HOW TO DISTINGUISH ALKALOIDS?

Chemical tests for Alkaloids:

Chemical tests for alkaloids are of two types

1. Group tests

2. Distinguishing test.

Group tests

Group test is for the whole class of alkaloids. It is of two types.

(i) Precipitating Test.

(ii) Colour Test.

Precipitating Test:

1. Wagner’s Reagents:

It is the solution of Iodine in potassium iodide.

When this reagent reacts with alkaloids it gives reddish brown ppt.

2. Mayer’s reagent:

It is the solution of potassium mercuric iodide. This

Reagent gives cream colour ppt with alkaloids.

3. Dargendorff’s reagents:

It is the solution of potassium bismuth iodide. It gives reddish brown ppt with alkaloids.

4. Hager’s reagent

It is a saturated solution of Picric acid. It gives yellow colour ppt with alkaloids.

5. Murexide test:

Caffeine and other purine derivatives do not precipitate like other alkaloids. There is specific test for this class, which is not useful for other classes.

The sample is mixed with the very small amount of potassium chlorate, and a drop of HCl, evaporated to dryness and exposed to ammonia vapors.

A purple colour is produced with the caffeine and other purine derivatives.

Colour Test

1. When Colchicine is treated with mineral acids, it gives yellow colour.

2. Certain indole alkaloids, when treated with H₂SO₄ and p-dimethyl amino benzaldehyde give bluish violet to red colour.

Distinguishing Tests:

Libermann’s test

This test is very useful in distinguishing various alkaloids. It is 10% solution of KMnO₄ in H₂SO₄. Whenever a microdrop of this solution is applied to given sample of alkaloid, the following different colour will show the alkaloids presence.

1. Atropine, Hyoscyamine, Hyoscine, Ephedrineq Orange colour.

2. Papaverine, Emetine Black colour.

3. Cocaine, Physostigmine Red colour.

4. Colchicine Yellow colour.

5. Yohimbine Blue colour.