When a combination style pH sensor is immersed into a aqueous liquid solution, a voltage potential develops at the outer gel layer of the glass membrane which forms a phase boundary between the glass membrane and the measured solution. This potential is dependent on the pH value of the measured solution and is therefore of primary interest. Unfortunately this potential cannot be measured individually as there are more phase boundaries in a pH measurement chain which all produce their individual voltage potentials. Only the resulting e.m.f. (electromotive force) of all single potentials added together is measurable and forms the mV value used to determine the actual pH measurement.
As can be seen from the picture to the right, there are six potentials which develop within the pH measurement chain, but only one potential – E1 – is dependent on the pH value of the liquid solution being measured. Ideally the potentials E2 to E6 should stay constant during the measurement time in order to enable the measurement of the variable potential of E1.
E2 is the asymmetry potential of the pH sensitive glass membrane. If the measurement and the reference electrode possess the same internal conducting system and if the sensor is immersed into a buffer solution having the same pH value as the internal buffer solution, the potential difference between the inside and the outside of the glass membrane should theoretically be 0 mV. In reality even a new pH sensor will show an asymmetry potential of a few millivolts. The asymmetry potential depends mainly on the different thickness of the gel layers and on the thickness of the glass membrane.
E3 is the potential which develops on the inner gel layer of the glass membrane and is dependent on the hydrogen ion concentration of the inner buffer solution. Since this buffer solution is sealed within the measurement electrode it does not change in value, therefore the potential E3 should remain constant at all times.
E4 and E5 are potentials which develop on the phase boundaries metal/buffer solution (measurement electrode) and metal/electrolyte (reference electrode). If both conductor systems are identical and the buffer solution and electrolyte have the same chloride ion activity, then E4 and E5 are identical and do not contribute to the total potential determined by the pH measurement device.
E6 is the diffusion potential of the reference liquid junction (diaphragm). This potential occurs at the boundary between two electrolytes, when both differ in concentration and composition. It is determined by the diffusion of ions having different polarity and different ionic mobility.
As stated earlier, potentials E2 to E6 should ideally be constant in order to determine E1. Since the individual potentials E2 to E6 are subject to certain errors, there is a resultant zero point error of the electrode assembly. This is why a zero point calibration is required before a pH measurement can commence and be repeated in regular time intervals during the measurement duration.