In an organic compound, phosphorus is estimated as (AFMC 2010) (a) Mg P O (b) Mg PO (c) H PO (d) P O Ans. (a) Sol. In Carius method, the weighed organic compound (containing phosphorus) is heated with conc. HNO followed by MgCl in the presence of NH Cl and NH OH . By this magnesium ammonium phosphate, MgNH PO is formed which on ignition gives magnesium pyrophosphate Mg P O . From the amount of magnesium pyrophosphate, the percentage of phosphorus is calculated as 2P Mg P O . → wt. of Mg P O % of P wt. of substance taken x x