What is an Ehmann’s reagent and how is it prepared?

(Last Updated On: October 14, 2022)
Visualization of indole intermediates in TLC using Ehmann's reagent
Visualization of indole intermediates in TLC using Ehmann’s reagent

Ehmann’s reagent is a mixture of Ehrlich reagent and Salkowski reagent in a ratio of 1:3 (v/v). It is used to detect indole intermediates during thin-layer chromatography. This reagent is greatly helpful to detect the presence of different types of indole intermediate. However, standard indole compounds must be run along with the sample to compare the retention factor and to determine the presence of intermediates in the sample.

To prepare Ehmann’s reagent, you need to prepare two individual reagents; 1) Ehrlich’s reagent and 2) Salkowski’s reagent. Ehrlich’s reagent is a 2 % solution of p-dimethylaminobenzaldehyde prepared in 100 ml of a mixture of 95 % ethanol and concentrated HCl in a 1:1 ratio (v/v).

Weigh 2 g of p-dimethylaminobenzaldehyde and dissolve it completely in 50 ml of 95 % ethanol and then add 50 ml of concentrated HCl and mix it well. Label this solution as Ehrlich’s reagent. To prepare a Salkowski reagent, see my post on how to prepare the Salkowski reagent.

Now, take 50 ml of Ehrlich’s reagent and add 150 ml of Salkowski reagent and mix it well and store it in a brown bottle and label it as Ehmann’s reagent. You can use it to spray over the TLC plates to visualize spots of different indole intermediates along with standards.

During the analysis, indole intermediates, such as indole-3-acetic acid, indole-3-acetamide, indole-3-acetonitrile, tryptamine, tryptophol, and indole-3-pyruvate will be used as standards. These indole standards will be run in the TLC along with the sample to be tested for the presence of indole intermediates.

After running a thin layer of chromatography, the TLC plate will be dried and then sprayed with Ehmann’s reagent. After spraying with the reagent, allow the TLC plate to get dry and wash the TLC plate with water two times to remove the unreacted Ehmann’s reagent. After washing with water, wrap the TLC plate with tissue paper to soak the water and allow it to dry. After drying, heat the plate to 90 ÂșC until all the spots are visible.

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