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Schiff Reagent and Test - Composition, Preparation, Structure, Reaction, FAQs

Schiff Reagent and Test - Composition, Preparation, Structure, Reaction, FAQs

Edited By Team Careers360 | Updated on Jul 05, 2022 02:23 PM IST

In this article we will be discussing Schiff's reagent, Schiff test, Schiff reagent formula, Schiff reagent test, Schiff reagent preparation, Rosaline structure and Schiff’s test for aldehydes.

Schiff Test

The Schiff reagent test is a test performed to detect presence of aldehydes in a given compound. This test is named after Hugo Schiff; a scientist who developed this test. This qualitative test for aldehydes is performed using Schiff reagent. The sample that needs to be examined is combined with Schiff reagent. If aldehyde is present, characteristic magenta-pink color is obtained.

Any compound with an aldehydic -CHO group will show positive Schiff test. Example: Human skin is stained due to presence of aldehydes in the tissues of skin. Although, sometimes aliphatic ketones might show positive results but the overall process for colorization is slow and takes time for pink color to emerge. Aromatic aldehyde reacts similarly as aliphatic ketones react with Schiff reagent but aromatic ketones do not produce color in presence of Schiff reagent.

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Schiff Reagent

Schiff reagent is a product of formulation of certain dyes such as fuchsin and sodium bisulphate which chemically react with aldehydes to form a bright pink product. Schiff reagent is sometimes referred to as leucofuchsin. The term ‘leuco’ means white or absence of color which is termed due to a very pale yellow color or nearly colorless solution. The fuchsin dye is decolorized by addition of sulphurous acid (or its conjugate base bisulphate) due to which it is also referred as fuchsin-sulphurous acid. Fuchsin dye is also known as rosaniline hydrochloride and is marked by its magenta color.

Schiff Reagent Composition

Schiff reagent is prepared by using fuchsin (<1%) dye in water (>98%) combined with sodium bisulfite (<1%) dissolved in solution of hydrochloric acid (<1%). The solution is shaken at intervals followed by decolorization with charcoal. The mixture is then filtered. Fresh activated charcoal must be used to ensure formation of perfectly colorless solution. If the solution is not colorless, the solution is refiltered.

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Schiff Reagent Preparation

The precise steps of preparation are stated below:

  1. In 900ml boiling water, dissolve 5g basic fuchsin.

  2. Cool down the solution until it reaches around 50°C temperature.

  3. Slowly add 100ml of 1M HCl to the dilute solution of fuchsin.

  4. Again cool down the solution to 25°C.

  5. To the cooled solution add 10g K2S2O5.

  6. Shake the above solution for 3 minutes and leave the solution mixture for incubation in a dark room for 24 hours.

  7. After incubation and resting, add 5g fine activated charcoal to the reaction mixture.

  8. Shake the solution for 3 minutes and filter.

  9. Crystal clear solution must be obtained or else refilteration and retreatment must be done.

  10. Store the prepared solution at 4°C in a bottle covered with foil. Schiff reagent precipitates a white crystalline substance if not stored properly. Thus, fresh batches at intervals of 2-3 weeks must be prepared to ensure accurate results.

Rosaniline Structure

Rosaniline structure

Rosaniline hydrochloride or fuchsin is a pinkish magenta color dye used in Schiff reagent to detect presence of aldehydes. In Schiff reagent, it is decolorized by sulphurous acid to form colorless solution.

Schiff reagent formula: Schiff reagent’s molecular formula is C20H19N3·HCl.

Schiff test reaction


Schiff test reaction

The reaction is initiates when rosaniline hydrochloride or fuchsin with characteristic magenta color is decolorized by adding sulphurous acid H2SO3. The decolorization is due to distortion of chromophore (chromophore is a term used to define atoms or group of atoms which are responsible for presence of color in a compound) by addition of sulphonic acid -SO3H group to the central carbon resulting in loss of quinoid ring.

This is known as Schiff reagent. When this new sulphonic acid substituted compound or Schiff reagent reacts with two molecules of aldehydes, condensation takes place which reforms the chromophore and restores the quinoid ring. Thus, pink-magenta color is reproduced indicating the presence of aldehydes.

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Frequently Asked Question (FAQs)

1. What is Schiff’s reagent?

Schiff reagent is a reagent used in Schiff’s test for aldehydes. Schiff reagent gives pink colour with aldehydes. It is composed of rosaniline hydrochloride or fuchsin and sulphurous acid.

2. Which of the following does not react with NaHSO3?



    1. CH3CHO

    2. C6H5COCH2.CH3

    3. CH3COCH3

    4. HCHO

  • Ans. B) C6H5COCH2.CH3 does not react with NaHSO3.

As discussed in the beginning, aromatic ketones do not react with Schiff reagent as they are less reactive than aliphatic ketones and both aromatic aldehydes and aliphatic aldehydes.

3. The reagent with which both acetaldehyde and acetone reacts is:
  1. Fehling’s reagent

  2. Tollen’s reagent

  3. Schiff reagent

  4. Grignard reagent

Ans. D) Grignard reagent

Grignard reagent can react efficiently with acetaldehyde to give secondary and with acetone to give tertiary alcohols.

Although acetone is an aliphatic ketone, it gives a very slow reaction with Schiff reagent. Thus, making it a not-so-good reagent for detecting ketones.

Tollen’s reagent, Fehling’s solution and Schiff reagent are effective for detection of aldehydes only as they are only reduced by aldehydes.

4. Benzaldehyde does not show positive test with:
  1. Fehling’s solution

  2. Tollen’s reagent

  3. Schiff reagent

  4. Benedict’s solution

Ans. b) Tollen’s reagent.

Tollen’s reagent is a powerful reagent which can be easily reduced by an aromatic aldehyde like Benzaldehyde and gives silver.

Fehling’s solution and Schiff reagent are weak reagents which cannot be reduced by aromatic aldehydes while Benedict’s solution is used for detecting sugars.

Aromatic aldehydes give a very slow reaction with Schiff reagent as discussed previously.

5. Does Benzaldehyde give Fehling test?

No, Benzaldehyde does not give Fehling test due to absence of alpha hydrogen and thus cannot form enolate.

6. Acetaldehyde cannot give:
  1. Iodoform test

  2. Fehling’s test

  3. Lucas test

  4. Tollen’s test

Ans. Acetaldehyde cannot give c) Lucas test.

Lucas test is a test performed to separate alcohols. The Iodoform test, Fehling’s test and Tollen’s test are identifying tests for aldehydes.

7. Glucose contains addition to aldehyde group:
  1. One primary OH and four secondary OH groups

  2. two primary OH and three secondary OH groups

  3. One secondary OH and four primary OH groups

  4. three primary OH and two secondary OH groups

Ans. A) One primary OH and four secondary OH groups.

8. What is schiff base test?

Schiff test is a qualitative test for aldehydes and is performed using Schiff reagent. The sample that needs to be examined is combined with Schiff reagent. If aldehyde is present, characteristic magenta-pink color is obtained. Any compound with an aldehydic -CHO group will show positive Schiff test. 

Schiff reagent is prepared by using fuchsin (<1%) dye in water (>98%) combined with sodium bisulfite (<1%) dissolved in solution of hydrochloric acid (<1%).

9. Name a chemical test to distinguish between benzaldehyde and acetaldehyde.

 Fehling Solution is a chemical test used to distinguish between acetaldehyde and  benzaldehyde.

Benzaldehyde does not give any reaction with fehling solution while acetaldehyde gives red colored precipitate with Fehling solution.

10. Differentiate between Schiff base and Schiff reagent.

Both Schiff base and Schiff reagent are named after Hugo Schiff. Both terms are used for naming a group of particular organic compounds. Schiff base refers to secondary ketimines or secondary aldimines while Schiff’s reagent is a reagent used to test for identification of aldehydes (and some ketones).

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