In an organic compound, phosphorus is estimated as

A pre-weighed amount of organic compound is heated with concentrated nitric acid ($HN{O}_3$) in a tube. 

Phosphorus present in the compound gets converted into phosphoric acid. 

Now the content of tube are heated with magnesia mixture ${\mathrm{MgSO}}_4+{\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{NH}}_4\mathrm{OH}$, 

so that magnesium ammonium phosphate ${\mathrm{MgNH}}_4{\mathrm{PO}}_4$ is formed which is collected, washed, 

dried and ignited to get magnesium pyrophosphate ${\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7$, which is then weighed.

$\underset{\underset{\mathrm{acid}}{\mathrm{Phosphoric}}}{{\mathrm{H}}_3{\mathrm{PO}}_4} \stackrel{\mathrm{Magnesia} \mathrm{mixture}}{\to }\underset{\underset{\mathrm{phosphate} (\mathrm{white} \mathrm{ppt}.) }{\mathrm{Mgnesium} \mathrm{ammonium}}}{{\mathrm{MgNH}}_4{\mathrm{PO}}_4+3\mathrm{HCl}}$

$2{\mathrm{MgNH}}_4{\mathrm{PO}}_4\stackrel{\mathrm{\Delta }}{⟶}{\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7\downarrow +2{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}$

Knowing the weight of ${\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7$, percentage of phosphorus can be calculated.