A solution with a set pH value of 7.00 - to be used for calibration of pH meters, monitors and controllers. We use our test probes alot and they are small and very sensitive pieces of equipment that do drift out of calibration rather often. We suggest that you check the calibration of your pH testing equipment at least once a week. A pH probe giving you false readings could cause drastic issues so maintain your equipment and it will be good to you not to mention you will get more life out it!