A Dipstick Sensor for Coulometric Acid-base Titrations

By performing an acid-base titration by coulometric generation of OHor H+ ions at an inert electrode in close proximity to the pH-sensitive gate of an ISFET, it is possible to determine the acid or base concentration of a solution using the ISFET as an indicator device for the equivalence point in the titration curve. Typical values for the titration time are 0.1 to 10 s for acid or base concentrations ranging from 0.5 X low3 to 10 X 10M3 mol/l. By placing the counter electrode for the supplied current on the backside of a small piece of printed-circuit board on which the sensor-actuator is attached, we obtain a dipstick-like structure. Introduction In many applications, the ISFET pH-sensitive device offers a good alternative for the accurate but vulnerable pH glass electrode. The high impedance of the glass electrode and its construction are properties that make some applications difficult. For instance, in on-line monitoring of the blood pH during surgery, the small ISFET has already proven its advantage: new information becomes available for the surgeon, which cannot be obtained otherwise [ 11. Another new mode of operation comes forward by using the small, planar and fast ISFET pH-sensor together with an actuator electrode 121. By performing a coulometric acid-base titration in a submicrolitre volume in the proximity of the sensor, it is possible to determine the acid or base concentration of a solution. In solutions of well-defined composition, the pH can be calculated from this measurement. If the nature of the protolyte is unknown, this relationship is more complicated. However, for some applications, such as in the food industry, it is desirable to obtain the total acid concentration instead of the pH. *Present address: Valle University, Department of Electrical Engineering, A.A. 25360, Cali, Colombia. 0250-6874/89/$3.50 0 Elsevier Sequoia/Printed in The Netherlands