Applicable to sulphate contents between 1 and 4000 mg/kg.
https://www.en-standard.eu/bs-4258-3-1989-methods-of-test-for-phosphoric-acid-orthophosphoric-acid-for-industrial-use-methods-of-test-for-phosphoric-acid-orthophosphoric-acid-for-industrial-use-determination-of-sulphate-content/
