Temperature:

A change in temperature may affect on pH measurements, because change in temperature affects the activities of the ions as well as the liquid-junction potentials. Thus, it is advised to calibrate the electrode at the temperature of the test solution.

Calibration procedures:

Buffer solution cannot be prepared more accurately than ± 0.01 pH units. Therefore, we cannot calibrate the electrode better than this.

Equipment related: These errors may due to the power fluctuations, parallax errors in reading analogue scales and many more.