About oxidation-reduction potential : Quinhydrone standard solution
The ORP measurement is not calibrated like the pH measurement, but the potential may fluctuate due to contamination of the metal electrode, etc. The quinhydrone standard solution is a stable solution with a constant redox potential that is used to determine if the performance of the metal electrode is normal. The redox potentials of quinhydrone solutions based on the respective reference electrodes are shown in Table 2.