During the audit of PH meter users, the following questions were raised: PH meter needs three-point correction, but two points are not enough. For the calibration with 7.004.01, if the third point is needed, which buffer should I use, such as 9.21 or 10.01,9.18,12.46,1.68? How can we be sure?

1. Actually, the third point correction of pH mainly depends on your sample. As you said, there are many kinds of calibration solutions from PH 1.68 to 12.46, and the appropriate calibration solution is determined according to the final pH range of the sample. We usually use 4.00, 6.86 and 9.18. If your sample is closer to alkaline, we need 9.18, 10.01 and 12.46. The calibration sequence is also different according to the situation of different instruments. Some require sequential calibration, while others do not. The instruments will be automatically identified, so please refer to the operating instructions of relevant instruments.

2. No matter what kind of pH meter, the point of pH=7 must be corrected, and the point of pH=7 must be corrected first when two points are corrected. The calibration starts from 7.0, and the selected standard solution is related to the PH value of the solution to be determined, so that the PH value of the solution can fall within the corrected PH range. Generally, two points can meet the requirements, and if the requirements are very high, the third point will be considered. Some instruments can correct three points, and there are modes to choose from, so you can use this mode directly. Some of them don't. Generally, two-point proofreading is adopted, that is, proofreading twice.

3. We usually use the calibration sequence of 7, 4 and 10. Check the acid first and then the alkali.