pH calibration procedure
A pH calibration is the procedure of adjusting the pH meter by measuring solutions of known pH values.
Why you need to calibrate
The characteristic of a pH electrode will change with time due to electrode coating and aging. And even a pH electrode would be stable over time, pH electrodes cannot be produced with identical characteristics.
In practice the response of a real pH sensor does not exactly follow the Nernst equation.
This difference between the theoretical and actual behavior of a pH
electrode must be compensated for. A calibration is required to match
the pH meter to the current characteristics of the used pH sensor.
How to perform a calibration
The pH calibration procedure is performed by measuring a known, well defined solution, a so-called pH buffer.
Two-point calibration
The most common pH electrode calibration method is the two-point calibration.
During a two-point calibration, the microprocessor based pH meter determines the real slope and offset error
for the actual pH electrode. This information is then used to adjust
the mV/pH-equation of the pH meter to match the characteristic of the
electrode in use. The two-point calibration is also called bracketing
calibration, since the two calibration points should bracket the range
of values that will be measured.
Multi-point calibration
To
achieve the best possible accuracy, the calibration should cover the
range of the desired measurement values. If the readings go beyond the
calibrated range, the pH meter assumes linearity and simply extrapolates
the value to be displayed. The true value may be slightly different.
More
advanced pH meters will let the user calibrate at three, four or five
and even higher numbers of pH values. A multi-point calibration mean, in
comparison to a two-point calibration, that you can calibrate your pH
tester on both sides of the zero point (pH 7.00). This will expand your
pH measurement range without the need of recalibrating.
Calibration interval
pH calibration must be repeated periodically. The time interval between calibrations depends on a number of factors, read more.
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One point calibration 
Sometimes a one point calibration is only needed. Typically, a one-point calibration is made by placing the electrode in pH = 7.00 buffer and adjusting …
