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.
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.
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 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.
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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The precise steps of preparation are stated below:
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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.
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.
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NCERT Chemistry Notes:
Schiff reagent is a chemical solution that is used to detect the presence of an aldehyde group in organic compounds. It is a solution of fuchsin dye treated with sulphurous acid.
Schiff Reagent is used in laboratory testing to identify and visualise aldehyde functional groups within various biological samples, such as tissues or carbohydrates. This is particularly useful in histological staining.
There are a few limitations and considerations while using Schiff's reagent
A variety of samples can be tested using Schiff reagent:
Schiff Reagent specifically detects aldehyde groups, whereas other tests, such as Benedict's test or Fehling's test, evaluate reducing sugars and may detect both aldehydes and certain ketones. Schiff’s test is unique in its distinct magenta colour change associated specifically with aldehyde detection and is particularly useful for certain types of carbohydrates.
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