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Lassaigne's test is used to detect the presence of sulphur, halogen and nitrogen in organic compounds. For conducting this test, take a fusion tube and add the organic compound into it along with a small piece of sodium. By doing so, sodium converts all the available metals into its ionic compounds.
Read Also: Cis-Trans Isomerism
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Key Terms: Lassaigne's test, sulphur, halogen, nitrogen, organic compounds, ionic compounds, metals
What is the Lassaigne Test ?
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Lassaigne’s test is a test developed by J L Lassaigne, for the detection of elements such as Nitrogen(N), Sulfur(S), Chlorine (Cl), Bromine (Br) and Iodine (I). This test is mainly done in two steps.
i) Preparation of sodium fusion extract (SFE).
ii) Detection of elements using sodium fusion extract
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Procedure of Preparing Lassaigne Extract
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Firstly, take a fusion tube and put a small piece of dry sodium into it. Heat the fusion tube in a burner. Then the sodium melts. Heat the fusion tube again after, adding a small amount of organic compound into it. The compound starts reacting with sodium metal. Continue heating the tube until it becomes red hot. Pour the matter into a china dish and crush it with a glass rod. Heat the above material to the boiling point. When reaching boiling point, stop heating and filter the insoluble matter. This filtrate is known as Lassaigne's Extract or Sodium Fusion Extract (SFE) . The extract obtained will be alkaline in nature. If the extract is tested to be not alkaline, add a small drop of NaOH solution into it to make it alkaline.
Read More: Preparation of Alcohols
Detection of Elements Using Sodium Fusion Extract
The extract prepared above can be used for the detection of metals like sulphur, nitrogen and halogens. The procedure for halogens, sulphur and nitrogen are in different ways which is explained below in detail.
Read More: Alcohols, Phenols, and Ethers
Lassaigne Test for the Detection of Halogens
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Firstly, take the Sodium Fusion Extract containing halogens in it. HNO3 acid is added inorder to acidify the compound. After that, the compound is being treated with AgNO3. The equations below show the reaction of Halogens to form the precipitate.
- Na + Cl → NaCl
NaCl + AgNO3 → AgCl + NaNO3
- Na + Br → NaBr
NaBr + AgNO3 → AgBr + NaNO3
- Na + I → NaI
NaI + AgNO3 → AgI + NaNO3
The colours of the precipitate vary with each other. The presence of halogen can be found by identifying the colours of the precipitate formed. If the precipitate formed is white and soluble in NH4OH, the presence of Chlorine (Cl) is confirmed. If the precipitate formed is yellow and sparingly soluble in NH4OH, it indicates the presence of Bromine (Br). Lastly, if the formed precipitate is yellow; but insoluble in NH4OH, this detects the element Iodine (I) in the compound.
Read More: States of Matter
Lassaigne Test for the Detection of Nitrogen
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The sodium fusion extract containing Nitrogen is boiled at first. It is acidified then with concentrated Sulphuric acid (H2SO4). A precipitate will be formed. If the precipitate has a prussian blue colour; it indicates the presence of Nitrogen in the organic compound.
The following reactions occur while doing so.
Na + C + N → NaCN
6NaCN + FeSO4 → Na4[Fe(CN)6] + Na2SO4.
(Sodium ferrocyanide)
Na4[Fe(CN)6] + Fe3+ → Fe4[Fe(CN)6]3
(Ferric Ferrocyanide)
The formation of Ferric Ferrocyanide indicates the presence of Nitrogen in the compound. If the compound contains no carbon but only nitrogen; the presence of nitrogen cannot be determined. It can only be found if both the elements are present in the compound.
This test cannot be conducted to detect the presence of nitrogen in diazonium salts. The reason is that, nitrogen in diazonium salts will break down if it undergoes boiling.
Read More: Oxidation and Reduction
Lassaigne Test for the Detection of Sulphur
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Sulphur in the organic compound reacts with sodium at first to form sodium sulphide. This sodium sulfide when reacts with sodium nitroprusside to form sodium thio nitroprusside which in turn causes the formation of violet colour which indicates the presence of sulphur in the organic compound. The reaction of Sodium fusion extract containing sulphur is given below.
Na + S → Na2S
Na2S + Na2[Fe(CN)5NO] (Sodium nitroprusside) → Na4[Fe(CN)5NOS] (Sodium thio nitroprusside)
Lead acetate test is another test conducted to detect the presence of sulphur in organic compounds.
Read More: Dioxygen
Things to remember
- The Lassaigne test is also known as the sodium fusion test.
- It mainly involves two steps including the preparation of Lassaigne Extract and Detection of Elements Using Sodium Fusion Extract
- The presence of Halogens, nitrogen and sulphur can be identified through this test.
- Prussian blue coloured precipitate indicates the presence of Nitrogen.
- White coloured precipitate soluble in NH4OH indicates the presence of Chlorine
- Pale Yellow coloured precipitate sparingly soluble in NH4OH helps in identifying the presence of Bromine.
- The yellow precipitate which is insoluble in NH4OH detects the presence of Iodine.
- The formation of a violet coloured precipitate indicates the presence of Sulphur in the organic compound.
Sample Questions
Ques. What is the other name of the Lassaigne Test? (2 marks)
Ans. The other name of the Lassaigne test is Sodium Fusion Test.
Explanation- It is thus known, due to the formation of sodium fusion extract in the procedure.
Ques. Is potassium used in the Lassaigne test instead of sodium? (2 marks)
Ans. No, potassium cannot be used in lieu of sodium in the Lassaigne test.
Explanation- Potassium is a highly reactive metal since it is dangerous to use this in the Lassaigne test.
Ques. What is the colour and nature of precipitate obtained in the detection of Chlorine? (1 marks)
Ans. In the detection of Chlorine, a white precipitate is obtained which is soluble in NH4OH.
Ques. Which is the compound responsible for the formation of Prussian blue colour in the detection of nitrogen? (2 marks)
Ans. In the detection of nitrogen, the Prussian blue colour indicates the presence of a compound known as ferric ferrocyanide.
Ques. When does the Lassaigne test for the detection of nitrogen fails? (2 marks)
Ans. The lassaigne test for the detection of nitrogen fails when the compound lacks the presence of the element carbon in it.
Ques. It is always better to use dried sodium in the Lassaigne test. Why? (2 marks)
Ans. It is always recommended to use dried sodium in the Lassaigne test, because if moist sodium is kept under a kerosene lamp for heating, it may cause an explosion.
Ques. What is the reason behind the alkaline nature of sodium fusion extract? (2 marks)
Ans. The organic compound is heated with sodium metal at first. Then it is extracted with the help of water. When unreacted metals combine with water, it always forms an alkaline nature.
Ques. Organic compounds are always mixed with sodium, while preparing Lassaigne extract. Give a reason. (2 marks)
Ans. While preparing Lassaigne extract, organic compounds are always mixed with sodium. Because, when these compounds are heated, they always form sodium salts. The aqueous solutions of these are then used in the detection of these elements
Ques. Why is sodium always used in the Lassaigne test? (2 marks)
Ans. Sodium is always used in the Lassaigne test because it is a reactive metal, its melting point is low compared to other elements and its compounds are soluble in water as well.
Ques. While storing, it is advised to keep sodium metal in kerosene. What is the reason behind this? (2 marks)
Ans. Sodium is a reactive metal which always undergoes reaction when coming in contact with oxygen and moisture. When keeping it in kerosene, the exposure of sodium with oxygen and moisture can be prevented and hence the un necessary chemical reactions.
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