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The Silver Mirror Test










What is the Silver Mirror Test?

Silver Mirror Test is a test that is used to differentiate between aldose and ketose sugars using the Tollen's reagent.

Tollen's reagent is an alkaline solution of ammoniacal silver nitrate and is used to test for aldehydes.

 *Ketones do not react with the Tollen's reagent.

 Silver ions in the presence of hydroxide ions form a brown precipitate of silver (I) oxide, Ag2O(s).
Ag2O










This precipitate dissolves in aqueous ammonia, forming the diamminesilver(I) ion, [Ag(NH3) 2 ]+.

The reactions that take place in this experiment are as follows:

 2Ag+(aq)  + 2OH-(aq)   →   Ag2O(s) + H 2 O(l)

 Ag2O(s) + 4NH3(aq) + H2O(l)   →  2[Ag(NH3)2]+(aq) + 2OH-(aq)

 What happens if Silver Nitrate is used without ammonia or why is ammonia used in this experiment?

 When ammonia is added to the silver nitrate solution, silver ions are less prone to reduction. Thus, silver is produced in a controlled manner.

 This can be explained using the following half reactions:

 Ag+  + e-   →   Ag ; E0 = +0.8V

 Ag(NH3)2+ + e-  →  Ag + 2NH3  ; E0 = + 0.37V

These two half reactions explain that ammonia forms a complex with the silver ion. This makes it more difficult to reduce than the silver ion as silver ions form more stable complexes with ammonia than with water.
When ammonia is not used in this experiment, colloidal silver metal would appear making the solution a black,cloudy liquid.

 Experiment:
 A mirrored fluorescence flask/test tube is used to perform the Tollen's test increase the effectiveness of this experiment (so that the .formation of the silver mirror is clearly visible).


Materials needed:

  • A selected volume (usually in the range 30 - 150 cm3) of  0.1 moldm-3 AgNO3.
  • A selected mass (usually 2-5 g) of a simple sugar (usually glucose - C6H12O6).
  • About 90% concentrated Ammonia solution (volume usually in the range -  2 - 10 ml).
  • Sodium hydroxide to provide OH- ions. (Usually 15 ml volume and 0.8 moldm-3 concentration).
  • Nitric acid (HNO3) to provide nitrate ions. (Usually 3 ml volume and a selected concentration).
  • Hot water to fill the flask
  • Usually a 500 ml flask is used for this experiment. But in rare experiments, a test tube may also be used.
  • A beaker (as the water bath) that can usually hold more volume than either the flask or the test tube.
  • Two other beakers of different volumes.
  • A stirrer (To stir the liquid to allow uniform solution distribution)
IMPORTANT:
ALTHOUGH SOME OF THE NUMERIC QUANTITIES (SUCH AS CONCENTRATION, VOLUME AND MASS) IN THE ABOVE LIST OF MATERIALS NEEDED ARE DISPLAYED IN A GIVEN RANGE, THESE QUANTITIES SHOULD BE MEASURED ACCURATELY SO THAT THEY OBEY THE STOICHIOMETRIC COEFFICIENTS OF THE FINAL REACTION. 

 Method:

  1. Dissolve glucose in hot water (distilled water) in one of the beakers provided.
  2. Add the silver nitrate solution to the other beaker.
  3. Add the concentrated ammonia solution to the silver nitrate solution while stirring.
  4. Now a brown precipitate of silver oxide is formed and this precipitate dissolves in excess concentrated ammonia solution
  5. Add the glucose solution to the ammoniacal silver nitrate solution in the beaker while stirring. This allows for the glucose solution to uniformly distribute inside the nitrate solution)
  6. Pour this mixture to the flask or the test tube
  7. Place this flask or the test tube on the final beaker (500 ml water bath) and heat the bath uniformly to a temperature close to 700C (water usually starts to evaporate rapidly beyond this temperature).

 Observation:
Within a few minutes of heating a highly reflective silver mirror is observed in the flask.



















Important:
Tollen's reagent will test for aldehydes but the aldehyde that is separated from a given solution cannot be identified. If identification of the aldehyde is needed, Brady's reagent should be used. As mentioned earlier, Tollen's reagent does not test for ketones.
Thus, the silver mirror test is used to isolate aldehydes from other compounds.

 Safety precautions:
  1. If the mixtures of aqueous silver nitrate, ammonia and sodium hydroxide are heated and allowed to react for several hours, silver nitride ( a highly explosive precipitate) may be formed. Thus, the mixture should be freshly prepared for the test and should be washed with water immediately after use.
  2. Concentrated nitric acid, Sodium hydroxide, Silver nitrate and Ammonia may cause severe burns and skin irritations if handled inappropriately.
  3. Concentrated solutions release ammonia vapor which may cause hazards if inhaled.

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