The accuracy of a pH measurement depends on many factors of which may or may not be controllable. In order that the non-controlled factors do not influence the pH measurement, the controllable factors must be monitored and kept as small as possible.
The calibration of the pH sensor is one of the most important factors. To ensure high measurement accuracy the user must follow correct calibration procedures, use new and accurate buffer solutions, and wait long enough for stable pH values to accept the calibration. The time interval between re-calibration should be as short as practically possible. Remember: no pH measurement can be more accurate than its calibrated accuracy.
It has been stated repeatedly that temperature influences the result of the pH measurement. In order to achieve high measurement accuracy the temperature difference between the pH buffer solutions used for calibration and the measured liquid medium must be kept as low as possible. The temperature of the measured medium must be determined accurately or, if possible, controlled at a constant value in order to perform an effective temperature compensation.
Ideally, the reference electrode must be kept pressurized by approximately 100 kPa (1 bar) above the process pressure to keep the reference electrolyte flowing and prevent the ingress of measured solution into the reference electrode body. The liquid junction must be kept clean at all times. The correct matching of the liquid junction to the application can drastically improve the pH measurement accuracy.
The glass membrane of the measurement electrode must also be kept clean at all times. Coating on the glass membrane will reduce the surface area directly impacting measurement accuracy and in extreme cases make a pH measurement impossible.
Accuracy expectations should not exceed practical, achievable results. Realistic accuracies of between +/- 0.03 pH units and +/- 0.05 pH units can be achieved with a new, well hydrated pH sensor connected to a properly grounded microprocessor-based pH transmitter or meter with a high impedance input circuit of at least 1012 ohms. In the laboratory under controlled conditions, accuracy of +/- 0.02 pH units are achievable.