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

Ever think about how chemists detect unknown compounds or the presence of certain molecules in solutions or medicines with the help of just a few drops? It seems difficult, but it is not; we have various reagents available that help to detect the presence of certain groups or molecules in a solution. Out of all these reagents, we are going to discuss the Schiff reagent. It is used to detect the presence of an aldehyde group in organic compounds. Schiff reagent was invented by Hugo Schiff, a German chemist. The Schiff test is performed with the help of Schiff reagent. It is a colourless or pale solution of fuchsin dye treated with sulphurous acid.

This Story also Contains

1. Schiff Test
2. Schiff Reagent
3. Composition of Schiff Reagent
4. Schiff Reagent Preparation
5. Rosaniline Structure
6. Schiff test reaction

When Schiff reagent comes into contact with an aldehyde, it changes its colour from colourless to pink. This test is simple yet widely applicable in various industries like food testing, forensic science, industrial quality control, etc.

Schiff Test

This qualitative test for aldehydes is performed using a Schiff reagent. The sample that needs to be examined is combined with Schiff reagent. If aldehyde is present, a characteristic magenta-pink color is obtained.

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

Schiff Reagent

Schiff reagent is a product of the formulation of certain dyes, such as fuchsin and sodium bisulfate, 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 the addition of sulfurous acid (or its conjugate base bisulfate) due to which it is also referred to as fuchsin-sulfurous acid. Fuchsin dye is also known as rosaniline hydrochloride and is marked by its magenta color.

Composition of Schiff Reagent

The Schiff reagent is prepared by using fuchsin (<1%) dye in water (>98%) combined with sodium bisulfite (<1%) dissolved in a 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 the formation of a 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:

• In 900ml boiling water, dissolve 5g basic fuchsin.
• Cool down the solution until it reaches around 50°C.
• Slowly add 100 mL of 1M HCl to the dilute solution of fuchsin.
• Again, cool down the solution to 25°C.
• To the cooled solution add 10g K₂S₂O₅.
• Shake the above solution for 3 minutes and leave the solution mixture to incubate in a dark room for 24 hours.
• After incubation and resting, add 5g of fine activated charcoal to the reaction mixture.
• Shake the solution for 3 minutes and filter.
• A crystal clear solution must be obtained, or else refiltration and retreatment must be done.
• 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 hydrochloride or fuchsin is a pinkish magenta color dye used in Schiff reagent to detect the presence of aldehydes. In Schiff reagent, it is decolorized by sulfurous acid to form a colorless solution.

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

Schiff test reaction

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

This is known as the 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, a pink-magenta color is reproduced indicating the presence of aldehydes.

Also, check-

• NCERT Exemplar Class 11th Chemistry Solutions
• NCERT Exemplar Class 12th Chemistry Solutions
• NCERT Exemplar Solutions for All Subjects

NCERT Chemistry Notes:

• NCERT Notes Class 11th Chemistry
• NCERT Notes Class 12th Chemistry
• NCERT Notes For All Subjects