions are necessary because glucose is oxidised more easily under basic conditions: 

RCHO + H2O →   RCOOH + 2H+ + 2e-

Tollens’ reagent and other similar tests, eg Benedict’s and Fehling’s, will test for aldehydes but will not identify individual compounds. They all rely on aldehydes being susceptible to oxidation whereas ketones are not readily oxidised.  

If identification is required, then the unknown compound must be reacted with Brady’s reagent (2,4-dinitrophenylhydrazine dissolved in acidified methanol). A bright orange or yellow precipitate will indicate the presence of aldehyde or ketone.  

If the precipitate is purified by recrystallisation, the melting point of the crystals can be measured and compared with tables of the melting points of 2,4-dinitrophenylhydra-zones of all the common aldehydes and ketones to identify the mystery compound. This reaction is an example of addition-elimination, which does not involve oxidation, and therefore will identify both aldehydes and ketones because both types of compound include a carbon-oxygen double bond.