Estimation of Phosphorous

Carius (Second Method) for Phosphorus

• A known mass of organic compound is heated with fuming HNO₃ in a Carius tube → phosphorus is converted to H₃PO₄.
• Magnesium mixture (MgCl₂ + NH₄Cl) is added → gives precipitate of magnesium ammonium phosphate (MgNH₄PO₄) → on heating gives magnesium pyrophosphate (Mg₂P₂O₇) → weighed.

\[\%P=\frac{\text{Atomic mass of P}}{\text{Molecular mass of Mg}_2P_2O_7}\times\frac{\text{Mass of Mg}_2P_2O_7\times100}{\text{Mass of compound}}\]

\[=\frac{62}{222}\times\frac{\text{Mass of Mg}_2\mathrm{P}_2\mathrm{O}_2\quad100}{\text{Mass of compound}}\]

\[=\frac{27.93\times\text{wt of Mg}_2\mathrm{P}_2\mathrm{O}_7\mathrm{ppt}}{w}\]

Molecular mass of Mg₂P₂O₇ = 222 g; 2 atoms of P = 62 g.