Hanna's 4.01 and 7.01 BUFFER and CALIBRATION solutions are made from established formulas and are standardized with a pH meter that is calibrated to NBS/NIST primary standard buffer solutions. The 20 mL sachets are ideal for infrequent calibrations. All Hanna solution bottles come with a pH Vs. temperature chart together with the production lot # and expiration date to guarantee maximum accuracy. Hanna's CLEANING and STORAGE solutions are made from the purest chemicals specifically for Hanna Instruments. You should clean your pH electrode after each use to prevent clogging and to maintain accuracy. Hnna storage solutions are prepared with high grade Potassium Chloride and Potassium Hydrogen Phthalate by titration with NIST 2201 standardized Silver Nitrate and NIST 84j standardized Sodium Hydroxide Solution, respectively. Storing your electrode in storing solution will extend it's life and accuracy.