This Bluelab HD4CAL500 calibration solution pH 4.0 500ml is essential for the calibration of pH and testing the cleanliness of the conductivity and temperature probe. The Bluelab solutions are manufactured specifically to the calibration process and referenced to high class laboratory standards. To insure an accurate reading, pH measuring equipment must be cleaned and calibrated regularly. pH readings are calibrated to pH7.0 first then to pH4.0. Bluelab HD4CAL500 solution must be stored with lids securely tightened. Evaporation will cause salt content in the solution to rise, decreasing the validity of the solution if no lid is replaced after use. This solution must also be in a cool place away from direct sun and heat, and away from children.