Can temperature compensation eliminate all errors caused by temperature?
It must be noted that the temperature compensation set on the pH meter only compensates for the slope term of the electrode (2.303RT/F). The standard potential of glass electrodes and liquid interface potential are also affected by temperature, which are not strictly linear with temperature. At the same time, the pH electrode also requires a certain amount of time to reach equilibrium at the new temperature. Therefore, whether it is manual temperature compensation or automatic temperature compensation, it is not very sufficient. According to the operational definition of pH measurement, in order to obtain precise measurement results, the sample solution and standard solution should be measured at the same and constant temperature, which is the principle of isothermal measurement. For pH measurements with general accuracy requirements, temperature compensation can be used when the temperature of the sample solution and the standard solution is different.
How much does temperature affect pH accuracy measurement?
For pH electrodes, temperature affects each pH value of 0.003 pH/℃. For example, a pH meter of 0.2 level is calibrated in a 30 ℃ pH buffer and then tested for a solution at 60 ℃ (assuming the pH range of the solution is between pH 6 and 8 and pH 7.00, with a difference of one pH unit). The maximum error in temperature effect is 30 × 0.003=0.09pH. If it is 3 pH units (within the range of pH 4-10), the maximum error is 0.27 pH, which shows that temperature has a significant impact on pH. Of course, we can also draw a conclusion from it that in order to reduce the error of temperature in pH measurement, we should pay attention to the following three points:
(1) Try to choose a buffer solution that is close to the pH value of the measured solution to calibrate the pH meter.
(2) Try to make the temperature of the calibration solution consistent or close to the temperature of the measured solution.
(3) A pH meter with temperature compensation should be selected.
PH meters with an accuracy higher than 0.1 pH have temperature compensation adjustment, while pH meters with a 0.2 level do not have temperature compensation. Some pH meters with a 0.2 level accuracy are also claimed to have a 0.1 level accuracy, but this is actually impossible. Some people have confused the concepts of resolution 0.1pH and accuracy 0.1pH. Even in one pH unit, the pH error at a distance of 60 ℃ is 0.003 × 60=0.18pH, therefore, a pH meter without temperature compensation has the highest accuracy of only 0.2pH.
