Laboratory tests

Liebermann-Burchard test

A reagent used in a colourimetric test to detect cholesterol (steroid), which gives a deep green colour. The formation of a green or green-blue colour after a few minutes is positive.

Molisch test

A sensitive chemical test, named after Austrian botanist Hans Molisch, for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid or hydrochloric acid to produce an aldehyde, which condenses with two molecules of a phenol, resulting in a violet ring. A mixture of α-naphthol and ethanol.

Salkowski's test

A test for cholesterol. When concentrated sulfuric acid is added to a chloroform solution of cholesterol, the chloroform layer shows a red to blue color and the acid layer shows.

Kraut's test

A test for glycerol, alkaloids, pseudo alkaloids and false alkaloids. One of the reagents used (choline) in Kraut's test for lipids is bismuth subnitrate with potassium iodide in 3M nitric acid. When Kraut's reagent reacts with a phospholipid, complexation involving the phosphorylated lipid occurs. This reaction gives a brick red precipitate.

Fehling's test

A test to detect reducing and non-reducing sugar. This test can be used to screen for glucose in urine, thus detecting diabetes. When a sugar solution containing a free aldehyde or ketone group is boiled with Fehling's solution, the characteristic result in a brick-red precipitate of Cu2O

Osazone test or phenylhydrazine test or Kowarsky test

An organic compound phenylhydrazine reacts with carbonyl carbon (or more specifically C1 and C2) of sugar to form the compound called osazones or phenylhydrazone. These osazone crystals have yellow colour characteristics shapes and melting point, time of formation and solubility. All reducing sugars form osazone with excess of phenylhydrazine (C6H5NHNH2) when kept at boiling temperature. This property is attributed to the presence of aldehyde or ketone group in their molecules. Needle-shaped yellow osazone crystals will be observed for glucose and fructose and mannose, whereas lactosazone shows mushroom/cotton ball shaped and maltose produces flower-shaped crystals.

Benedict's test

Determines the presence of non- reducing sugars. It is often conducted on a food sample which tested negative for reducing sugar. If reducing sugar is present, a heavier precipitate is often observed when the test for non-reducing sugar is conducted. In the presence of simple sugars, the blue solution turns green, yellow, and brick red, depending on the amount of sugar present. Any carbohydrate that is able to exist in a solution, even to a small extent, has a reducing ability because they are able to form a free aldehyde or ketone group.

Seliwanoff's Test

Identification test for fructose (for Keto sugar). timed colour reaction specific for ketohexoses. Thus it is used to distinguish aldoses from ketoses. In the presence of HCl ketohexoses undergo dehydration to yield 4-hydroxy methyl furfural more rapidly than aldohexoses. Further these furfural derivatives condense with resorcinol to form a red coloured complex.

Anthrone's test

Is another general test for carbohydrates. Its principle is same as that for molisch's test except that the furfurals and hydroxy -methyl furfurals give condensation products with anthrone that are bluish green in colour.

Ninhydrin test

Ninhydrin is a chemical that detects ammonia as well as amines (primary and secondary). Once ninhydrin reacts with these chemicals, it produces a deep blue or purple color - Ruhemann's purple. Ninhydrin is also the same chemical used to detect fingerprints.

Mucic acid test

This test is highly specific for galactose which is either independently present in solutions or obtained by the hydrolysis of lactose. Galactose is converted to Saccharic acid on heating with HNO3 (a strong oxidizing agent). Mucic acid (galactaric acid, a dicarboxylic acid) which is formed from galactose due to the oxidation of both aldehyde & primary alcoholic group at C1&C6. It is the only Saccharic acid which is insoluble in cold water and thus helps in the identification of galactose. Result: The presence of insoluble crystals in the bottom of the tube indicates the presence of mucic acid.

Barfoed's test

This test is performed to distinguish between a reducing mono- and disaccharide. Monosaccharides are more reactive (easily oxidized) reducing agents than disaccharides and thus react in about 1-2 min while the reducing disaccharides take 7-12 min to get hydrolysed in the acidic solution and then react. Thus, the difference in reducing property can be detected. Thin brick red precipitates, at the bottom or sides of the tube indicates the presence of a reducing monosaccharide.

Bial's Orcinol test

This test is specific for pentoses and the compounds containing pentoses and thus useful for the determination of pentose sugars. Reaction is due to the formation of furfural in the acid medium which condenses with orcinol in the presence of ferric ions to give a blue green coloured complex.

Acrolein test

Used to detect the presence of glycerol or fat. When fat is treated strongly in the presence of a dehydrating agent like potassium bisulphate (KHSO4), the glycerol portion of the molecule is dehydrated to form an unsaturated aldehyde, acrolein that has a pungent irritating odour.

Iodine test

chemical test for the presence of starch using a potassium iodide solution. A colour change to purple/black indicates a positive result. In the absence of starch, the brown colour of the aqueous solution remains

Anthrone's Test

principle is same as that for molisch's test except that the furfurals and hydroxy -methyl furfurals give condensation products with anthrone that are bluish green in colour.