I would suggest that you ...

I would suggest that you ensure that your suppliers provide chemicals that comply with the relevant standards for chemicals used within the water industry.

The UK standards can be found here: http://dwi.defra.gov.uk/drinking-water-products/approved-products/Annex2.pdf. They are usually based on ISO ? EN standards, so you should have equivalent versions translated to your local conditions; if not, then use the EN standards and request compliance with them from your suppliers.