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

What Is The Schiff’s Test?

This qualitative test for aldehydes (used to check the presence of aldehydes) is performed using Schiff’s reagent. The sample that needs to be examined is combined with Schiff’s reagent. If aldehyde is present, a characteristic magenta-pink color is obtained.

Any compound with an aldehydic -CHO group will show a positive Schiff’s 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 to aliphatic ketones with Schiff’s reagent, but aromatic ketones do not produce color in the presence of Schiff’s reagent.

Schiff's Reagent

The name of Schiff’s reagent is Fuchsine or basic fuchsine. The IUPAC name of Schiff's reagent is 4-[((4-Dimethylaminophenyl)imino)methyl]phenol. Schiff’s 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’s 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's Reagent

The Schiff’s 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.

Schiff’s's Reagent Preparation

The precise steps of preparation are stated below:

• In 900 mL of 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’s 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’s reagent to detect the presence of aldehydes. In Schiff’s reagent, it is decolorized by sulfurous acid to form a colorless solution.

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

Schiff’s 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 the 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 the sulphonic acid -SO₃H group to the central carbon, resulting in loss of the quinoid ring.

This is known as Schiff’s reagent. When this new sulphonic acid substituted compound or Schiff’s 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.

Some Solved Examples

Question.1 What is Schiff's reagent primarily used to detect?

a) Alcohols
b) Aldehydes
c) Ketones
d) Carboxylic acids

Solution:

Schiff’s test is a chemical test used to check for the presence of aldehydes in a given analyte.

Hence , the correct answer is option b) Aldehydes

Question.2 What is the composition of Schiff’s reagent?

a) Fuchsine + Sulfuric acid
b) Fuchsine + Sulfur dioxide
c) Fuchsine + Sodium hydroxide
d) Fuchsine + Acetic acid

Solution: Schiff's reagent is a solution used primarily for detecting aldehydes. It is composed of the Fuchsine (or Basic Fuchsine) and Sulfur Dioxide (SO₂)

Hence , the correct answer is option b) Fuchsine + Sulfur dioxide

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