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1. This method is for the determination of sulfur present in fertiliser extracts in the form of sulfates.
2. This method applies to the determination of sulfates present in the extractions performed in methods 15, 16, 17 and 18.
3. Gravimetric determination as barium sulfate.
4.—4.1 One volume of hydrochloric acid (ρ = 1.18 g/ml) and one volume of water.
4.2 Barium chloride solution BaCl2.2H2O: 122 grams per litre.
4.3 Silver nitrate solution: 5 grams per litre.
5.—5.1 Crucibles.
5.2 Hot water bath.
5.3 Drying oven set at 105°C ± 1°C.
5.4 Electric furnace set at 800°C ± 50°C.
6.—6.1 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.
6.2 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).
6.3 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.
7. One mg of barium sulfate corresponds to 0.137 mg of S or to 0.343 mg of SO3.The percentage S content of the fertiliser is obtained as follows:
where:
w = the mass of the barium sulfate precipitate in milligrams,
v1 = the volume of the extraction solution in millilitres,
v2 = the aliquot volume in millilitres,
m = the mass of the test sample in grams.
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