Silver mirror test is a test used to distinguished between aldehydes and ketones. Before we continue with the test, let us understand the basic explanations or definitions of Aldehydes and Ketones.
Aldehydes and ketones are both carbonyl group. The carbonyl group (-CO) consists of a carbon atom that is joined to an oxygen by a double bond.
Aldehyde = R – CO –H
Ketone = R – CO – R (note R is a methyl group)
The difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. Ketones don’t have that hydrogen.
It is the presence of Hydrogen in aldehydes that makes it easy to oxidised
Aldehydes are oxidized easily, even by weak oxidizing agents such as Ag+. While Ketones are not oxidized by these reagents.
The silver mirror test is the reaction of a sample with a solution containing silver-ammonia complex ions. When the aldehydes are oxidized by these reagents, the silver ions are reduced to metallic silver, which forms a black precipitate, and if the test tube is clean, a silver mirror on the test tube.
The reaction is as follows:
2Ag (NH3)2 + + 2OH – + R – C – H = 2Ag(s) (silver mirror) + R-C-O – + NH4+ + 3NH3 + H2O
Experiment: Silver mirror test
Materials: glycerol, Conc. NH4OH 0.25 10 X 75 mm test tube 0.10 M AgNO3 50 mL beaker 0.80 M KOH
- Add about 1 mL of 0.1M silver nitrate solution to a test tube
- Add 0.8M of NaOH into a beaker and add about 2 to 3 d drops NH4OH into it
- Add a small amount of this mixture into the test tube containing a solution of silver nitrate. Notice the formation of brown precipitate and shake the test tube.
- Add more mixture of NaOH/NH3 in drops and shake until the precipitate is dissolved
- Prepare a solution of glycerol with water in a ratio of 1:3 inside a test tube
- Mix the solution of the dissolved precipitate and the solution of glycerol together
- Add hot water in a beaker and insert the test tube with the mixture
- Observe and record your observation
NOTE: Do not forget to rinse the test tube immediately you are though with the experiment
Observation: if the glass wall of the test tube is clean, a silver precipitate will be deposited but if it is not, a black precipitate will be deposited.