Tollens Test: Overview, Questions, Preparation

Bernhard Tollens, a German chemist who discovered this reagent and its benefits, is named after the Tollens Reagent. This test is often called the silver mirror test. Tollen test will help us identify the aldehydes and ketones groups. To conduct the tollens test, we require ammonia and silver nitrate.  

This specific reagent, named after the German chemist Bernhard Tollens, is used in research for aldehydes and ketones. We know that Tollens Reagent has a relatively short shelf life, and the reagent is not available in the market to sell. For this reason, this reagent is often prepared directly in the laboratory. Tollens test is popularly known as the Silver Mirror test due to the formation of a thin layer of metallic silver on the test tube.

• Stage 1: We need to add a few drops of diluted sodium hydroxide in the preparation of silver nitrate. In the case of nitrate solutions, there is a silver aqua complex in which water works as a substance. These silver aqua complexes are now changed by hydroxide ions into silver oxides. When brown solid from this solution, this silver oxide ( Ag2O) gets precipitates.

The chemical reaction will be as under:  The chemical reaction will be as under:

2AgNO3 + 2NaOH → Ag2O (brown ppt) + 2NaNO3 + H2O

• Stage 2: At this stage, we have to use liquid ammonia to dissolve the brown precipitation of silver oxide produced in step 1. The solution formed from this aqueous ammonia addition contains the complex [Ag(NH3)2]+. The primary component of Tollen Reagent is this complex.

 The reaction will be written as

Ag2O (brown ppt) + 4NH3 + 2NaNO3 + H2O → 2[Ag(NH3)2]NO3 + 2NaOH

When aldehyde is introduced to the Tollens reagent, two things occur, namely the aldehyde is oxidised by the Tollens reagent and forms a carboxylic acid.

Generally, the Tollens Test is carried out in clean and transparent test tubes made of glass. The silver ions present in the Tollen reagent are reduced to metallic silver. This is because the reduction of the silver ions into metallic silver forms a silver mirror on the test tube.

Weightage of the chapter in Class 12 is 7 marks.

• Tollen's Test: Aldehydes give a positive silver mirror to Tollen's test while ketones give no reaction.
• The Tollens test is a reaction used to separate aldehydes from ketones because aldehydes can be oxidised into carboxylic acid while ketones can not.
• Tollen test’s active ingredient is Di-ammine-silver(I) complex ( [Ag(NH₃)₂]⁺ ).

[Image courtesy: NCERT]

Q: What is the test for Tollens used for?

Q: What is the reagent of Tollens and its usage?

Q: How do you differentiate aldehydes and ketones from each other?

Q: How are you going to screen for ketones?

Q: What is a Class 12 toll reagent?