Molisch

The Molisch test is a widely used chemical assay in biochemistry and microbiology for detecting the presence of carbohydrates in a sample. Developed by Austrian biochemist Hans Molisch in 1908, this test is simple, rapid, and highly sensitive, making it valuable for both qualitative and semi-quantitative analysis.

The procedure involves adding a few drops of Molisch’s reagent—a solution of α-naphthol in ethanol—to the sample, followed by the careful addition of concentrated sulfuric acid along the side of the test tube. The reaction produces a characteristic purple or violet ring at the junction of the two layers, indicating the presence of carbohydrates. This color change results from the dehydration of carbohydrates by sulfuric acid, forming furfural or its derivatives, which then react with α-naphthol.

The Molisch test is not specific to a single type of carbohydrate; it detects most monosaccharides, disaccharides, and polysaccharides, including glucose, fructose, sucrose, and starch. However, it does not distinguish between different carbohydrate types, requiring additional tests for specific identification. The test is particularly useful in clinical and environmental microbiology for screening samples where carbohydrate presence is suspected.

While the Molisch test is highly reliable, false positives can occur with certain non-carbohydrate compounds, such as some phenolic substances. False negatives are rare but may arise if the sample contains very low concentrations of carbohydrates or if the test conditions are not properly controlled. Variations of the test, such as the use of different reagents or conditions, have been explored to improve specificity.