pH temperature relationship
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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.
Slope error
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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