pH calibration is the process of using a pH standard solution in a pH indicator to memorize the pH measurement standard, which is indispensable for a pH indicator. There are individual differences in each pH electrode, _and if measured over a long period of time, the indicated value may deviate due to dirt _or wear on the pH electrode. In order to maintain the reliability of the indicated value, it is necessary to calibrate regularly not only at the start of use _and when replacing the pH electrode, but also during use.
The following 6 types of pH standard solutions are specified for calibration (JIS). Our instrument uses two-component calibration, a neutral phosphate pH standard solution with a pH of 6.86 _and a phthalate pH standard solution with a pH of 4.01. Since the pH value of each standard solution fluctuates depending on the water temperature, there is no problem if you are using a pH electrode with temperature compensation *, but if you are using a pH electrode without temperature compensation, consider the pH deviation due to water temperature. It is necessary to calibrate.

* With temperature compensation: A resistance temperature detector [Tohkemy: Pt100Ω] is built in, _and by measuring the water temperature at the same time as the pH, the deviation due to the water temperature is automatically corrected during pH calibration.

The pH calibration solution must be sealed in a high-quality hard glass _or polyethylene container _and stored in a cool and dark place. When using a product that has been stored for a long period of time, it is necessary to confirm that the pH value is the same by measuring _and comparing it with a new homogeneous solution. In addition, the calibration solution once used cannot be used repeatedly due to reasons such as interference due to electrode immersion _and absorption of carbon dioxide in the atmosphere, which lowers the pH value.

Name	pH value at 25 ℃
Oxalate pH standard solution	1.67
Phthalate pH standard solution	4.01
Neutral phosphate pH standard solution	6.86
Phosphate pH standard solution	7.41
Borate pH standard solution	9.18
Carbonate pH standard solution	10.02