pH

In a laboratory, pH meters are generally calibrated once in the morning and once in the afternoon. In the field, calibrate for each set of samples or whenever field conditions change. Calibrate to two points that bracket the expected sample pH and are at least 3 pH units apart using the following sequence:
* Ensure that the meter is on, the electrode is connected and all needed materials are present.
* Engage the instrument calibration mode.
* Remove the electrode from its storage solution and rinse it with de-ionized water.
* Blot the electrode dry with a paper towel, (do not wipe).
* Immerse the electrode in pH buffer 7.0 (constantly mixed by a stir plate).
* Allow the meter to stabilize, record the reading.
* Calibrate the meter to buffer 7.0.
* Remove electrode from the 7.0 buffer, rinse with de-ionized water and blot dry with paper towel.
* Immerse the electrode in a second pH buffer (4.0 or 10.0).
* While mixing constantly with stir plate, allow the meter to stabilize and then record the reading.
* Calibrate meter to second buffer value.
* Remove the electrode from the second buffer, rinse it with de-ionized water and blot dry with paper towel. ƒ Immerse the electrode in the check pH buffer (buffer not yet used).
* While mixing, allow the meter to stabilize and then record the reading.
* The reading of the check buffer must be within 0.1 pH unit of actual value, if not, the calibration must be repeated.
* Remove the electrode from the buffer, rinse it with de-ionized water and blot dry with paper towel.
* The pH meter is now calibrated.

Use sufficient sample volume to cover the electrode and give adequate clearance for a stir bar. Mix the sample constantly, but be cautious not to introduce CO2 by overly rapid mixing (no vortex). Record the pH value when the reading is stable. Some pH meters indicate when the reading is stable with a light or through a “chirp”. Record the sample temperature to the nearest degree Centigrade (º C). Repeat measurement on another sample aliquot until values differ by < 0.1 pH unit.

Be aware that if the reading is not in the range bracketed by the calibration buffers used, the meter will need to be recalibrated using another buffer. For example, if you calibrated to the 7.0 and the 10.0 buffers, and then checked the calibration with the 4.0, the sample reading must be between 7.0 and 10.0. If the sample reading were below 7.0, you would need to go back and recalibrate to the 7.0 and the 4.0 buffers, and check the calibration with the 10.0 buffer.

ƒ Equipment
* Do not store electrodes dry. Store in solution indicated by manufacturer.
* Maintain electrolyte in reference electrodes. Refill when volume is low.
* Uncover fill hole of the reference electrode (if so equipped) so that electrolyte can flow freely.
ƒ Buffers
* Record buffer date of purchase, date opened and expiration date.
* Purchase buffers in quantities that will be consumed within six months.
* Buffers should be used before their expiration date.
* Store buffers in the dark at room temperature.
* Discard buffers that are not clear or that show growth.
ƒ Samples Measurements
* Meter must be calibrated with two buffers that bracket the sample pH.
* Performance evaluations on externally supplied standards should be run at least once a year.