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4-(Dimethylamino)benzaldehyde (DMAB) Reagent Grade | CAS 100-10-7 | Ehrlich’s Reagent Component
  • 4-(Dimethylamino)benzaldehyde (DMAB) Reagent Grade | CAS 100-10-7 | Ehrlich’s Reagent Component
4-(Dimethylamino)benzaldehyde (DMAB), Reagent Grade

ASC-8069

4-(Dimethylamino)benzaldehyde (DMAB), Reagent Grade

$35.00 AUD ex GST
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Laboratory Reagent

Chromogenic aldehyde for Kovács' and Ehrlich's reagents and the bacterial indole test

Cat. No. ASC-8069CAS 100-10-7Assay ≥ 98.5%Reagent gradeIndole test

At a glance

  • CAS No.100-10-7
  • FormulaC9H11NO
  • Mol. weight149.19 g/mol
  • Assay≥ 98.5%
Australian stock. SDS available. Batch COA on request.

Overview

4-(Dimethylamino)benzaldehyde (DMAB, p-dimethylaminobenzaldehyde) is the active component of Kovács' and Ehrlich's reagents. In microbiology it is used in the indole test, which shows whether an organism breaks down tryptophan to indole. The test is a standard step in identifying Escherichia coli and other Enterobacterales.

DMAB is also used in analytical chemistry as a chromogenic reagent, including spectrophotometric methods for hydrazine and urea.

Key features

  • Assay ≥ 98.5% for consistent colour development
  • For Kovács' and Ehrlich's indole reagents (recipes on the Technical Data tab)
  • Supplied from Melbourne stock

Principle of the Reaction

Under strongly acidic conditions, DMAB reacts with indole at its 3-position to form a red to red-violet dye (a rosindole cation). Other indoles and pyrroles give the same type of reaction.

In the indole test, bacteria with the enzyme tryptophanase break down tryptophan in the medium and release indole. When Kovács' reagent is added, the indole moves into the alcohol layer, where it reacts with DMAB and forms a red ring.

Ehrlich's reagent (ethanol-based) is more sensitive than Kovács' reagent. It is preferred for weak indole producers, anaerobes and non-fermenters, usually after extraction of the culture with xylene.

Applications

Field Typical use
Clinical and food microbiology Indole test for the identification of Enterobacterales and other bacteria
Water testing Confirmation of E. coli by indole production at 44 °C
Biochemistry Detection of indoles, pyrroles and tryptophan metabolites (Ehrlich reaction)
Analytical chemistry Chromogenic reagent for spectrophotometric determination of hydrazine and urea

Frequently Asked Questions

What is the difference between Kovács' and Ehrlich's reagent?

Both use DMAB in acid. Kovács' reagent uses amyl alcohol and is the standard for the tube indole test. Ehrlich's reagent uses ethanol, is more sensitive, and is preferred for weak indole producers, usually with xylene extraction.

How do I make Kovács' reagent?

Dissolve 5 g DMAB in 75 mL isoamyl alcohol, then slowly add 25 mL concentrated hydrochloric acid. Store in a brown bottle at 2–8 °C. Full instructions are on the Technical Data tab.

Why has my reagent turned brown?

DMAB degrades with light, air and time. Discard brown reagent and check new reagent with positive and negative controls.

Is DMAB the same as DMACA?

No. DMACA (p-dimethylaminocinnamaldehyde) is a different, more sensitive reagent used for the rapid spot indole test.

Intended use: For laboratory, analytical and research use only. Not for human or veterinary use, food, feed or consumption.

Kovács' Reagent

The standard reagent for the tube indole test.

Component Amount
4-(Dimethylamino)benzaldehyde 5 g
Isoamyl alcohol (or n-amyl alcohol) 75 mL
Hydrochloric acid, concentrated 25 mL
  1. Dissolve the DMAB in the alcohol, warming gently in a 50–55 °C water bath if needed.
  2. Cool to room temperature.
  3. Add the hydrochloric acid slowly, with mixing.
  4. Store in a brown glass bottle at 2–8 °C. Discard if the reagent turns brown.
Prepare in a fume hood. Concentrated hydrochloric acid is corrosive and amyl alcohol is flammable.

Ehrlich's Reagent (for indole)

More sensitive; used with xylene extraction for weak indole producers.

Component Amount
4-(Dimethylamino)benzaldehyde 1 g
Ethanol, 95% 95 mL
Hydrochloric acid, concentrated 20 mL
  1. Dissolve the DMAB in the ethanol.
  2. Add the hydrochloric acid slowly, with mixing.
  3. Store in a brown glass bottle at 2–8 °C.

How It Is Used

Test Procedure Positive result
Indole test, Kovács' method Grow the organism in tryptone water for 24–48 h at 35–37 °C. Add 0.5 mL Kovács' reagent and shake gently. Red or pink ring in the upper layer within 1 minute; yellow = negative
Indole test, Ehrlich's method Add 1 mL xylene to a 48 h culture, shake and let the layers separate. Run 0.5 mL Ehrlich's reagent down the side of the tube. Red ring at the interface

Controls: Escherichia coli ATCC 25922 (positive) and Klebsiella aerogenes ATCC 13048 (negative). Use a tryptophan-rich medium; media containing glucose can give weaker reactions.

Limitations

  • The indole test is presumptive. Confirm identification with further biochemical or molecular tests.
  • Prepared reagents deteriorate with light and time; test them with positive and negative controls before use.

Literature and References

  1. Kovács N. Eine vereinfachte Methode zum Nachweis der Indolbildung durch Bakterien. Z Immunitätsforsch Exp Ther. 1928;55:311–315.
  2. MacFaddin JF. Biochemical Tests for Identification of Medical Bacteria, 3rd ed. Lippincott Williams & Wilkins; 2000.
  3. US Food and Drug Administration. Bacteriological Analytical Manual, Reagents: Kovacs' Reagent.
  4. Watt GW, Chrisp JD. A spectrophotometric method for the determination of hydrazine. Anal Chem. 1952;24:2006–2008.

Hazard Identification

Warning. Classified as hazardous according to the GHS (Rev. 7) as adopted in Australia. Pictogram: GHS07 (exclamation mark).
  • H302 Harmful if swallowed.
  • H315 Causes skin irritation.
  • H317 May cause an allergic skin reaction.
  • H319 Causes serious eye irritation.
  • P261 Avoid breathing dust. P280 Wear protective gloves and eye protection.
  • P302+P352 If on skin: wash with plenty of water. P305+P351+P338 If in eyes: rinse cautiously with water for several minutes; remove contact lenses if easy to do; continue rinsing.

First Aid

Route Action
Inhalation Move to fresh air. Seek medical advice if symptoms persist.
Skin Wash with plenty of soap and water. If irritation or a rash occurs, seek medical advice.
Eyes Rinse cautiously with water for several minutes. Seek medical advice if irritation persists.
Ingestion Rinse mouth. Do not induce vomiting. Seek medical advice.
Poisons Information Centre (Australia, 24 h): 13 11 26

Handling, Spills and Disposal

  • Avoid generating dust and avoid contact with skin and eyes. Repeated skin contact may cause sensitisation.
  • Spills: sweep up without raising dust and place in a labelled container for disposal.
  • Dispose of unused product and prepared reagents as chemical waste under local regulations.
  • Reagents made with concentrated acid and alcohol carry the additional hazards of those components.

Quality Specification

Test Specification
Appearance White to light yellow or beige crystalline powder
Assay (titration) ≥ 98.5%
Melting range 72–76 °C
Solubility Soluble in ethanol, acetone and dilute acids; practically insoluble in water
The information on this page is provided in good faith as a guide to the properties and use of this product and does not constitute a warranty of fitness for a particular purpose. Users are responsible for confirming suitability for their application. Ausamics Pty Ltd · ABN 56 676 640 467 · 36/25 Trafalgar Road, Epping VIC 3076, Australia · support@ausamics.com

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