Testing For Carbohydrates

A) Test for reducing sugars 1. Place 1 cm3 of glucose solution in a test tube. Add 2 cm3 of Benedict’s of Fehling’s solution. 2. Place the test tube in a water bath of boiling water for five minutes 3. Place 2 cm3 Benedict’s or Fehlings’s solution in another test tube. Add 1 cm3 of distilled water. Place the mixture in the water bath as in 2 above. 4. shake each tube at intervals and observe the colour of each liquid carefully, the series of colour changes in the first test tube, the colour of the grains ( precipitate) which settles at the bottom of the tube, the colour changes in the second test tube B) Test for non-reducing sugar 1. Repeat the Benedict’s or Fehling’s test with 1 cm3 of sucrose (table sugar) solution. Observe any colour change 2. To 1 cm3 of sucrose solution in another test tube add three drops of dilute hydrochloric acid. Place in a water bath and boil for about three minutes 3. Cool the test tube in a beaker of cold water. Add sodium hydrogen carbonate slowly, until fizzing stops. This indicates that the solution has been neutralized. Confirm this with universal indicator paper. 4. Add 2 cm3 of Benedict’s or Fehlings’s solution, Place in the water bath of boiling water for five minutes. Observe the colour changes .i.e are they green, yellow, red and finally reddish-brown or orange. The hydrochloric acid has hydrolysed the sucrose to monosaccharide molecules, which can react with the test reagent Benedict’s or Fehling’s solution. C) Test for Starch 1. To 1 cm3 of starch suspension in water in a test tube add two drops of iodine solution. Observe any colour change. 2. To 1 cm3 of distilled water in a test tube add two drops of iodine solution. Observe any colour