The frequency that you should calibrate your pH meter depends on many factors such as the type of your test samples, performance of electrodes, and the requirement of the accuracy. For High-Accuracy measurements (≤ ±0.02pH), the meter should be calibrated before test every time; For general-accuracy Measurements (≥±0.1pH), once calibrated, the meter can be used for about a week or longer. In the following cases, the meter must be re-calibrated before use:
- The electrode hasn’t been used for a long time or the electrode is brand new.
- After measuring strong acid (pH<2) or strong base (pH>12) solutions.
- After measuring fluoride-containing solution and strong organic solution
- There is a big difference between the temperature of the test sample and the temperature of the buffer solution that is used in the last calibration.
And there is a simple way to quickly check if your pH meter needs a calibration at the moment. All you need to do is dip the pH probe into the calibraiton standard solutions such as pH 4.00 or pH 7.00, and see if the measurements have a big difference to the standard value. For most applications (if not for scientific research), if the difference is within 0.05 pH, for example the meter reads 6.97 in 7.00 solution or 4.03 in 4.00 solution, then it means the meter is in good condition and a calibration is not a must at the point. If the difference is over 0.05 pH, for example the meter reads 6.92 in 7.00 or 4.08 in 4.00, then a calibration is necessary and highly recommended before testing.