# Fertilisers (Sampling and Analysis) Regulations (Northern Ireland) 1996, Paragraph 7: PROCEDURE


Extraction
7.1 Weigh to the nearest 0.001 g, 10 g of the prepared sample (5 g for potassium salts containing more than 50% of potassium oxide or 20 g for fertilisers containing less than 5% of potassium oxide) and place in a 600 ml beaker with approximately 400 ml of water. Bring to the boil and maintain heat for 30 minutes. Cool, transfer quantitatively into a 1 litre graduated flask, make up the volume, mix and filter into a dry receiver. Discard the first 50 ml of the filtrate.
Note:
If the filtrate is dark in colour, transfer by pipette, an aliquot portion containing at the most 100 mg of K2O and place in a 100 ml beaker, add bromine water and bring to the boil to eliminate any surplus bromine. After cooling transfer quantitatively to a 100 ml graduated flask, make up to the volume, filter and determine the potassium in an aliquot portion of the filtrate.
Determination
7.2 Transfer by pipette an aliquot portion of the filtrate containing 25-50 mg of potassium (see Table on page 70) into a 250 ml beaker; make up to 50 ml with water.
To remove interferences, add 10 ml of the EDTA solution (4.5), several drops of the phenolphthalein solution (4.4) and stir in, drop by drop, sodium hydroxide solution (4.3) until a red colour persists. Finally add a few more drops of sodium hydroxide to ensure an excess (usually 1 ml of sodium hydroxide is sufficient to neutralise the sample and ensure an excess).
Boil gently for 15 minutes to eliminate most of the ammonia. Add water to make the volume up to 60 ml.
Bring the solution to the boil, remove the beaker from the heat and add 10 ml formaldehyde (4.1). Add several drops of phenolphthalein solution (4.4) and, if necessary, more sodium hydroxide solution until a distinct red colour appears. Cover the beaker with a watch glass and place it on a steam bath for fifteen minutes.
Weighing the crucible
7.3 Dry the filter crucible (5.1) to constant weight in the oven at 120°C (5.2) (about 15 minutes). Allow the crucible to cool in a desiccator and weigh it.
Precipitation
7.4 Remove the beaker from the steam bath, stir in drop by drop 10 ml of the STPB solution (4.6). This addition should take about 2 minutes; allow to stand for at least 10 minutes before filtering.
7.5 Filtering and washing
Filter under vacuum into the weighed crucible, rinse the beaker with the wash liquid (4.7), wash the precipitate three times with the wash liquid (60 ml in all of the wash liquid) and twice with 5 to 10 ml of water.
Drying and weighing
7.6 Wipe the outside of the crucible with a filter paper and place in the oven (5.2) for one and a half hours at a temperature of 120°C. Allow the crucible to cool in a desiccator to ambient temperature and weigh rapidly.
Blank test
7.7 Carry out a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.
Control test
7.8 Carry out the determination on an aliquot portion of an aqueous solution of potassium chloride, containing at the most 40 mg of K2O.

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Source: https://www.legislation.gov.uk/nisr/1996/513/schedule/2/part/I/chapter/11./paragraph/7 (the official version)
Page: https://openlegislation.uk/fertilisers-sampling-and-analysis-regulations-northern-ireland-1996/schedule-2/part-I/chapter-11./paragraph-7
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.