Answer:
Estimation of Halogens
The estimation of
halogens (Cl, Br or I) inorganic compounds is usually done by Carius method.
Principle:
The method is based on the fact that when an organic compound containing
halogen (Cl, Br or I) is heated in a sealed tube (Carius tube) with
fuming nitric acid in presence of excess of silver nitrate, silver halide is
formed. From the mass of silver halide formed, the percentage of the halogen can
be calculated.
For
example:
\[\text{Percentage}\,\text{chlorine}=\frac{35.5}{143.5}\times
\frac{\text{Mass}\,\text{of}\,AgCl}{\text{Mass}\,\text{of}\,\text{organic}\,\text{compound}}\times
100\]
Estimation
of sulphur:
When an
organic compound containing sulphur is heated with fuming nitric acid, sulphur
is quantitatively oxidised to sulphuric acid. This is precipitated as barium
sulphate by adding barium chloride solution. From the amount of barium
sulphate, percentage of sulphur can be calculated.
Percentage sulphur
=
\[\frac{32}{232}\times
\frac{Mass\,of\,BaS{{O}_{4}}}{Mass\,of\,compound}\times 100\]
Estimation
of Phosphorus
It is also estimated
by Carius method. The organic compound containing phosphorus is heated with
fuming nitric acid. Phosphorus is oxidised to phosphoric acid. It is
precipitated as magnesium ammonium phosphate,\[MgN{{H}_{4}}P{{O}_{4}}\], by the
addition of magnesia mixture
\[(MgS{{O}_{4}}+N{{H}_{4}}OH+N{{H}_{4}}Cl)\].
The magnesium ammonium phosphate is washed, dried and ignited when it is
converted to magnesium pyrophosphate \[(M{{g}_{2}}{{P}_{2}}{{O}_{7}})\]
\[P\xrightarrow[\Delta
]{HN{{O}_{3}}}{{H}_{2}}P{{O}_{4}}\]
\[\underset{Phosphoric\,acid}{\mathop{{{H}_{2}}P{{O}_{4}}}}\,\xrightarrow[(M{{g}^{2+}}+N{{H}_{4}}OH)]{Magnesia\,mixture}\underset{\begin{smallmatrix}
Magnesium\,ammonium \\
phosphate
\end{smallmatrix}}{\mathop{MgN{{H}_{4}}P{{O}_{4}}}}\,\]
\[2MgN{{H}_{4}}P{{O}_{4}}\xrightarrow{Heat}M{{g}_{2}}{{P}_{2}}{{O}_{7}}+2N{{H}_{3}}+{{H}_{2}}O\]From
the mass of magnesium pyrophosphate, the percentage of phosphorus can be
calculated.
Percentage of phosphorus \[=\frac{62}{222}\times
\frac{\text{Mass of }M{{g}_{2}}{{P}_{2}}{{O}_{7}}}{\text{Mass of
compound}}\times 100\]
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