All Hanna solution bottles come with a pH Vs. temperature chart together with the production lot # and expiration date to guarantee maximum accuracy. Formulation Hanna pH buffers are made of potassium dihydrogen phosphate and disodium hydrogen phosphate and are standardized with a pH meter calibrated to NIST primary standard buffer solutions (NIST 186Ie & 186IIe and NIST 158 g S.R.M.).