Aqua Master Calibration Solution pH 4.01 500ml (Case of 8)Description Aqua Master Calibration Solution pH 4. 01 500ml (Case of 8): This high precision calibration solution has a pH of 4. 01. It is traceable to the NIST standard, ensuring the highest level of accuracy for your pH measurements. Storage and Usage: To maintain its accuracy, this solution should be stored between 59 77F. Please make sure to close the bottle immediately after use and avoid returning any liquid back into the bottle after removal.