Estimation of Phosphorus

• Heating a known mass, (m) g, of the organic compound containing Phosphorus with fuming nitric acid in a hard glass tube known as Carius tube in a furnace.
• Phosphorus present in the compound is oxidized to Phosphoric acid.

Using ammonia and ammonium molybdate

• When ammonia and ammonium molybdate are added to it, phosphoric acid is precipitated as ammonium phosphomolybdate, (ce{(NH4)3 PO4 .12MoO3}), which is filtered, washed, dried and weighed.
• If mass of (ammonium phosphomolybdate) formed = (m_1), mass of (phosphorus) present in it = $$ frac{text {mass of phosphorus in one mole of ammonium phosphomolybdate, i.e. 31} }{text{molar mass of ammonium phosphomolybdate i.e. 1877}} times m_1 $$ Let it be, (m_2) g
• Mass % = ({m_2 over m} times 100)%

Using magnesia

• When magnesia mixture is added to it, phosphoric acid is precipitated as ( ce{(MgNH4PO4}), which yields magnesium pyrophosphate (ce{Mg2P2O7}) on ignition. It is filtered, washed, dried and weighed.
• If mass of (magnesium pyrophosphate) formed = (m_1), mass of (phosphorus) present in it = $$ frac{text {mass of phosphorus in one mole of magnesium pyrophosphate, i.e. 62} }{text{molar mass of magnesium pyrophosphate i.e. 222}} times m_1 $$ Let it be, (m_2) g
• Mass % = ({m_2 over m} times 100)%