Electrochemical transducers are often used to monitor the enzymatic reaction by potentiometric or amperometric measurements of the product or cosubstrate concentration. When the enzymatic reaction results in a change in pH, a pH electrode can be used as a potentiometric transducer. A schematic representation of an enzyme biosensor based on pH electrode is given in Fig.1. The sensitive surface of the pH electrode is in contact with an enzymatic layer and is immersed in a solution containing the substrate. The substrate migrates toward the interior of the layer and is converted into reaction products, with proton H+ production or consumption, when it reacts with the immobilized enzyme.