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
- 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
- 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
|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.