Hanna\’s buffer solutions are made from established formulas and are standardized with a pH meter calibrated to NBS/NIST primary standard buffer solutions. The 20 ml sachets are ideal for infrequent calibrations while for heavy-duty applications, you can choose from the popular 230 ml, 1 liter, or 1 gallon containers. Now the pH 4 buffers carry lot # andexpiry date as well as pH/C chart on every bottle. HI 7004L/C and HI 8004L/C are also supplied with an analysis certificate. Hanna pH 4 buffers contain potassium hydrogen phthalate. They are standardized using a pH meter calibrated by primary standard buffer solutions (NIST 186 Ie, NIST 186 IIe and NIST 185g S.R.M.). The pH value is certified traceable to NIST 185g S.R.M.