Introduction
Carbohydrates consist of carbon, hydrogen and oxygen with the last two elements hydrogen and oxygen usually present in the same proportion as in water. Carbohydrates arose as one of the products of photosynthesis. Carbohydrates constitute a large proportion of the plants biomass and are responsible as cellulose for rigid framework, and as starch for providing an important food reserve
Identification Tests for Carbohydrates
Some of the useful tests for sugars and other carbohydrates are:
1. Reduction of fehlings solution:
To a heated solution of the substance add drop by drop a mixture of equal parts of fehlings solution No.1&No.2. A brick red precipitate confirms the presence of reducing sugar which includes all monosaccharides and some dissacharides (e.g. lactose, maltose , cellobiose and gentiobiose ). Non reducing sugars including some disaccharides like sucrose and polysaccharides will however require boiling with acid to produce same reaction. Non reducing sugars/ carbohydrates on boiling with acid will be converted to reducing sugars.
2. Molisch’s test:
All carbohydrates give a purple colour when treated with α naphthol and concentrated sulphuric acid. With a soluble carbohydrate the purple ring appears immediately but with an insoluble carbohydrate like cotton wool (cellulose), the purple ring will not appear till the acid is shaken to bring it in contact with the cotton wool.
3. Osazone formation:
Osazone are sugar derivatives formed by heating a sugar solution with phenylhydrazine hydrochloride, sodium acetate and acetic acid. The yellow crystals formed if viewed under microscope are sufficiently characteristic for certain sugars to be identified. Note that glucose and fructose form the same osazone (Glucosazone, m.p. 205ºC). sucrose does not form an osazone but under the conditions of the above test, sufficient hydrolysis takes place for production of glucosazone.
4. Resorcinol test for Ketones:
This is known as Selivanoff’s test. A crystal of resourcinol 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).
5. Test for pentoses:
Heat a solution of the substance in a test tube with an equal volume of hydrochloric acid containing a little phloroglucinol. Formation of a red colour indicates the prescence of pentoses.
6. Keller-Killiain test for deoxysugars:
Deoxysugars are found in the cardiac glycosides such as Digitalis and Strophantus spp. The sugar is dissolved in acetic acid containing a trace of Ferric chloride and transferred to the surface of concentrated sulphuric acid. At the junction of the two liquids, a reddish brown colour is produced which gradually turns blue.
7. Enzyme reactions:
Since certain carbohydrate reactions are brought about by specific enzymes. Such enzymes may be used for their identification.
8. Chromatography:
Chromatographic methods are particularly suitable for examination of drug extracts which may contain a number of carbohydrates often in very small amounts. For examples, TLC methods have been used to study the products of hydrolysis of polysaccharides complexes such as gums and mucilages.