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sodium fusion test
16
Lassaigne's Test
or Sodium fusion test
Structure of the presentation :
Prepared by :
Aiman akram
&
Lawa azad
What is the Lassaigne's test?
The sodium-fusion test forms an essential part of most qualitative organic analyses since it represents a rapid method for the detection of the
presence of various elcments in the substance tested. The elements usually tested for are nitrogen, sulfur, and the halogens-others being tested for usually only if their presence is suspected from other evidence.
Since its original use by Lassaigne in 1843
Lassaigne's test
Test for Nitrogen :
The extract is boiled with FeSO4 and acidified with concentrated H2SO4. The appearance on Prussian blue colour indicates the presence of nitrogen.
The following reactions occur,
Test for Nitrogen
Fe2+ + 6CN– → [Fe(CN)6]4-
Fe2+ + H+→ Fe3+ + e–
[Fe(CN)6]4- + 4Fe3+ → Fe4[Fe(CN)6].H2O
The acid helps in the oxidation of ferrous ions to ferric ions. The formation of ferriferous cyanide indicates the presence of nitrogen
To a small portion of the Lassaigne’s extract, add 2 ml of freshly prepared ferrous sulphate solution and heat. Now add 2-3 drops of ferric chloride solution and acidify with conc.
Appearance of a prussian blue coloration
Presence of Nitrogen in the compound.
Test for Sulphur :
To a small portion of Lassaigne's extract, add a few drops of sodium nitroprusside solution. Appearance of a purple colouration. Presence of sulphur in the compound.
The following reaction occurs:
Test for Sulphur
S2- + [Fe(CN)5NO]2- → [Fe(CN)5NOS]4-
Lead acetate Test
Acidify a small portion of Lassaigne's extract with a few drops of acetic acid and add a few drops of lead acetate solution. A black precipitate forms. A black precipitate appears. Presence of sulphur in the compound.
Test for Halogens :
The extract is acidified with HNO3 and then treated with AgNO3. A white precipitate soluble in NH4OH indicates the presence of Cl, a yellowish precipitate sparingly soluble in NH4OH indicates the presence of Br, and a yellow precipitate insoluble in NH4OH indicates the presence of I .
Test for Halogens
AgNO3 + NaX → AgX ↓ + NaNO3