Explain the determination of sulphur present in coal sample and write its significance.

Principle: the sulphur in coal is converted to sulphate and weighed as BaSO₄.

Procedure

1. A known mass (W g) of coal is burnt in a bomb calorimeter in O₂, which converts S to SO₂/SO₃. These dissolve in the water in the bomb as H₂SO₄ (sulphate).
2. The bomb washings are collected, acidified with dilute HCl and heated.
3. Excess BaCl₂ solution is added, which precipitates BaSO₄: \( \mathrm{H_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2HCl} \)
4. The precipitate is filtered, washed free of chloride, dried, ignited and weighed.

Calculation: 233 g of BaSO₄ contains 32 g of S, so

\( \%S = \dfrac{\text{wt. of BaSO}_4 \times 32 \times 100}{233 \times W} \)

Significance

• Sulphur adds a little to the calorific value, but it is undesirable overall.
• It burns to SO₂ and SO₃, which cause air pollution and acid rain and corrode equipment, boilers and chimneys.
• Coal with high sulphur is unsuitable for metallurgical coke, because S makes steel brittle.
• Good coal should therefore have low sulphur, under about 0.5–1%.