Question

biochem

please explain what each test distinguishes and observes.

What types of carbohydrates does Bial's test distinguish between? What is observed in Bial's test with each type of carbohydrate? 5. What two types of carbohydrates does Seliwanoff's test distinguish between? What is observed in Seliwanoff's test with each type of carbohydrate? What carbohydrate gives a positive test with I, solution? What is observed in a positive test?

Answer 1

1) Bial’s Test is to determine the presence of pentoses (5C sugars). The components of this reagent are resorcinol, HCl, and ferric chloride. In this test, the pentose is dehydrated to
form furfural and the solution turns bluish and a precipitate may form.

Procedure:
- To 5 mL of Bial’s reagent, add 2-3 drops of sugar solution and boil.
- Upon boiling, note the green-blue color formed.

2) Seliwanoff’s Test distinguishes between aldose and ketose sugars. Ketoses are distinguished from aldoses via their ketone/aldehyde functionality. If the sugar contains a ketone group, it is a ketose and if it contains an aldehyde group, it is an aldose. This test is based on the fact that, when heated, ketoses are more rapidly dehydrated than aldoses.

Procedure:
- Heat 1 mL of sugar solution with 3 mL Seliwanoff’s reagent (0.5 g resorcinol per liter 10% HCl) in boiling water.
- In less than 30 seconds, a red color must appear for ketoses.
- Upon prolonged heating, glucose will also give an appreciable color.

3) Iodine test is an indicator for the presence of starch. Iodine solution (iodine dissolved in an aqueous solution of potassium iodide) reacts with starch producing a blue-black color.
Procedure:
- To 2-3 mL of polysaccharide solution, add 1-2 drops of iodine solution.
- Observe the different colors obtained for each of the polysaccharide solutions.