Question

What test would you use to distinguish between the members of the following pairs of carbohydrates? Describe chemistry and observations behind the tests you will use to distinguish between these pairs of carbohydrates. 1. Glycogen and Starch 2. Pentose (Arabinose) and Glucose 3. Sucrose and Lactose 4. Glucose and Fructose

Answer 1

1. Glycogen and starch can be distinguished by the famous iodine test. Lugol's iodine reagent can be employed to detect the presence of either glycogen or starch. Starch with Lugol's iodine reagent yields blue-black color while glycogen and Lugol's iodine gives brown-blue color. Starch can bind more iodine because of less branching and gives intense color whereas glycogen having more branching cannot bind more iodine so it gives brown blue color.

2. Pentose and glucose can be differentiated using Bial's test. Pentose sugars react with bial reagent to form a green colored product or precipitate. The formation of a brown precipitate is because of the presence of hexose sugar like glucose.

3. Woehlk test is used to know the presence of lactose.

4. Selivanoff's test is used for distinguishing between glucose and fructose. Fructose gives a positive test with the reagent by giving dark red color while glucose on reacting with selivanoff's reagent gives a light pink color.