Estimation of Phosphorus -- Carius Method

Phosphorus is estimated by yet another variant of the Carius method: a known mass (w) of the phosphorus-containing organic compound is heated, sealed in a tube, with fuming nitric acid, again oxidising its carbon to CO2 and hydrogen to H2O as a side reaction, while its phosphorus is specifically oxidised to phosphoric acid. That phosphoric acid is then precipitated in one of two ways, giving two parallel calculation routes. In the main route, heating with further concentrated nitric acid and then adding ammonium molybdate solution precipitates the phosphorus as ammonium phosphomolybdate, (NH4)3PO4.12MoO3 (molar mass 1877 g/mol), which is filtered, washed, dried and weighed (mass x); since 1877 g of that precipitate contains 31 g of phosphorus, %P = (31/1877) x (x/w) x 100. In the alternative route, the phosphoric acid is instead precipitated by adding magnesia mixture (a solution of magnesium chloride, ammonium chloride and ammonia) as magnesium-ammonium phosphate, which is washed, dried and then ignited to convert it fully to magnesium pyrophosphate, Mg2P2O7 (molar mass 222 g/mol, weighe …