If the customer is following Standard Methods (5910 B), a phosphate buffer is mentioned. How is this buffer made?
Phosphate buffer for Organic Constituents (UV-254)
Dissolve 4.08 dried anhydrous KH2PO4 and 2.84 dried anyhydrous Na2HPO4 into 800 mL organic-free water. Verify that the pH is 7.0 and dilute to 1 L with organic-free water. Store in a brown glass bottle at 4 degrees C. Prepare fresh weekly or more frequently if microbial growth is observed.