Lassaigne Test

Lassaigne Test is a set of procedures used to detect the presence of nitrogen, sulphur and halogens in an organic compound. In the theoretical approach, we use different symbols in the molecular formula of a compound to show the presence of a particular element in it but in the practical approach, we use a variety of tests to identify the presence of specific elements through observable reactions or characteristic behaviours when brought in contact with a specific reagent. For instance, when heated, certain metals exhibit certain characteristic flame colours, indicating their presence.

In this article, we will learn about the Lassaigne Test, its principle, the preparation of the Lassaigne Extract, the procedure for the Lassaigne Test, its applications, and some problems based on it.

What is Lassaigne Test?

Lassaigne Test is a qualitative analysis tool to detect the presence of nitrogen, sulphur, and halogens in an organic compound using sodium metal as a reagent. The organic compound for which the test is to be carried out, is heated in a fusion tube in the presence of sodium metal. The sodium metal converts the elements present in the compound into an ionic form called as sodium fusion extract. This extract is then further treated with specific reagents to check for some observable changes that infer the presence of a particular element in the compound.

Principle of the Lassaigne Test

The principle of Lassaigne Test is that when sodium metal is heated with an organic compound, it converts all the elements present in the compound into an ionic form. The extract obtained in the ionic form can further be treated with specific reagents to get observable changes which infer the presence of specific elements in the compound. Let us have a look on how different elements in organic compounds get converted when heated in presence of sodium in a fusion tube.

Conversion of Organic Compounds to Ionic Forms

• Na + C + N → NaCN
• 2Na + S → Na₂S
• Na + X → NaX, where X = Cl, Br or I

These ionic salts can be extracted by boiling the fused mass with distilled water. The extract is therefore called the sodium fusion extract.

Detection of Elements via Formation of Coloured Compounds

The ionic salts are further treated with some specific reagents to form different coloured compounds which indicates presence or absence of certain elements in the organic compound under consideration. This is discussed in detail as follows.

Procedure for the Lassaigne Test

To proceed with Lassaigne Test we need to prepare Lassaigne Extract. After the extract is prepared we perform test for confirmation of element present.

Preparation of Lassaigne Extract

The preparation of Lassaigne Extract is discussed below:

• A small piece of sodium metal is taken and cut into further small pieces which are then placed in a dry boiling tube.
• A small amount of the organic compound test sample (approximately 0.5 g) is then added to the boiling tube.
• A few drops of distilled water are also added to moisten the mixture.
• The boiling tube is then heated over a burner gently until ionic products are formed with the sodium metal.
• The contents of the boiling tube are then emptied into a clean test tube. The mixture obtained in this step is called the Lassaigne Extract or Sodium Fusion Extract.
• Furthermore, steps are different for test for different elements in the organic compound, discussed as follows.

Test for Nitrogen

• After the sodium fusion extract has been taken in a clean test tube, distilled water is added to it and the contents are well shaken to dissolve the sodium cyanide properly.
• The solution is then filtered to get rid of any impurities.
• The filtrate is then acidified with hydrochloric acid (HCl).
• Few drops of freshly prepared ferrous sulphate solution are then added.
• The solution is then gently heated.
• If a blue precipitate (Prussian Blue) is formed, it confirms the presence of nitrogen in the organic compound.

Test for Sulphur

• Same steps are followed here until the filtration of the solution formed by dissolving the sodium fusion extract with distilled water.
• Here, the filtrate is acidified with dilute acetic acid.
• A few drops of lead acetate solution are then added.
• After a few minutes, the formation of a black precipitate (lead sulphide) indicates the presence of sulphur in the organic compound.

Test for Halogens

• Same steps are followed in this case also to obtain the filtrate.
• The filtrate is acidified with dilute nitric acid (HNO₃).
• A few drops of silver nitrate (AgNO₃) solution are then added.
• The colour of precipitate formed indicates the presence of different halogens in the organic compound, listed as follows:
• White precipitate: Chlorine (Cl).
• Cream precipitate: Bromine (Br).
• Yellow precipitate: Iodine (I).

Examples of Lassaigne Test

The Lassaigne Test can be applied to different organic compounds for test of elements such as sulphur, nitrogen and halogens in them. Some examples of Lassaigne Test are discussed as under:

• For Amines (-NH₂ group): Amines are the organic compounds which contain -NH₂ functional group. The presence of nitrogen can thus be detected through Lassaigne Test.
• For Thiols (-SH group): Thiols are the organic compounds which contain -SH functional group. The presence of sulphur can thus be detected through Lassaigne Test.
• For Nitroalkanes (-NO₂ group): Nitroalkanes are the alkanes containing -NO₂ functional group. The presence of nitrogen can be detected in them through Lassaigne Test.
• For Haloalkanes and Haloarenes: Haloalkanes or Haloarenes contain halogen as a functional group. Hence, the presence of halogens in them can be detected through Lassaigne Test.

Applications of Lassaigne Test

Lassaigne Test can be applied to various organic compounds as it can be used to detect presence of nitrogen, sulphur, and halogens in them with use of sodium metal. Some of the practical applications are listed as follows:

• Impurities analysis: Impurities containing nitrogen, sulphur and halogen elements can be detected through Lassaigne test and further actions can be taken to remove them accordingly.
• Soil, Water and Air analysis: Lassaigne Test can be used to identify organic pollutants in soil, water and air samples and analyze their impact on environment.
• Manufacturing Industry: The test can be used to detect unwanted elements in production of different chemicals which helps in quality control and compliance to the standards.
• Research Purposes: The test is used by researchers to determine composition and properties of various organic compounds.
• Forensic Analysis: Lassaigne test is also used in forensic science to determine the elements present in the substances found at crime scenes for investigation purposes.

Conclusion

In conclusion, it can be said that Lassaigne Test is a qualitative analysis tool that is used to detect the presence of nitrogen, sulphur and halogens in organic compounds. The test uses sodium metal as a reagent which is broken into smaller pieces and mixed with the organic compound sample. The mixture is then heated in a test tube. Further, it is emptied into a clean test tube where distilled water is added to it and then this mixture is filtered to remove any impurities. The filtrate is then analyzed for presence of different elements such as nitrogen, sulphur, and halogens using the procedures discussed above. The test has practical applications in various fields such as impurities analysis, quality check and control, forensic analysis and research purposes.

Sample Questions on the Lassaigne Test

Following are some questions based on the Lassaigne Test:

Question 1: A student upon performing the Lassaigne test on a sample, observed the formation of a black precipitate when lead acetate solution was added to the sodium fusion extract. What does this observation indicate?

Answer:

This indicates the presence of sulphur in the organic compound tested as formation of black precipitate is due to formation of lead sulphide upon reaction with lead acetate.

Question 2: How is the Lassaigne test be used to distinguish between different halogens present in the organic compounds?

Answer:

In Lassaigne Test, silver nitrate is added to the sodium fusion extract to detect presence of halogens in the organic compound. Silver nitrate forms different halides upon reaction with sodium fusion extract. Thus, different colored precipitates are formed which indicate presence of different halogens listed as follows:

• White precipitate: Chlorine (Cl)
• Cream precipitate: Bromine (Br)
• Yellow precipitate: Iodine (I)

Question 3: What are some sources of error in the Lassaigne Test? How can these be avoided?

Solution:

Sources of error in Lassaigne Test are contamination, incomplete fusion, and improper acidification. These can be avoided by ensuring cleanliness, proper mixing during fusion and precise acidification using appropriate acids.

Frequently Asked Questions (FAQs) on Lassaigne Test

What is the principle of Lassaigne Test?

The principle of Lassaigne Test is that when sodium metal is heated with an organic compound, it converts the elements into an ionic form which upon reaction with specific reagents yield different coloured precipitates which indicate presence of different elements.

What is the purpose of Lassaigne Test?

Lassaigne Test is used to detect the presence of nitrogen, sulfur, and halogens in organic compounds.

Can Lassaigne test be used to have information about amount of elements present in the organic compound samples?

No, the Lassaigne’s Test is used only for qualitative analysis of elements present in the organic compounds, i.e. to detect presence or absence of certain elements in the samples.

What are the safety considerations to be kept in mind while performing the Lassaigne’s Test?

Safety measures such as wearing lab coat, gloves, goggles and proper ventilation must be ensured while performing the Lassaigne’s Test, especially while handling sodium and acidic solutions.

Why metallic sodium is used in the Lassaigne’s Test?

Metallic sodium acts as a reducing agent when reacts with organic compounds and forms solube salts. These salts can then be extracted and tested for presence of nitrogen, sulphur and halogens.