This method is for the extraction of water-soluble sulfur contained in fertilisers, in various forms.
This method applies to all fertilisers for which a declaration of the water-soluble sulfur trioxide is required.
Soluble forms of sulfur are dissolved in cold water and converted into sulfate by oxidation with hydrogen peroxide in an alkaline medium.
Diluted hydrochloric acid:
One volume of hydrochloric acid (p = 1.18 g/ml) plus one volume of water.
4.2 Sodium hydroxide solution containing at least 30% NaOH (p = 1.33g/ml).
500 ml graduated Stohmann flask.
Rotary shaker, 30 to 40 turns per minute.
Electric hot plate with adjustable temperature.
See method 1.
Test sample.
(a)Where fertilisers contain a maximum of 3% of sulfur (S) i.e. 7.5% SO3, together with a maximum of 4% of calcium (Ca) i.e. 5.6% CaO, weigh out 5 g of fertiliser to within 1 mg.
(b)Where fertilisers contain more than 3% of sulfur (S) together with more than 4% of calcium (Ca), weigh out 1 g of fertiliser to within 1 mg.
Place the test sample in a 500 ml flask (5.1).
Preparation of the solution.
Add approximately 400 ml of water. Stopper. Shake on the rotary shaker (5.2) for 30 minutes. Make up to volume with water and mix. Pass through a dry filter into a dry container. Reject the first portion of the filtrate. Stopper if the solution is not to be used immediately.
Oxidation of the aliquot portion to be analysed.
Take an aliquot of the extraction solution not exceeding 50 ml and, if possible, containing between 20 and 100 mg of sulfur (S).
Make up the volume to 50 ml with water, if necessary. Add 3 ml of sodium hydroxide solution (4.2) and 2 ml of hydrogen peroxide solution (4.3). Cover with a watch glass and boil gently for one hour on the hot plate (5.3). Keep adding 1 ml increments of hydrogen peroxide solution for as long as the reaction continues (maximum quantity 5 ml).
Then leave to cool. Remove the watch glass and wash the underside into the beaker. Add approximately 20 ml of dilute hydrochloric acid (4.1). Make up to approximately 300 ml with water.
Determine the content of sulfates on the whole of the oxidised solution in accordance with method 23.