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.