Laboratory Protocol for Carbohydrates
For polysaccharide extract a. Repeat procedure A. 2a – A. 2d with 10 mL of the polysaccharide extract from Expt. 6 but use 10 drops conc. HCl. B. General Tests for Carbohydrates Test the ff. carbohydrate solutions: 1% glucose, fructose, maltose, sucrose, lactose, agar-agar, gum arabic, glycogen, cotton, starch, polysaccharide solution from clams, and all hydrolysates from
Part A - Molisch Test
- Add 2 drops Molisch reagent to 1 mL sugar solution. Mix thoroughly.
- Incline the tube and gently pour 3 mL conc H2SO4 down the side of the tube.
- Note the color at the interface of the 2 layers.
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Benedicts's Test
Add 1 ml of the solution to be tested to 5 ml of Benedict's solution, and shake each tube. Place the tube in a boiling water bath and heat for 3 minutes. Remove the tubes from the heat and allow them to cool. Note precipitation, if there is any, and the color of the precipitate formed.
Barfoed's Test
- Add 1 ml of the solution to be tested to 3 ml of freshly prepared Barfoed's reagent.
- Place test tubes into a boiling water bath and heat for 3 minutes.
- Remove the tubes from the bath and allow to cool.
Do not heat the tubes longer than 3 minutes, as a positive test can be obtained with disaccharides if they are heated long enough.
Lasker and Enkelwitz Test
- Add 1 ml of the solution to be tested to 5 ml of Benedict's solution in a test tube and mix well.
- Heat the test tube in a 55 oC water bath for 20 minutes.
- Note changes after 10 mins and up to 20 mins.
Orcinol Test
- Add 1 ml of the solution to be tested to 3 ml of Orcinol reagent.
- Gently heat the tube to boiling. Allow the tube to cool.
- Note color of the solution or if there is any precipitate formed.
Mucic Acid Test
- Add 10 drops conc HNO3 to 3 ml of the solution to be tested and mix well.
- Heat on a boiling water bath until the volume of the solution is reduced to about 1 ml.
- Remove the mixture from the water bath and let it cool in an ice bath.
- Note the formation of crystals, if any.
Caution: Perform the reaction under a fume hood.
Iodine Test for Starch and Glycogen
Add 2 drops of Lugol's iodine solution to 10 drops of solution to be tested in a spot plate. Note color changes.
Post-Lab Questions:
- Correlate the results of the iodine test on the polysaccharides with their structures.
- Correlate the results of the tests on cotton with the structure of cellulose.
- Give the balanced chemical equation, the positive result and the product/s responsible for the positive result of each color reaction test.
- Conclude on the type of carbohydrate in your polysaccharide extract based on the results of its color reactions.
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