Third-point calibration of laboratory pH meters
The following questions were raised during the audit of the unit using the pH meter: The pH meter needs three points of calibration, and two points are not enough. When calibrating with 7.004.01, if a third point is needed, which buffer should be used, 9.21 or 10.01, 9.18, 12.46, 1.68, etc.? How to determine?
1. In fact, the third point of pH calibration mainly depends on your sample conditions. As you said, there are many types of calibration solutions ranging from pH 1.68 to 12.46. The appropriate calibration solution is determined based on the final pH range of the sample. What we commonly use are 4.00, 6.86, and 9.18. If your sample is more alkaline, you need 9.18, 10.01, and 12.46. The calibration sequence varies according to the conditions of different instruments. Some require calibration in sequence, while others do not. The instrument will automatically recognize it. You need to refer to the relevant instrument instruction manual.
2. No matter what kind of pH meter it is, the point pH=7 must be calibrated, and when calibrating at two points, the point pH=7 must be calibrated first. When doing calibration, start from 7.0. The selected standard solution is related to the pH value of the solution to be measured, so that the pH value of the solution can fall within the calibration pH range. Generally, two points can meet the requirements. If the requirements are very high, the third point will be considered. Some instruments can calibrate three points and have optional modes. You can use this mode directly. For some, two-point and two-point proofreading is generally used, that is, proofreading twice.
3. We usually use the calibration sequence of 7, 4, and 10. Calibrate acid first and then calibrate base.
