The zero point of a combination pH sensor is the pH value at which the entire electrode assembly potential Etotal is equal to 0 mV. Theoretically the zero point of the pH measurement chain is determined by the internal buffer solution of the measurement electrode, which under normal circumstances has the value of pH 7. If the pH value of the measured medium also equals pH 7, then the potential difference of the pH measuring chain should be 0 mV.
In practice however this is seldom the case, because Etotal is the resultant of the sum potentials E1 to E6. Each potential reacts differently to temperature changes and to the composition of a measured liquid solution. Therefore it is difficult, if not impossible, to produce a pH sensor with an accurately defined and reproducible zero point.
The zero point tolerance, as stipulated by the German Industrial Standard (DIN), may vary within –30 mV and +30 mV. Many manufacturers of pH probes deliberately set their electrode assembly zero point (pH 7 = 0 mV) slightly lower (approximately pH 6.8) since the zero point tends to drift upwards during the ageing process of the electrode assembly.
The repeatability of a pH sensor assembly (the uncertainty factor) is seldom stated by sensor manufacturers. Experience has shown that the repeatability of a pH electrode assembly seldom exceeds +/- 0.02 pH (approx. 1.16 mV).
The exact zero point deviation of a pH sensor has to be established by the user, prior to a pH measurement, and must then be compensated for by a zero adjustment at the pH meter/transmitter. Microprocessor based pH meters adjust the zero point of an electrode assembly automatically during the calibration procedure.
Causes of Zero Point Drift
The zero point check and adjustment should be repeated at specific time intervals during the measurement process, as the zero point tends to drift due to the following reasons:
- Penetration of the measured liquid solution into the reference electrolyte via the liquid junction. The ingress of liquid will either poison or dilute the reference electrolyte. Both actions will change the chloride ion activity of the electrolyte, resulting in a change of the reference potential.
- A change of the measurement electrode buffer solution. Due to the exposure to high temperature the glass membrane of the measuring electrode releases alkali hydroxide into the inner buffer solution which gradually increases its pH value. This resulting pH drift issue is well documented in the sterilization process commonly used in bioprocesses.
- Increase in electrical resistance due to corrosion of the contact metals used in the sensor electrical connector or related mating cable.