Coulometric Sensurs, the Application of a Sensor- Aci’uato3% System for Long-term Stability in Geemical S~s~~*

We present a new type of chemical transducer, the co~orn~~c sensor, This sensor is in fact an integrated sensor-actuator system that is able to measure the concentration of acids and bases by means of a coulometric titration. An ISFET is used as the sensor to monitor the pH changes induced by the actuator, which is a gold electrode that fits closely around the ISFET’s gate area. Coubmetry is an absolute method ztnd therefore the output of the new sensor is only determined by its d~e~ons and is not subject to changes in offset and sensitivity of the indicator electrode It is thus expected that the operation of the senscz will be stable for a long time, so that only a onetime calibration is needed. As a first example of this new class of chemical clucks, a carbon dioxide sensor. is presented. It is shown that the stabil= ity is some orders of magnitude better than that for a ‘classical’ potentiometric sensor. Direct potentiometric measurement of ion concentrations is a powerful method and is widely used. Because of the high speed of the direct method, it is very useful for continuous rno~~~. However, ~~ntiome~c methods require a rigid control over experimental variables such as temperature and ionic strength if accurate measurements are to be made. lf, for example, a 1 mV error is made in the reading of the electrode potential for the detection of a univalent ion, this will result in a 4% error in concentration, 86 can be seen from the logarithmic term in the Nernst equation. For divalent ions this error will even be doubled. It will be clear that any drift in the potential of the ion-sensitive elec’trode or in the potential of the reference *Based on a paper presented at the 2nd International Meeting on Chemical Sensors, Bordeaux, France, July 7 10,1986. 0250-6874/88/$3.60 @ Elsetier Sequoia/Printed in The Netherlands