AMCO AEPA-1 Standards are safe, EPA approved and non-toxic. No mixing, no diluting. Accuracy within 1%. Stable, does not settle out. Composed of submicron solid polymer beads suspended un ultra-pure water. Guaranteed one-year shelf life. Approved by EPA, Standard Methods, ASTM, ASBC and EBC. Traceable to NIST 1690 and 1691. Non Ratio.