Question

Which method is used for the estimation of phosphorus in organic compounds?
A. Dumas’s method
B. Combustion method
C. Kjeldahl’s methods
D. Carius methods

Answer 1

The method in which a known mass of the organic compound is heated with fuming nitric acid $(HN{O_3})$ in a sealed tube which oxidizes the phosphorus to phosphoric acid and where $C$ is converted into $C{O_2}$ and $H$ into ${H_2}O$. The phosphoric acid is then precipitated by heating it with concentrated nitric acid $(HN{O_3})$

Complete answer: The method which is used for the estimation of phosphorus in organic compounds is the Carius method. In this method a known mass of the organic compound which contain phosphorus is heated with fuming nitric acid $(HN{O_3})$ in a sealed tube i.e. carius tube where $C$ is converted to $C{O_2}$ and $H$ into ${H_2}O$. The phosphorus present in the organic compound is oxidized to phosphoric acid $({H_3}P{O_4})$ which is then treated with concentrated $(HN{O_3})$and then adding ammonium molybdate $({\left( {N{H_4}} \right)_2}Mo{O_4})$. This results in the formation of precipitate which is ammonium phosphomolybdate $({\left( {N{H_4}} \right)_3}P{O_4}.12Mo{O_3})$.
The precipitate i.e. ammonium phosphomolybdate formed is filtered, washed, dried and then weighed. After this, the percentages of Phosphorus can be determined.
The following are the chemical reaction involves in this method
$C\, + \,2O \to \,C{O_2}$
$2H\, + \,O \to \,{H_2}O$
$2P\, + \,5O \to {P_2}{O_5}$
${P_2}{O_5}\, + \,{H_2}O \to 2{H_3}P{O_4}$
${H_3}P{O_4}\, + \,\,12{(N{H_4})_2}Mo{O_4}\, + \,\,21HN{O_3}\xrightarrow{{heat}}\,{(N{H_4})_3}P{O_4}.12Mo{O_3}\,\, + \,\,12N{H_4}N{O_3}\,\, + \,12{H_2}O$
Hence, it is explained that Carius method is used for the determination of phosphorous in an organic compound.
Also, there is an alternative method used for the estimation of phosphorous in organic compound. In this method, the phosphoric acid formed is treated with magnesia mixture. A Magnesia mixture contains$MgC{l_2}$, $N{H_4}Cl$ and ammonia$\left( {N{H_3}} \right)$. After this, the phosphoric acid is precipitated as magnesium – ammonium phosphate$(MgN{H_4}P{O_4})$. The precipitate is ignited to form magnesium pyrophosphate $(M{g_2}{P_2}{O_7})$ which is washed, dried and then weighed. By knowing the mass of precipitate and organic compound taken, the percentage of phosphorous can be calculated.
The reactions involved in this method are as follows:
$MgC{l_2}\,\, + \,\,N{H_4}Cl\,\, + \,{H_3}P{O_{4\,}}\,\xrightarrow{{}}\,MgN{H_4}P{O_4}\,\, + \,\,\,3HCl$
Then, ignition of magnesium – ammonium phosphate to form magnesium.
$2MgN{H_4}P{O_4}\,\,\xrightarrow{{}}M{g_2}{P_2}{O_7}\,\, + \,\,2N{H_3}\,\, + \,\,{H_2}O$
Hence, the correct answer is option (D).

Note:
Always remember that the Carius method is not used only for estimation of phosphorus. It is also used for estimation of sulphur and halogens. Also, Duma’s method and kjeldahl’s method are used for the estimation of nitrogen. Liebig’s combustion is used for the estimation of carbon and oxygen. Aluise’s method is used for the estimation of oxygen. These are some important methods which you should remember.