Multiple choice

In the Carius method for quantitative elemental analysis, phosphorus in an organic compound is estimated in the form of ____________________.

  1. magnesia mixture

  2. ammonium phosphomolybdate

  3. phosphoric acid

  4. magnesium ammonium phosphate

  5. magnesium pyrophosphate
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E Correct answer
Explanation

Phosphorus present in an organic compound is estimated by Carius method. A known mass of the organic compound is heated with fuming nitric acid in a Carius tube for about 2 hours when P present in the compound is oxidised to phosphoric acid. It is precipitated as magnesium ammonium phosphate (Mg(NH4) PO4) by adding magnesia mixture (MgCl2, NH4Cl and a little aqueous NH3). The precipitate is filtered, washed, dried and then ignited to give magnesium pyrophosphate (Mg2P2O7). It is then weighed. From the masses of organic compound and magnesium pyrophosphate obtained, the percentage of P in the organic compound can be calculated.

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