Question

How will you estimate phosphorous in an organic compound?

Answer 1

Carius method:

Procedure:

A known mass of organic compound (wg) containing phosphorus is heated with fuming HNO₃ in a sealed tube where C is converted into CO₂ and H to H₂O. Phosphorous present in the organic compound is oxidised to phosphoric acid which is precipitated as ammonium phospho molybdate by heating with conc. HNO₃ and by adding ammonium molybdate.

H₃PO₄ + 12(NH₄)₂MoO₄ + 21 HNO₃ \(\overset{\Delta}{\rightarrow}\) (NH₄)PO₄.12 MoO₃ + 21 NH₄NO₃ + 12 H₂O

The precipitate of ammonium phospho molybdate is filtered, washed, dried and weighed.

Calculation:

Mass of organic compound = Wg

Mass of ammonium phospho molybdate = x g

Molar mass of ammonium phospho molybdate 1877 g

1877 g of ammonium phospho molybdate contains 31g of phosphorous

x g of ammonium phospho molybdate contain = \(\frac{31}{1877}\times\) x g of phosphorous

% of phosphorous = \(\big(\frac{31}{1877}\times\frac{x}{w}\times100\big)\%\) of phosphorous

In an alternate method, phosphoric acid is precipitated as magnesium-ammonium phosphate by adding magnesia mixture. The ppt. is washed dried and ignited to get magnesium pyrophosphate which is washed, dried and weighed.

Weight of magnesium pyrophosphate = y g

Molar mass of magnesium pyrophosphate = 222 g

222 g of magnesium pyrophosphate contains 62 g of P

y g of magnesium pyrophosphate contain = \(\frac{62}{222}\times\) y g of P

% of phosphorous = \(\big(\frac{31}{1877}\times\frac{y}{w}\times100\big)\%\)