# Fertilisers (Sampling and Analysis) Regulations (Northern Ireland) 1996, Paragraph 6: PROCEDURE


Preparation of the solution for titration
6.1 Place an aliquot portion of the solution containing not less than 20 mg Cu in a 500 ml Erlenmeyer flask.
Drive off any excess oxygen present by boiling briefly. Make up to volume of about 100 ml water. Add 5 ml of nitric acid (4.1), bring to boiling and allow to boil for about half a minute.
Remove the Erlenmeyer flask from the heating apparatus, add about 3 g of urea (4.2) and resume boiling for about half a minute.
Remove from the heating apparatus and add 200 ml of cold water. Where necessary, cool the contents of the Erlenmeyer flask to ambient temperature.
Gradually add ammonium hydroxide solution (4.4) until the solution becomes blue, then add 1 ml in excess.
Add 50 ml of ammonium bifluoride solution (4.3) and mix.
Add 10 g of potassium iodide (4.6) and allow it to dissolve.
Titration of the solution
6.2 Place the Erlenmeyer flask on a magnetic stirrer. Insert the rod into the Erlenmeyer flask and adjust the stirrer to the desired speed.
Using a burette, add standard sodium thiosulfate solution (4.5) until the brown colour of the iodine released from the solution becomes less intense.
Add 10 ml of the starch solution (4.8).
Continue to titrate with the sodium thiosulfate solution (4.5) until the purple colour has almost disappeared.
Add 20 ml of the potassium thiocyanate solution (4.7) and continue titration until the violet blue colour has completely disappeared.
Note the volume of thiosulfate solution employed.

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Source: https://www.legislation.gov.uk/nisr/1996/513/schedule/2/part/I/chapter/26g./paragraph/6 (the official version)
Page: https://openlegislation.uk/fertilisers-sampling-and-analysis-regulations-northern-ireland-1996/schedule-2/part-I/chapter-26g./paragraph-6
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.