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Tollens’ Test

Definition: What is Tollens’ Test?

Tollens’ test, also known as a silver-mirror test, is a qualitative laboratory test that is used to differentiate between an aldehyde and a ketone. It uses the fact that aldehydes can be more easily oxidized than ketones. This test cannot distinguish between an aldehyde and an α-hydroxy ketone (e.g., hydroxy acetone) [1-4].

During the process, two events occur. First, the aldehyde gets converted into carboxylic acid. Second, the Ag+ ions are reduced to Ag metal, which leaves a shiny appearance inside the test tube, indicating a positive result.

Tollens Test

The history of this reaction goes back to 1882 when a work done by German chemist Bernard Tollens was published in a German journal.

Examples of Tollens’ Test

Tollens Test Benzaldehyde

Tollens’ Reagent

Tollens reagent is a mild oxidizing chemical reagent that is used in the Tollens’ test. It is a colorless, basic, and aqueous solution containing silver ions coordinated to ammonia, forming a diamine silver(I) complex [Ag(NH3)2]+. Tollens’ reagent is prepared using a two-step procedure. [1-3]

Tollens Reagent

Mechanism of Tollens Test [5]

Tollens Test Mechanism


Q.1. Why does fructose reduce Tollens’ reagent?

Ans. In an aqueous solution, fructose is enolized and converted into aldehyde in a basic medium. As aldehydes generally reduce Tollen’s reagent, thus fructose also reduces Tollen’s reagent.


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5 responses to “Tollens’ Test”

  1. Pooja says:

    So much helpful…

  2. Shohayeb says:

    What is the importance of the Tollen Test, Can anyone give me 10 reasons of it?

    • Satyam Bhuyan says:

      As mentioned in the article, the test is used to differentiate between an aldehyde and a ketone.

  3. Matías says:

    Hello, [Ag(NH3)2]+ and OH- are forming an ionic pair?
    I assume that the reaction would not occur in an acid medium or at pH 7?

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