How to perform a
pH meter calibration
A proper performed pH meter calibration is essential to get an accurate and repeatable measurement.
pH meters will perform a calibration by guiding the user through a
sequence of calibration steps. This feature is of great help and will
reduce the possibility for user error during the calibration. However, a
calibration procedure will be highly dependent on the manufacturer of
the pH meter, so for best results, the procedure described in the
operating manual of the pH meter should be used.
Buffers - your reference values
A pH meter calibration is performed by measuring a series of reference standards, so-called pH buffers that have known and accurate pH values at different temperatures.
Proper cleaning of the electrode prior to calibration is essential. If you have stored your electrode, make sure it is ready for use.
The main steps for a generic pH calibration procedure:
- Turn the pH meter on and allow adequate time to warm up (check the operating manual).
- Select two pH buffers that bracket the expected sample pH. The first buffer should be pH 7.00 (zero point adjustment) and the second buffer should be near the expected sample pH (pH 4 or pH 9.21). If you will measure both acidic and basic samples, and only perform one pH calibration, you should use a multi-point calibration.
- Before starting calibration, be sure the sensor and the buffer solution are at the same temperature. If not, allow time for temperature equilibration.
- Pour the necessary amount of buffer solutions into individual glass beakers. Buffer solutions will remain stable in a glass beaker for a maximum of 2 hours. Note: To minimize the risk of contamination, calibrations should never be carried out in the storage container.
Close the buffer containers promptly to avoid carbon dioxide absorption. Do not pour used buffer back into the storage container. Discard it.
- Place the electrode into the first buffer. When the reading is stable, set the pH meter to the pH value of the first buffer at the measured temperature. Most modern pH meters have an "auto-read" function for early detection of a stabilized reading.
It is not strictly necessary to stir samples for pH measurement. However, if the buffer solutions are stirred, then the sample should also be stirred in the same way.
- Between buffers, rinse the electrode with distilled water and then with the next buffer.
If you, for some reason, don't want to rinse the pH electrode with the next buffer; Rinse the electrode with distilled water and gently blot it dry with a lint-free tissue. Avoid rubbing or wiping the electrode bulb.
- Repeat step 3 for the next buffer.
- When the pH meter calibration is done, rinse the electrode and place into the sample and make your pH measurement.
Is your pH electrode working properly?
Some pH meters also give recommendations regarding the used pH electrode. The electrode may need to be cleaned or replaced.
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