HANNA instruments` buffer solutions are prepared according to precise formulas and are standardized with a pH meter calibrated according to the NIST standards.The 20 mL sachets are single-dose whereas, for more demanding applications, you may choose between packages of 230 mL, 500 mL, 1 Liter or 1 Gallon (3.78 L).All pH 4 solutions show batch number, expiration date and the correlation table between pH and temperature. HI 5004, HI 6004, HI 7004L/C, HI 8004L/C and HI 70004C are supplied complete with certificate of analysis.This buffer value is widely used in water purification plants, in the food industry and where ever the pH is expected to be slightly acidic.