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Azo dye test

The azo dye test is a test used to determine whether an organic compound contains a primary aromatic amine group. Amines have an amino functional group and can be grouped into primary, secondary, and tertiary amines.

In this test, a primary aromatic amine reacts with nitrous acid, which is generated in situ by the reaction of sodium nitrite with hydrochloric acid (HCl), at 0–5°C to produce a diazonium salt. The diazonium salt couples with 2-naphthol (also called beta naphthol) to make a scarlet red dye, which is slightly soluble in water.

The reaction is as follows:

There are two reactions. The first reaction shows aniline (a primary aromatic amine) reacting with sodium nitrite and hydrochloric acid to form benzene diazonium chloride (this is the diazonium salt). The amine group has changed to a positively charged nitrogen nitrogen triple bond with a negatively charged chloride ion. The second reaction shows benzene diazonium chloride reacting with beta naphthol to form beta naphthol azo dye which is a scarlet red color.

Materials

  • Test tubes

  • Test tube rack

  • Dilute hydrochloric acid

  • Ice

  • Sodium nitrite solution

  • 2-Naphthol (beta naphthol)

  • Sodium hydroxide (NaOH) solution

  • Compound to be tested

Safety

Hydrochloric acid and sodium hydroxide are corrosive. Sodium nitrite is oxidizing, toxic, and an environmental hazard. 2-naphthol is harmful and an environmental hazard.

Procedure

  1. Dissolve nearly 0.2 g of the compound in 2 mL of dilute hydrochloric acid in a test tube.

  2. Cool the contents of the test tube in ice.

  3. Add 2 mL of 2.5% cold aqueous sodium nitrite solution. This is the diazonium salt solution. Maintain the temperature of the reaction mixture below 5°C during diazotization, as diazonium salt is unstable at higher temperatures.

  4. In another test tube, dissolve 0.2 g of 2-naphthol in dilute sodium hydroxide solution.

  5. Add the diazonium salt solution prepared in step 3 into the cold 2-naphthol solution slowly with shaking. Always add diazonium salt solution into the alkaline solution of 2-naphthol and not vice-versa.

  6. The formation of a scarlet red dye confirms the presence of a primary aromatic amine.

References:

  • NCERT Lab Manual for Functional Group tests