DETECTION OF NITROGEN BY LASSAIGNE TEST

Experiment No. 5: DETECTION OF NITROGEN BY LASSAIGNE TEST

OBJECTIVE:

TO DETECT THE FOREIGN ELEMENT PRESENT IN GIVEN ORGANIC COMPOUND

APPARATUS REQUIRED

Fusion tube
Tong
Burner
Porcelain basin
Beaker
Test tube

CHEMICALS REQUIRED

Sodium metal
Given organic compound
NaOH
FeSO₄
FeCl₃
HCl
Lead acetate
Sodium nitroprusside
AgNO₃

THEORY

Organic compounds are covalent compounds and are generally insoluble in water, carrying out molecular reactions that are very slow. So, the given organic compound is fused with a highly reactive metal, such as sodium to give a water-soluble ionic compound, which carries out ionic reactions which are fast. This test is known as the Lassaigne test. Nitrogen, sulphur and halogens are known as hetero elements or foreign elements.

Foreign elements are those elements other than carbon, hydrogen and oxygen which form the basic constituent of organic compound. Foreign elements are covalently bonded with the carbon of organic compounds.

If nitrogen is present, it forms sodium cyanide

Na + C + N → NaCN → Na⁺ + CN^–

If sulphur is present, it forms sodium sulphide

Na + S → Na₂S → 2Na⁺ + S^—

If halogen is present, it forms sodium halide

Na + Cl₂ → 2NaCl → Na⁺ + Cl^–

Sometimes, when both nitrogen and sulphur are present in the same organic compound then sodium sulphocyanide is formed.

Na + C + N + S → NaCNS → Na⁺ + CNS^–

PROCEDURE

A freshly cut pea-shaped sodium metal was taken out from the kerosene oil and dried by pressing it between the folds of filter paper. The metal is placed in a fusion tube with a tong and was heated gently until the sodium melted to a shiny globule. Then a pinch of organic compound was added to it and heated strongly until the lower end of the tube became red hot. The hot tube was then plunged into the porcelain basin containing 10 ml of water. The particles were grinded with the pestle. The solution was foiled and filtered. The filtrate is called sodium extract solution (Lassaigne solution).

OBSERVATION TABLE

Experiment	Observation	Inference
1. Detection of nitrogen: 2 ml of sodium extract solution was taken in a clean and dry test tube and a few drops of NaOH solution was added. Then freshly prepared FeSO₄ solution was added and boiled and cooled. Finally, FeCl₃ solution was added and acidified with conc. HCl.	Prussian blue colour was obtained	Presence of nitrogen
2. Detection of sulphur: i. 2 ml of sodium extract solution was taken in a clean and dry test tube and a few drops of acetic acid solution were added followed by a few drops of lead acetate.	No any ppt. was obtained	Absence of sulphur
ii. 2 ml of sodium extract solution was taken in a clean and dry test tube and few drops of sodium nitroprusside solution was added.	No any ppt. was obtained	Absence of sulphur
3. Detection of nitrogen and sulphur together: 2 ml of sodium extract solution was taken in a clean and dry test tube and a few drops of ferric chloride solution was added.	No any ppt. was obtained	Absence of nitrogen and sulphur together
4. Detection of halogen: 2 ml of sodium extract solution was taken in a clean and dry test tube and acidified with nitric acid and a few drops of AgNO₃ solution was added to it.	No any ppt. was obtained	Absence of halogen

Reactions involved

Na + C + N \rightarrow NaCN\\ FeSO_{4} + NaOH \rightarrow \underset{Dirty\ green\ ppt}{Fe(OH)_{2} }+ Na_{2}SO_{4}\\ Fe(OH)_{2} + NaCN \rightarrow Fe(CN)_{2} + NaOH\\ Fe(CN)_{2} + 4NaCN \rightarrow Na_{4}[Fe(CN)_{6}]\\ 3Na_{4}[Fe(CN)_{6}] + 4FeCl_{3} \rightarrow \underset{\underset{(Prussian\ blue)}{Ferric\ ferrocyanide}}{Fe_{4}[Fe(CN)_{6}]_{3}} + 12NaCl

RESULT

The given sample of organic compound contains nitrogen as foreign element.

CONCLUSION

We can detect the foreign element present in given organic compound by Lassaigne test.

PRECAUTION

Large amount of sodium metal was not taken.
The contents of basin was boiled only after the completion of unreacted sodium.
If dark colour was formed, the compound is not decomposed completely and the process was repeated.

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