Tests for Carbohydrates

TESTS FOR CARBOHYDRATES

The following are some of the more useful tests for sugars and other carbohydrates.

Reduction of Fehling’s Solution

To the solution of carbohydrate, equal quantity of Fehling’s solutions A and B is added. After heating, brick red pre-cipitate is obtained.

Molisch Test

The test is positive with soluble as well as insoluble car-bohydrates. It consists of treating the compounds with α-naphthol and concentrated sulphuric acid which gives purple colour. With a soluble carbohydrate this appears as a ring if the sulphuric acid is gently poured in to form a layer below the aqueous solution. With an insoluble, carbohydrate such as cotton wool (cellulose), the colour will not appear until the acid layer is shaken to bring it in contact with the material.

Osazone Formation

Osazones are sugar derivatives formed by heating a sugar solution with phenylhydrazine hydrochloride, sodium acetate and acetic acid. If the yellow crystals which form are examined under the microscope they are sufficiently characteristic for certain sugars to be identified. It should be noted that glucose and fructose form the same osazone (glucosazone, m.p. 205°C). Before melting points are taken, osazones should be purified by recrystalization from alcohol. Sucrose does not form an osazone, but under the condi-tions of the above test sufficient hydrolysis takes place for the production of glucosazone.

Resorcinol Test for Ketones (Selivanoff’s Test)

A crystal of resorcinol is added to the solution and warmed on a water bath with an equal volume of concentrated hydrochloric acid. A rose colour is produced if a ketone is present (e.g. fructose, honey or hydrolysed inulin).

Test for Pentoses

Heat a solution of the substance in a test tube with an equal volume of hydrochloric acid containing a little phloroglu-cinol. Formation of a red colour indicates pentoses.

Keller-Kiliani Test for Deoxysugars

A Deoxysugar (found in cardiac glycosides) is dissolved in acetic acid containing a trace of ferric chloride and trans-ferred to the surface of concentrated sulphuric acid. At the junction of the liquids a reddish-brown colour is produced which gradually becomes blue.

Furfural Test

A carbohydrate sample is heated in a test tube with a drop of syrupy phosphoric acid to convert it into furfural. A disk of filter paper moistened with a drop of 10% solution of aniline in 10% acetic acid is placed over the mouth of the test tube. The bottom of the test tube is heated for 30–60s.

A pink or red stain appears on the reagent paper.