The pH versus temperature characteristic of a pH sensor

The pH versus temperature relation of a pH sensor is described, according to Nernst equation, with the following equation:

E = E0 - kT·pH

The slope factor of the equation, "kT", will change when temperature changes. And that mean the millivolt (mV) output of the pH electrode will change with temperature.

pH versus temperature graphs

ph versus temperature

The figure above shows the effect on the electrode signal at various temperatures. The isothermal point of a pH electrode is defined as the intersection point of different temperature lines, see the figure. The isopotential point, or the zero point, is then point where electrode output is 0.0 mV.

For an ideal pH electrode, this would represent an isothermal intersection at the zero point (0 mV at pH 7) for different temperatures.

Important practical significance

When performing pH measurements it's important that the pH electrode has reached thermal equilibrium with the measured media in order to obtain a stable measurement value. And this is more important the farther the pH gets away from 7.

When a pH electrode is transferred between solutions of different temperatures, or the temperature of the measured solution changes rapidly during measurement, you will notice a slow response or drift from the pH meter. The resulting drift will continue until the sample and the pH electrode have reached thermal equilibrium.

Temperature Compensation

temperature compensation

To be able to get accurate pH values, you or at least the pH meter needs to know the actual temperature and compensate for the temperature effect according to Nernst equation.

automatic temperature compensation

Temperature compensation is contained within all pH meters, and it may be either manual or automatic. With manual compensation, you have to make a separate temperature measurement and manual enter the temperature value in the pH meter.

Modern pH meters have automatic temperature compensation (ATC). The signal from a separate temperature sensor is fed into the pH meter, so that it can accurately determine the pH value at that temperature.

Uncompensated temperature error

uncompensated temperature error

If an uncompensated pH meter is set to 25°C but is measure a sample of 20°C and 8.0 pH, the pH meter would measure a voltage of -58.2 mV but would expect -59.2 mV. The value on the display would be 7.98 pH. An error of 0.02 pH.

For a measurement of 4.00 pH at 75°C (probably close to a typical worst case), an uncompensated pH meter would read about 4.50 pH. An error of 0.5 pH.

As you can see, when working near 7.0 pH, temperature compensation is not a significant factor. However, the farther the pH gets away from 7, the larger the error introduced.

Conclusion of the pH versus temperature relationship:

To make an accurate pH measurement you need to use temperature compensation. Exception is if you measure a pH of about 7.

Real pH electrodes differ from an ideal pH electrode

What we have discussed so far is the temperature behavior of an ideal pH electrode. Unfortunately, reality is seldom perfect. Real-world pH electrodes are not ideal. . .

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