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


Add 25 ml of 6 M nitric acid (4.3) and 2.5 g of sodium bismuthate (4.5) to the 400 ml beaker containing the test solution. Stir vigorously for three minutes on the magnetic stirrer (5.2).
Add 50 ml of 0.3 M nitric acid (4.4) and stir again. Filter in vacuo through a crucible (5.1), the bottom of which is covered with Keiselguhr (4.6). Wash the crucible several times with the 0.3 M nitric acid (4.4) until a colourless filtrate is obtained.
Transfer the filtrate and the washing solution into a 500 ml beaker. Mix and add 25 ml of 0.15 M ferrous sulfate solution (4.8). If the filtrate turns yellow after the addition of ferrous sulfate, add 3 ml of 15 M orthophosphoric acid (4.7).
Using a burette, titrate the excess ferrous sulfate with 0.02 M potassium permanganate solution (4.9) until the mixture turns pink, the colour remaining stable for one minute. Perform a blank test under the same conditions, omitting only the test sample.
Note:
The oxidized solution must not come into contact with rubber.

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Source: https://www.legislation.gov.uk/nisr/1996/513/schedule/2/part/I/chapter/26i./paragraph/7 (the official version)
Page: https://openlegislation.uk/fertilisers-sampling-and-analysis-regulations-northern-ireland-1996/schedule-2/part-I/chapter-26i./paragraph-7
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.