Introduction to azo dye test

Azo dyes are a group of organic compounds having the functional group -N=N-. The most common use of azo dyes is as a coloring material. Yellow and red azo dyes are the common dyes used in food industries. The azo dye test determines whether an organic compound contains a primary aromatic amine or not. The main reagents for the azo dye test are sodium nitrate, hydrochloric acid, and beta-naphthol.

In the azo dye 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 slightly soluble in water. The color appearance of the final products confirms that the compound consists of a primary aromatic amine.

The complete reaction is as follows:

Image source: Labster: Azo dye test

Why do students face difficulties in learning the azo dye test?

The following are the three primary reasons that make the azo dye test challenging to understand.

1. Lack of background knowledge

Before delivering the main topic to students, it is necessary to clear the basic knowledge first. If you do not do this, the students may be confused and unable to understand the main topic. The first thing that may confuse the students is what azo means and what azo dyes are. The second thing that may confuse the students will be the difference between azo dye and azo dye test. Clearing this point makes this topic more accessible. By the name, students assume this is a test to find out all types of azo dyes, which is invalid. So make it clear that this test only detects the primary aromatic amines. The structure of primary, secondary, and tertiary amine also makes the topic difficult for students, so to clear this point is also necessary.

2. The in-situ reaction

If you use NaNO₂ instead of HNO₂, an in-situ reaction occurs in which HCl and NaNO₂ react to produce nitrous acid. This may confuse the students as the reagents they use are HCl, NaNO₂, and beta naphthol, but in the final reaction, beta naphthol reacts with nitrous acid. The reaction with nitrous acid will confuse students, so drawing the complete reaction mechanism is necessary.

3. Theoretical imagination

The azo dye test is a practical topic because it involves the color change of materials. Understanding it theoretically is challenging because students get confused about why and how the color change occurs. They have no idea about the colors of the reagents, so make them clear about the colors of the reagents as well. Tell them why and how the color change occurs. If you teach in a classroom, use the color makers to express the final products. It is impossible to take several markers of different colors. However, yellow and red color board markers make the topic enjoyable for students to understand as most of the azo dyes are yellow and red-like colors.

Discover Labster's Azo Dye Test virtual lab!

Preview of Identify the reagents simulation.

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5 Ways to make azo dye test exciting for students

1. Briefly explain the basics.

To make any topic interesting, you must briefly explain the basics. Starting from the name is a good option, so what azo means as azo refers to the functional group -N=N-, so any compound having this group will be known as an azo compound. Next, go for azo dye, which means any dye containing the azo group is called the azo dye. The second thing that may confuse the students will be the difference between azo dye and azo dye test. Clearing this point makes this topic more accessible. By the name, students assume this is a test to find out all types of azo dyes, which is invalid. So make it clear that this test only detects the primary aromatic amines. It is not helpful to detect other amines like secondary and tertiary. Draw structures of primary, secondary, and tertiary amines to help understand the topic easily. Give them some information about what beta or 2-naphthol is. Why is it called beta or 2-naphthol, and what is the primary use of 2-naphthol? Give them information about the positions like alpha, beta, and gamma. This information will make them clear in understanding the main topic.

2. Explain the reaction mechanism in a stepwise manner

Aniline is the primary aromatic amine given in the test books to take the identity test so that you can take it as an example. Explain the reaction mechanism in a stepwise manner.

Step 1:

In step 1, the aniline, which we assume as an unknown, will be dissolved in HCl, and cool the mixture to 0 - 5 ℃.

Step 2:

In the 2nd step, add nitrous acid to the mixture of aniline and hydrochloric acid; this will produce diazonium salt.

Step 3:

In step 3, add the diazonium salt to the basic mixture of 2-naphthol; this will produce an orange-red material. The colored products confirm that the unknown compound is aniline.

3. Use color board markers.

Use the color makers to express the final products and the reagents. It is impossible to take several markers of different colors. However, yellow and red color board markers make the topic enjoyable for students to understand as most of the azo dyes are yellow and red-like colors. So you can write the reagents like aniline with a blue marker because most experts use blue color to express nitrogen. Write other reagents with a black marker and the product with a red marker; this increases students’ attention towards the board.

4. Conduct well-designed experiments

After the theoretical studies, you must go for a lab experiment. Explain the experimental procedure to the students as this experiment involves acids, so extra precautions should be taken. Give the experimental procedure to the students, and provide them with two unknown compounds (one should be a primary aromatic amine). Ask them to identify whether there is any primary aromatic aniline or not.

Experiment

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

Dissolve nearly 0.2 g of the compound in 2 mL of dilute hydrochloric acid in a test tube.
Cool the contents of the test tube in ice.
Add 2 mL of 2.5% cold aqueous sodium nitrite solution. Maintain the temperature of the reaction mixture below 5°C during diazotization, as diazonium salt is unstable at higher temperatures.
In another test tube, dissolve 0.2 g of 2-naphthol in a dilute sodium hydroxide solution.
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, not vice-versa.
The formation of a scarlet red dye confirms the presence of a primary aromatic amine; if the color does not appear, it means that it is something else but not a primary aromatic amine.

5. Use virtual lab simulations.

With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment.

In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.

In Labster’s interactive Azo Dye Test: Identify primary aromatic amines, students first learn the basics of the azo dye test and apply their concepts to find out whether the unknown compound consists of any primary aromatic amine or not.