The Milwaukee Instruments Calibration Solution 7.01 is a must have solution for ensuring the proper use of electrodes which in turn, will give accurate pH readings. The solution is what you will keep your pH meter in as it needs to remain wet in between testing. For the freshwater hobbyist, it is recommended to use this 7.01 calibration solution. Calibrating a pH meter in RO or distilled water will damage the meter. This is why purchasing the right calibration solution is very important.