What is Molisch’s Test?
Molisch test is a colourimetric method for the analysis of the presence of carbohydrates in a given analyte. This test is named after Austrian botanist Hans Molisch. Molisch’s test is done by using Molisch reagent. A solution of naphthol in ethanol (95%) is known as Molisch reagent. It’s also known as the purple ring test. Other than the Molisch reagent, concentrated sulfuric acid is also used in the test.
The Molisch test gives a positive result for all carbohydrates. However, tetrose and triose are exceptions. In Molisch’s test, monosaccharides give a positive test faster. While disaccharides and polysaccharides react slowly with Molisch reagent and give the positive test in delay. Thus, Molisch’s test for carbohydrates is very useful in the detection of the presence of carbohydrates in a substance.
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Requirements
1. Reagent
Molisch reagent: Dissolve 3.75 gm of α-naphthol in 25 ml of Ethanol 99%. This particular reagent should be prepared fresh.
Concentrated sulphuric acid
Test sample
2. Materials Required
Test tubes
Test tube stand
Pipette
Distilled water
Molisch’s Test Principle
Carbohydrates reacting with mineral acids such as sulfuric acid get dehydrated and form furfural derivatives. These compounds react with naphthol and get condensed to form a purple-coloured product, as illustrated below by taking the example of glucose.
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Molisch’s Test Procedure
Molisch’s test procedure can be explained by the following points –
2ml of Sample is taken in a test tube.
Now, 2 drops of Molisch’s reagent are added to the sample and mix it.
Molisch Reagent preparation - Molisch’s reagent can be prepared by adding naphthol in 95% ethanol.
Now, add slowly concentrated sulphuric acid in sloping the test tube by its sides without mixing vigorously so that it will get time to react and will form a layer.
Observation
A purple ring appears at the interface between the acid and test layers which confirms the presence of carbohydrates.
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A Positive Result
The presence of carbohydrates is indicated if the test solution creates purple or purplish-red tinted layers. As a result, the Molisch test results are positive.
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A Negative Result
If the test solution does not yield purple or purplish-red tinted layers, the test solution is devoid of carbohydrate molecules. As a result, the Molisch test results are negative.
Applications of Molisch’s Test
Molisch’s test is used to determine the presence of carbohydrates or sugars in the substance. The most practical use of this test is that if some food product is labelled as sugar-free, then you can easily test the product by Molisch’s test that if the product contains carbohydrate (or sugar) or not. The presence of some glycoproteins and nucleic acids can also be determined by Molisch’s test.
Limitations of Molisch’s Test
Although almost all carbohydrates and even some glycoproteins and nucleic acids can be detected in a substance by Molisch’s test, a few carbohydrates are exceptions. Such as tetrose and triose sugars don’t give Molisch’s test.
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Precautions
Drop the acid into the test tube by dropping it down the test tube's wall. The acid will not react quickly as a result of this, and your entire treatment will be smooth and simple.
Handle the strong acid with caution, as it is extremely harmful to the skin.
Shaking the reddish-violet ring after it has formed will cause the ring to vanish.
FAQs on Molisch’s Test
1.Give some uses of the Molisch Test.
A few of the uses of the Molisch Test can be given as follows.
The Molisch test is performed to identify whether or not a sample is composed of carbohydrates.
It can be used to distinguish carbohydrates from other biomolecules as a by-product of several processes.
2.List the limitations of the Molisch Test.
The limitations of the Molisch Test can be given as follows.
Due to the lack of the five carbon atoms required for furfural synthesis, trioses and tetroses do not yield a good result in this reaction.
The Molisch test is not a carbohydrate-specific test. Citric acids, lactic acids, oxalic acids, formic acids, and other organic acids can create furfurals or furfural-like compounds.
3.What is the objective of the Molisch Test?
Molisch’s test is used to determine the presence of carbohydrates in a sample and to distinguish carbohydrates from other biomolecules.