Set of pH buffers 4, 7 and 9 for the calibration of the pH meter. Each satchel make 250 ml buffer solution that will be stable for at least 6 months. Buffers are supplied as a set so you get the 4, 7 and 9 when you order one set. Keep the solutions in a cool place and keep bottle closed when storing. You can purchase distilled water at a local store to make up the buffers.