Tollens' Reagent Testing
Distinguishing Between Aldehydes & Ketones
- Weak oxidising agents can be used to distinguish between an aldehyde and a ketone
- The aldehyde will be oxidised to a carboxylic acid, but the ketone will not undergo oxidation
- Tollens’ reagent contains the silver(I) complex ion [Ag(NH3)2]+
- This is formed when aqueous ammonia is added to a solution of silver nitrate
- Tollens’ reagent is also known as ammoniacal silver nitrate
- 2cm depth of 0.1 mol dm-3 silver nitrate solution is added to a test tube
- Drops of 2.0 mol dm-3 sodium hydroxide are added until a brown precipitate of silver(I) oxide appears
- Concentrated ammonia is then added drop by drop until the brown precipitate just disappears and the reagent is ready to use
- A few drops of the suspected aldehyde are added to the test tube
- The test tube is gently warmed with Tollens’ reagent
- The silver(I) complex ion solution, [Ag(NH3)2]+, is colourless
- As the aldehyde is oxidised, it causes the [Ag(NH3)2]+ ions to become reduced to solid metallic silver, Ag
- This is why a positive test result is called a “silver mirror”
Positive Test Result:
- When Tollens’ reagent is gently warmed with an aldehyde, the silver mirror is formed
- This is the positive test result
- When Tollens’ reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens’ reagent, so no reaction takes place
- This is a negative test result
The Ag+ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms
Practical tip
- Always make sure you use very clean glassware for a Tollens' test to ensure you get the best possible result
