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Detection of Phosphorus

The organic compound is fused with sodium peroxide (Na2O2). Phosphorus present in the organic compound gets converted into sodium phosphate by this treatment. The fused mass is extracted with water. The aqueous extract is heated with concentrated nitric acid and ammonium molybdate solution. A yellow precipitate of ammonium phosphomolybdate (NH3)3[PMO12O40] indicates the presence of phosphorus in the organic compound.

Detection of Sulphur

The organic compound is fused with sodium. The sulphur present in the organic compound is converted into sodium sulphide. Thus, Lassaigne's solution contains sodium sulphide.
2Na    +    S     Na2S
     from organic
  1. Lead Acetate Test for Sulphur
    The sodium extract is acidified with acetic acid and lead acetate is added. A black precipitate of lead sulphide (PbS) is formed if sulphur is present in the organic compound.
Na2S + Pb(CH3COO)2  PbS + 2CH3COONa
  1. Nitroprusside Test for Sulphur
    A few drops of a solution of sodium nitroprusside are added to the sodium extract. A violet or pink colour indicates the presence of sulphur in the organic compound.
Na2S   +   Na2[Fe(CN)5NO]     Na4[Fe(CN)5NOS]
              sodium nitroprusside                violet
When both nitrogen and sulphur are present, then fusing the compound with sodium forms NaCNS (sodium thiocyanate, also called sodium sulphocyanide). The presence of sulphur and nitrogen can be confirmed by adding a few drops of ferric chloride to Lassaigne's solution. A blood-red colour due to ferric sulphocyanide confirms the presence of both sulphur and nitrogen.
FeCl3 + 3NaCNS   3NaCl     +   Fe(CNS)3

Detection of oxygen

There is no test that can directly indicate the presence of oxygen in an organic compound. The presence of oxygen is indicated indirectly by the presence of groups containing oxygen, e.g. groups such as -OH, -CHO, > C=O, -COOH, -NO2, etc.

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