Twist the capsule, open and pour the granules in 100ml of DI (deionised) water, shake the solution so the granules dissolve and the solution is ready to use. Once made up the buffer should last approximately one week, before it's too weak to use. NIST certified. Tolerance margin of ± 0,02 if used with deionised water. Tests show the same results even if the volume is not totally accurate, no need to measure with an exact volumetric flask and a pipette. If to use as a side information it can be used with distilled water and even tap water. No expiring date