This document describes Lassaigne's test to detect nitrogen, sulfur, and halogens in organic compounds. Sodium fusion is used to convert elements in organic samples to soluble sodium salts. Tests are then performed on the fusion extract solution. Nitrogen is detected using ferrous sulfate and ferric chloride to form a Prussian blue or green precipitate/color. Sulfur is detected using sodium nitroprusside to form a violet color or lead acetate to form a black precipitate. Halogens like chlorine, bromine, and iodine are detected using silver nitrate to form characteristic precipitate colors soluble in ammonium hydroxide. The document provides procedures and expected observations for each elemental

1. Organic and Inorganic Analysis​ 1
©Department of Chemistry, BUET
Course
126:
Exp
No-04
​
Name of the experiment:
Detection of Elements: Lassaigne's Test

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©Department of Chemistry, BUET
Course
126:
Exp
No-04
​
Objectives​
⮚The objective of this experiment is to detect the presence of nitrogen, sulphur, and
halogen (chlorine, bromine and iodine) in organic compounds by Lassaigne's test.
⮚ To use reagent grade chemicals to identify elements in organic compounds.
⮚ To use simple apparatus to identify elements in organic samples.
⮚ To understand the reaction and its mechanism.
⮚ To observe change of colour for different reactions related with to this analysis.
This experiment is designed for this course to make the student capable-

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©Department of Chemistry, BUET
Course
126:
Exp
No-04
Theory​
⮚In this experiment, organic compounds are fused with metallic sodium to convert the
elements (N, S, X) into water soluble sodium salt. As we know, organic compounds are non-
polar and insoluble in water so, to make it polar and to form soluble ion, sodium fusion method
is used. Then prepared ionic solution (ions) react with respective reagents to detect elements
such as nitrogen, sulphur and halogens.
⮚There are several ways to detect element in organic compounds such as spectroscopic
analysis, elemental analysis etc. But these techniques are expensive and time consuming. On
the other hand, by using Lassaigne test we can identify nitrogen, sulphur and halogens present
in organic compounds easily. This is inexpensive and can detect element within a short time.

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©Department of Chemistry, BUET
Course
126:
Exp
No-04
Detection of Nitrogen​
Detection of Nitrogen:
If nitrogen is present in the compound, the Lassaigne's extract would contain sodium cyanide
formed during fusion. Sodium cyanide is converted to sodium ferrocyanide on treating with
ferrous sulphate. On further treating it with ferric chloride, a prussian blue complex,
ferricferrocyanide is formed.

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©Department of Chemistry, BUET
Course
126:
Exp
No-04
Reactions ​
Reactions for detection of nitrogen
Prussian Blue Colour
Prussian Blue PPT
and Green PPT

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©Department of Chemistry, BUET
Course
126:
Exp
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Detection of Sulphur​
Detection of Sulphur
1. Sodium nitroprusside test
During the preparation of Lassaigne's extract, sulphur from the organic compound reacts
with sodium to form sodium sulphide. It gives a purple colour with sodium nitroprusside
due to the formation of sodium thionitroprusside.
Sodium thionitroprusside (Violet Colour)

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©Department of Chemistry, BUET
Course
126:
Exp
No-04
Test of Sulphur
2. Lead acetate test
Sodium sulphide formed during the preparation of Lassaigne's
extract reacts with lead acetate to yield lead sulphide as black
precipitate.
Black ppt of Lead Sulphide

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©Department of Chemistry, BUET
Course
126:
Exp
No-04
Test of Chlorine​
Chlorine:
During the preparation of Lassaigne's extract, chlorine form the organic compound reacts with
sodium to form sodium chloride. Sodium chloride gives a white precipitate of silver chloride with
silver nitrate solution. The precipitate is soluble in ammonium hydroxide.
White ppt of Silver Chloride

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Bromine
b. Bromine
Sodium bromide formed during the preparation of Lassaigne's
extract reacts with silver nitrate to form pale yellow precipitate
of silver bromide, which is sparingly soluble in ammonium
hydroxide.
Pale yellow ppt of Silver Bromide

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Iodine
c. Iodine
Sodium iodide formed during the preparation of sodium fusion
extract reacts with silver nitrate solution to form yellow
precipitate of silver iodide, which is insoluble in ammonium
hydroxide.
Yellow ppt of Silver Iodide

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Preparation of stock solution ​
Procedure:
⮚ At first, take a small oil free sodium metal in a dry fusion tube.
⮚ Add very little amount of supplied organic sample to the sodium metal into the fusion tube.
⮚ Then burn this fusion tube with sodium and sample properly up to make it red color.
⮚After burning properly, put it in a mortal with small amount of water and make it powder (in
solution) with pestle.
⮚ Then filter this solution by using a filter paper.
⮚ Keep this filtrate as stock solution.
This stock solution will be used for detecting nitrogen, sulfur and halogen.

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©Department of Chemistry, BUET
Course
222:
Exp
No-02
Preparation of stock solution
Sodium metal under
liquid paraffin
Experiment :
(Video)

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Nitrogen​
a. Test for nitrogen (N):
⮚ Take 1 or 2 mL of stock solution in a test tube
⮚ Add 2-3 drops of freshly prepared saturated solution of FeSO4 or directly you can add solid
FeSO4.
⮚ Boil the mixture for about 1 minute and 2-3 drops of FeCl3 solution.
⮚ Add dilute HCl to render the solution acidic until any precipitate forms.

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©Department of Chemistry, BUET
Course
222:
Exp
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Test of Nitrogen​
Observation: Prussian blue precipitate or green colour solution was observed.
Inference: Positive (Nitrogen present). If no blue ppt or green solution was found, Negative
(Nitrogen absent)
Positive Positive

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Sulphur​
b) Test for sulphur (S):
i) Lead acetate test:
Procedure:
⮚ Acidify a portion of the stock solution by adding acetic acid dropwise
⮚ Add 2-3 drops of saturated lead acetate solution.
Black ppt of PbS
Observation: Black precipitate was observed.
Inference: Positive (sulphur present).

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Sulphur​
ii) Observation: Violet ppt was
found,
Inference: Sulphur present
Violet colour of sodium nitropruside
ii) Sodium nitropruside test:
Procedure:
⮚Take 1-2 mL of the stock solution
⮚ Add 2-3 drops of freshly prepared sodium nitropruside solution.

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Combined Test of Sulphur and Nitrogen ​
c) Combined test of N and S :
Procedure:
⮚ Take 1 or 2 mL of stock solution in a test tube and boil the solution for about 1 minute
⮚ Add 2-3 drops of FeCl3 solution.
Observation: Reddish brown/Red precipitate was observed.
Inference: Positive (Nitrogen & sulphur present).
Reddish brown/Red
precipitate of ferric
thiocyanate

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Halogen
c) Test for halogen (Cl, Br, I):
i. Silver nitrate test:
⮚ Acidify a portion of the stock solution with dilute nitric acid, boil until it has been reduced to
half of its original volume.
⮚ Cool and add 1% silver nitrate (AgNO3) solution.
⮚ A heavy white precipitate indicates the presence of chloride, pale yellow indicates presence
bromide and yellow indicates presence of iodide in the supplied sample.

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Experiment

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Test of Halogen​
ii) Further confirmation by ammonia solution:
After forming precipitate, add dilute ammonia solution to the precipitate (ppt). If the ppt is
white and readily soluble in ammonia solution, chloride is present. If it is pale yellow and
sparingly soluble in conc. ammonia solution, bromide is present. If it is yellow and insoluble in
ammonia solution the iodide is present.
AgBr = Sparingly soluble in NH4OH
AgI = Insoluble in NH4OH

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©Department of Chemistry, BUET
Course
222:
Exp
No-04
Result for unknown sample
Observation:
Sample A: A-----colour ppt was formed
Sample B: A-----colour ppt was formed
Sample B: A-----colour ppt was formed
Inference:
1. Sample A might be contain-
2. Sample B might be contain-
3. Sample C might be contain-
Results