Paragraph 6 PROCEDURE

Pipette an aliquot portion of one of the extraction solutions indicated at (2) containing between 20 and 100 mg of S or 50 and 250 mg of SO₃.

• Place this aliquot portion in a beaker of suitable capacity. Add 20 ml of dilute hydrochloric acid (4.1). Make up to about 300 ml with water.

Bring the solution to the boil. Add, drop by drop, about 20 ml of the hot barium chloride solution (4.2) while stirring the solution vigorously. Boil for a few minutes.

• Place the beaker, covered with a watch glass, in a boiling water bath (5.2) for an hour. Then leave standing hot (~60°C) until the supernatant liquor is clear. Decant the clear solution through a slow filtration ash-free filter. Wash the precipitate several times with hot water. Continue to wash the precipitate on the filter until the filtrate is chloride free. This can be checked by using silver nitrate solution (4.3).

Place the filter paper and precipitate in a crucible (5.1) previously weighed to the nearest 0.1 mg. Dry in the oven (5.3) and ash at approximately 800°C for half an hour (5.4). Allow to cool in a desiccator and weigh to within 0.1 mg.