A pH calibration is the procedure of adjusting the pH meter by measuring solutions of known pH values.
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.
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.
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.
pH calibration must be repeated periodically. The time interval between calibrations depends on a number of factors, read more.
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