The silver mirror test
Demonstrations designed to capture the student's imagination, by Colin Baker of Bedford School.
In this issue: the silver mirror test
Bernhard Christian Gottfried Tollens (1841-1918) was a German chemist whose name has been recognised through the silver mirror test using Tollens' reagent. He developed this test to differentiate between aldose and ketose sugars.
Tollens' reagent is an alkaline solution of ammoniacal silver nitrate and is used to test for aldehydes. Silver ions in the presence of hydroxide ions come out of solution as a brown precipitate of silver(I) oxide, Ag2O(s). This precipitate dissolves in aqueous ammonia, forming the diamminesilver(I) ion, [Ag(NH3)2]+. Ketones do not react with Tollens' reagent.
2Ag+(aq) + 2OH-(aq) Ag2O(s) + H2O(l)
Ag2O(s) + 4NH3(aq) + H2O(l) 2[Ag(NH3)2]+(aq) + 2OH-(aq
4 g Glucose, C6H12O6;
150 cm3 0.1 mol dm-3 AgNO3;
10 cm3 Concentrated ammonia solution (0.88), NH3;
10 cm3 Distilled water;
250 cm3 Round bottom flask; One-litre beaker (water bath);
250 cm3 Beaker; and 50 cm3 beaker.
Dissolve 4 g of glucose in 10 cm3 distilled water in a 50 cm3 beaker. Place 150 cm3 0.1 mol dm-3 silver nitrate in a 250 cm3 beaker. Add 5 cm3 concentrated ammonia solution (0.88) to the silver nitrate solution while stirring. A brown precipitate of silver oxide is formed which dissolves when a second 5 cm3 measure of concentrated ammonia solution is added. Add the glucose solution to the ammoniacal silver nitrate solution in the beaker while stirring and then pour this mixture into a 250 cm3 round bottom flask. Place this flask into a 70 oC water bath. Within five minutes the inside of the flask should be coated with a highly reflective silver mirror.
Silver nitrate is poisonous if swallowed or inhaled; the solid/ solution stains (brown/black marks) the skin, which cannot be removed with soap and water but disappears as new skin grows. Ammonia contact with the eyes can cause long-term damage. The solution is corrosive and may cause burns. Concentrated solutions release ammonia vapour, which is hazardous if inhaled.
Most recipes for this demonstration involve sodium hydroxide to provide alkaline conditions for the precipitation of silver(i) oxide. I use concentrated ammonia solution which provides sufficient hydroxide ions to cause precipitation followed by the formation of the diammine-silver(i) ion. Some sources recommend that the glassware is scrupulously clean, but I find that the temperature of the water bath is more important because the rate of deposition is the critical factor in a successful monstration.
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 + 2NH3 E° = +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.