Iridium oxide as actuator material for the ISFET-based sensor-actuator system

Acid or base concentrations can be determined by performing an acid-base titration with Coulometrically generated OH- or H+ ions at a noble-metal actuator electrode in close proximity to the pH-sensitive gate of an ISFET. The ISFET is used as the indicator electrode to detect the equivalence point in the titration curve. The potential of the actuator electrode during the generation of the titrant is relatively high for the anodic water electrolysis (or relatively low for the cathodic reaction). Consequently other redox couples which are possibly present in the sample solution can interfere with the water electrolysis. This reduces the efficiency of the current to titrant generation on which this measurement relies. To overcome this problem, iridium oxide has been used as a new electroactive material for the actuator electrode. The reversible redox reaction in this metal oxide occurs at a favourable potential and is attended by the exclusive uptake or release of protons, making a titration possible. It is shown that a Coulometric titration in the presence of Cl¿ ions, formerly not possible with the noble-metal actuator electrode because of the redox interference, can now successfully be carried out with iridium oxide as the actuator material. Calculations show that the ISFET pH-sensor is well suited to determining accurately the equivalence point in the steep part of the titration curve, because of its short response time.