Real-world pH electrodes are not ideal and will have a pH temperature relationship that differs from the theoretically value.
For the ideal pH electrode the isothermal point, the intersection point of the different temperature lines, will coincide with the isopotential point (zero point). For real pH electrodes the isothermal point hardly ever coincides with the zero point of the electrode.
An ideal electrode at 25°C will produce 0.0
mV when placed in a solution with a pH of 7.00. But a real electrode
will have an actual reading which varies from 0 mV. This variation is
called the electrode’s offset error.
The developer of a high quality pH electrode really wants to bring the isothermal intersection and the zero point as close together as possible, since the nearer they are to pH 7 the smaller the error in the temperature compensation. The closer your pH electrode is an ideal electrode, the better it is.
How to decrease the offset error
The offset error increases with an increasing temperature difference between the calibration and the actual measuring. The error can be as high as 0.1 of a pH unit. The offset error cannot be eliminated by the automatic temperature compensation (ATC) feature in the pH meter. It can only be eliminated when the temperature of the calibration and the measured sample are identical.
As stated by Nernst equation a perfect electrode produces an output potential according to:
E = E0 - 0.592·pH (at T = 25 °C)
This mV/pH relation is the equation of a straight line with a slope factor of 0.592 V. Any variation from this ideal value is specified as the electrode’s span error or slope error. It is essential to compensate for the slope error during the calibration procedure.
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