Temperature and
pH measurement

Temperature plays a significant role on pH measurement. This is a well-know fact for most users of pH test equipment. However, the temperature affects not only your sensor but also your sample. All solutions will change their pH value with temperature. This is a result of the shifting of the chemical equilibrium of the components, mainly of dissociation.


Temperature compensation

The automatic temperature compensation (ATC) in a pH meter corrects for the change in mV per pH unit generated by the glass electrode as defined by the Nernst equation.

pH temperature relation

The pH change with temperature is not an error. It is the true pH of the solution at the new temperature. At the table above you can see typical pH values for solutions at different temperatures. The conclusion that we can draw is that the effect of temperature is greatest for highly basic solutions.

temperature and ph

Pitfall

A common mistake is when you take a sample from a process tank and make the pH measurement at the laboratory. You are probably not measuring at the same temperature (?), and this means that you do not have the same pH value.

How big the difference is between the two pH values, depends, of course, on your measured solution.

Solution temperature compensation

If you want to compare pH values measured at different temperatures you need to use a function called solution temperature compensation. Solution temperature compensation converts the pH at the measurement temperature to the pH at a reference temperature. The reference temperature is almost always 25°C.

Note:
Do not confuse solution temperature compensation with the automatic temperature compensation used to correct the pH electrode, see at the beginning of this page.


The change in pH with temperature is called the solution temperature coefficient (pH/°C). For example, if a solution has a temperature coefficient of -0.035 [pH/°C], the pH decreases 0.035 units for every °C increase in temperature. Usually the temperature coefficient must be determined empirically.

Summary

• The pH value of a solution depends on the temperature.
• A pH value without a temperature value is meaningless.
• Solution temperature compensation converts measured pH to the pH at 25°C.
• Only pH values taken at the same temperature or converted using solution temperature compensation can be compared to each other.


Remember

To achieve highest accuracy, calibrate and measure at the same temperature.


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pH temperature compensation 
The response of an ideal pH electrode is defined by the Nernst Equation: E = Eo - kT·pH However, the slope or Nernst factor (kT) will change when …

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temperature and ph



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