Laboratory instruments - Third point calibration of pH meters
The following issues were raised during the audit of the pH meter user unit: the pH meter requires three points of calibration, and two points are not enough. If the correction is done using 7.004.01, which other buffer should be used, such as 9.21 buffer or 10.01, 9.18, 12.46, 1.68, etc? How to determine?
1. In fact, the amount of pH correction at the third point mainly depends on your sample situation. As you mentioned, there are many calibration solutions available from pH 1.68 to 12.46, and the appropriate calibration solution is chosen based on the final pH range of the sample. We commonly use 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 depending on the situation of different instruments. Some require calibration in sequence, while others do not. The instrument will automatically recognize it and please refer to the relevant instrument user manual.
2. No matter what type of pH meter it is, the point of pH=7 must be corrected, and the point of pH=7 must be corrected first when calibrating at two points. Starting 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 calibrated pH range. Generally, two points are sufficient to meet the requirements, and the third point is only considered if the requirements are high. Some instruments can calibrate three points and have a mode to choose from, which can be used directly. For some that are not available, it is generally done through two point and two point proofreading, which means proofreading twice.
3. We usually use a calibration sequence of 7, 4, and 10. First calibrate the acidity and then calibrate the alkalinity.
