This Bluelab HD7CAL250 250 ml pH 7.0 calibration solution is essential for the calibration of pH and testing the cleanliness of conductivity and temperature probes. Bluelab solutions are manufactured specifically to the calibration process and held to high laboratory standards. To ensure an accurate reading, pH measuring equipment must be cleaned and calibrated regularly. This solution must be stored with lids securely tightened, as evaporation will cause salt content in the solution to rise, decreasing the effectiveness of the solution. This calibration solution must also be kept in a cool, dry place away from direct sun and heat and away from children.