Estimation of Halogens and Sulphur

Principle: A known mass of the organic compound is heated with fuming nitric acid ( $\ce{HNO3}$ ) in the presence of silver nitrate ( $\ce{AgNO3}$ ) in a sealed Carius tube inside a furnace.

The carbon and hydrogen in the compound are oxidised to $\ce{CO2}$ and $\ce{H2O}$. The halogen forms the corresponding silver halide ( $\ce{AgX}$ ), which is insoluble and precipitates out:
$\ce{Org-X + AgNO3 ->[HNO3, \Delta] AgX v + \ other\ products}$

The precipitate is filtered, washed, dried, and weighed.

Calculation:
Let mass of compound = $m$ g; mass of $\ce{AgX}$ formed = $m_1$ g.

Since 1 mol of $\ce{AgX}$ contains 1 mol of halogen X:
$\% \text{X} = \frac{\text{atomic mass of X}}{\text{molar mass of AgX}} \times \frac{m_1}{m} \times 100$

Principle: The organic compound is heated in a Carius tube with sodium peroxide or fuming nitric acid. Sulphur in the compound is oxidised to sulphuric acid ( $\ce{H2SO4}$ ).

The $\ce{H2SO4}$ is then precipitated as barium sulphate ( $\ce{BaSO4}$ ) by adding excess barium chloride solution:
$\ce{H2SO4 + BaCl2 -> BaSO4 v + 2HCl}$

The precipitate is filtered, washed, dried, and weighed.

Calculation:
Let mass of compound = $m$ g; mass of $\ce{BaSO4}$ = $m_1$ g.

Molar mass of $\ce{BaSO4}$ = 137 + 32 + 64 = 233 g/mol; contains 32 g of S per mole.

$\% \text{S} = \frac{32}{233} \times \frac{m_1}{m} \times 100$