The ISFET has two reference electrodes: an internal reference and an external reference, that give separate reference potentials to the ISFET and show separate pH values (pH Internal and pH External). After the corrections for temperature and salinity are applied, the values from the internal and external are similar, and let the user verify the validity of the sensor’s measurements.
The internal reference electrode inside the DuraFET® is immersed in a bath of saturated potassium chloride (KCl) gel and is physically separated from the environment. The KCl gel exposes the Ag/AgCl internal electrode to a relatively constant chloride concentration. The sensor can therefore measure pH regardless of environmental salinity. If accurate salinity and temperature data are not available, the internal cell is generally more accurate.
The external reference electrode has a Ag/AgCl reference electrode in direct contact with seawater. The potential of this electrode varies with pH and chloride concentration, so unless the chloride concentration is known, the external reference is not stable. To correct this, salinity can act as an approximation of chloride concentration. If accurate salinity data is available, it can be applied to the pH external data and significantly reduce measurement errors, and give the most accurate and stable pH data.