Tests for Carbohydrate

Requirements:
 

Glasswares: Test tube, test tube stand, beaker, wash bottle, burner, glass rod, funnel, filter paper, and spatula.

Chemicals: Molisch reagent, concentrate sulphuric acid, hydrochloric acid,  iodine solution, Benedict reagent, fehling solution, barfoed’s reagent, selivanoff reagent, glacial acetic acid, phenylhydrazine hydrochloride, sodium acetate.

Theory for chemical tests

Carbohydrates are defined chemically as aldehyde or ketone derivatives of polyhydroxy alcohols or compounds which yield these derivatives on hydrolysis.  “Carbohydrate" means hydrated carbon, which indicates the relative proportion of carbon, hydrogen and oxygen atoms in a carbohydrate. For example glucose has a molecular formula C6H12O6 .It also indicates that these are the only three elements in this group of natural compound. However, like the complex protein, some carbohydrate also may have amino group. Carbohydrates also have two major functions in living organisms: (1) they provide energy needed for life processes. (2) Some are parts of structural unit of cell and tissue especially in plant. Simple carbohydrates are glucose, fructose, (both occur in fruits), lactose (occur in milk) and sucrose (cane sugar). Starch (rice, potato, maize) is a high molecular weight carbohydrate. Carbohydrates are classified on the basis of units of sugar present in the molecule as follows (1) monosaccharide (2) disaccharides (3) oligosaccharides (4) polysaccharides.

Procedure for chemical tests for carbohydrate

Different chemical tests for carbohydrate are given below-

Molisch test

Take two mL sample solution in test tube and add two drops of molisch reagent. Mix well and inclined the test tube and add one mL concentrate sulphuric acid by the side of the test tube. Appearance of reddish violet colour confirms the presence of caebohydrate.

Iodine test

Take two mL sample solution in test tube followed by addition of two drops of dilute hydrochloric acid and few drops of iodine solution. Appearance of blue color indicate the presence of starch.

Benedict’s test

Take three mL benedict’s reagent in test tube then add eight drops sample solution. Mix the content and heat for three minutes. Appearance yellowish green precipitate confirms the presence of reducing sugar.

Fehling test

Take two mL fehling solution in test tube, warm the solution and add one mL sample solution. Mix well and heat the solution. Brownish red colour confirms the presence of reducing sugar.

Barfoed’s test

Take two mL barfoed reagent in test tube followed by addition of one mL sample solution, and heat the solution up to boiling. Appearance of reedish orange precipitate indicates presence of monosaccharides.

Selivanoff’s test

Take two mL sample solution in test tube and add three mL selivanoff reagent. Mixed well and gently heat the solution. No red color confirms the presence of aldose sugar.

Osazone test

Take three mL sample solution in test tube and add ten drops glacial acetic acid followed by addition of few amount of phenyl hydrazine hydrochloride and double amount of sodium acetate. Mix and warm the solution and filter the solution and filtrates is keep in boiling water bath. Needle shape crystal confirms the presence of glucose.

Result: In the given sample carbohydrate (glucose) is present; it is confirmed by the different test such as molisch test, iodine test, benedict’s test, fehling test, barfoed test, selivanoff test, osazone test.