Calibrating a pH meter using buffers

A pH meter requires proper calibration in order to give accurate pH readings. The meter must accuratetely translate voltage measurements into pH measurements.

How a pH meter is calibrated to give accurate pH readings?

A pH meter is calibrated by immersing its electrode(s) into buffers (test solutions of known pH) and by adjusting the meter accordingly. Since pH measurements are affected by temperature, the temperature must remain constant during the time of the calibration. Modern pH meters have build-in thermometers and automatically correct their own pH measurements as the temperature changes.

What are the main steps for a generic  pH meter calibration?

The following procedures are used to calibrate pH meters (analytical chemistry):

·         A 2 or 3-point calibration : Two or three buffers solutions are used respectively. They are usually sufficient for initial calibration. After this initial calibration the meter can accurately measure pH values in between.

·         A 1-point calibration: A buffer with a pH close to the expected sample pH is used. The pH measurements are not as accurate as with the 2 or 3 point calibration.

Procedure for an 1-point calibration:

1. Place the pH buffer solution (normally pH = 4.01 solution) into a small beaker. Place a magnetic stirrer and a temperature probe into the buffer solution in the beaker.
2. Measure the temperature of the buffer solution if the pH meter does not have its own temperature probe. Adjust the temperature control of the pH meter according to the buffer’s measured temperature (remember pH measurements are temperature dependent).
3. Remove the electrode protective cap(s). Rinse the electrodes and the temperature probe with distilled water using a squeeze bottle.
4.  Dab dry the bottom of the glass bulb with a tissue paper. Do not wipe the glass bulb (scratches and static charges affect the electrode’s response).
5. Place the electrode(s) and temperature probe into the well stirred pH buffer solution. The porous frit must be covered with the buffer solution.
6. Adjust the slope/sensitivity control to read the true pH of the buffer solution (modern pH meters do this automatically).

Procedure for an 2-point calibration:

1. Place the pH buffer solution (normally pH = 7.0 solution) into a small beaker. Place a magnetic stirrer and a temperature probe into the buffer solution in the beaker.
2. Follow steps 2-6 of the 1-point calibration.
3. Follow steps 1-6 of the 1-point calibration using the pH = 4.01 buffer solution
4. Keep repeating steps 2 and 3 until practically no adjustments are required

Procedure for a 3-point calibration:

1. Follow steps 1-6 of the 2-point calibration procedure
2. Use a third pH buffer, whose pH value is as close as possible to the suspected pH of the sample, and follow the 1-point calibration.

The third point is used to confirm that the pH meter has been calibrated correctly.