This standard BS 5551-4.2.3:1987 Fertilizers. Chemical analysis. Determination of phosphorus. is classified in these ICS categories:
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65.080 Fertilizers
Specifies reagents, apparatus, procedure and reporting of results.
https://www.en-standard.eu/bs-5551-4-2-3-1987-fertilizers-chemical-analysis-determination-of-phosphorus-method-for-determination-of-phosphorus-content-quinoline-molybdophosphate-gravimetric-method/