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pH Meter Calibration

A pH meter electrode needs to be calibrated regularly. It is recommended that you do this at least once a day before you start measuring. The calibration is necessary to adjust the slope and offset of an electrode to their true values for the measuring system in question.

Calibration Points for pH meters

Since an electrode is characterized by both its zero point and its slope, it is advisable to do a minimum of a two point calibration for reliable measurements and better precision.

When measurements are performed over a large range of pH values it is recommended that one takes at least 3 calibration points. Most pH meters can do 3–5 point calibrations. It is important to note that one should only measure samples within the chosen region of calibration.

How to successfully calibrate your phMeter

1. Switch on the pH meter and select the correct buffer group or buffer values for the calibration.
2. Set the meter to manual temperature correction if no temperature probe is attached.
3. Select the correct temperature for the buffers if no automatic temperature correction is done.
4. Prepare the buffer solutions intended for calibration by pouring a sufficient amount of the solutions into clean beakers.
5. Make sure that the buffer solutions are used in the correct order for the calibration unless the pH meter has auto-buffer recognition
6. Take the electrode out of its holder and visually inspect it to see if there are any obvious problems with the electrode. Make sure that you have opened the electrolyte filling hole to ensure that there is no pressure build up or reduction in the electrode and to ensure that theelectrolyte can slowly flow into the sample.
7. Rinse the electrode with distilled or deionized water.
8. Take the first buffer solution, stir gently and immerse the electrode.
9. Press the calibration (or equivalent) button on the pH meter.
10. Wait until the measurement is stable. METTLER TOLEDO instruments have automatic endpoint algorithms which freeze the measurement  automatically as soon as the value is stable.
11. Take the electrode out of the buffer solution and rinse it.
12. Take the second buffer solution, stir gently and immerse the electrode.
13. Press the calibration (or equivalent) button on the pH meter.
14. Wait until the measurement has reached an endpoint.
15. Take the electrode out of the buffer solution and rinse it.
16. For a third calibration point, repeat steps above and end the calibration procedure on the pH meter by pressing the appropriate button.
17. Take the electrode out of the buffer solution, rinse it and store it in its holder.

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Source of article: A Guide to pH measurement by Mettler Toledo