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Lassaigne’s test
This test is employed todetect the presence of nitrogen, sulphur, halogens, and phosphorous in anorganic compound. These elements are present in the covalent form in an organiccompound. These are converted into the ionic form by fusing the compound withsodium metal.
The cyanide, sulphide,and halide of sodium formed are extracted from the fused mass by boiling it indistilled water. The extract so obtained is called Lassaigne’s extract. ThisLassaigne’s extract is then tested for the presence of nitrogen, sulphur,halogens, and phosphorous.
(a) Test for nitrogen
Chemistry of the test
In the Lassaigne’s testfor nitrogen in an organic compound, the sodium fusion extract is boiled withiron (II) sulphate and then acidified with sulphuric acid. In the process,sodium cyanide first reacts with iron (II) sulphate and forms sodiumhexacyanoferrate (II). Then, on heating with sulphuric acid, some iron (II)gets oxidised to form iron (III) hexacyanoferrate (II), which is Prussian bluein colour. The chemical equations involved in the reaction can be representedas
(b) Test for sulphur
Chemistry of the test
In the Lassaigne’s testfor sulphur in an organic compound, the sodium fusion extract is acidified withacetic acid and then lead acetate is added to it. The precipitation of leadsulphide, which is black in colour, indicates the presence of sulphur in thecompound.
Chemistry of the test
The sodium fusionextract is treated with sodium nitroprusside. Appearance of violet colour alsoindicates the presence of sulphur in the compound.
If in an organiccompound, both nitrogen and sulphur are present, then instead of NaCN,formation of NaSCN takes place.
Na + C + N + S → NaSCN
This NaSCN (sodiumthiocyanate) gives a blood red colour. Prussian colour is not formed due to theabsence of free cyanide ions.
(c) Test for halogens
Chemistry of the test
In the Lassaigne’s testfor halogens in an organic compound, the sodium fusion extract is acidifiedwith nitric acid and then treated with silver nitrate.
If nitrogen and sulphurboth are present in the organic compound, then the Lassaigne’s extract isboiled to expel nitrogen and sulphur, which would otherwise interfere in thetest for halogens.