Modeling of enzyme-potentiometric sensors involving acid- or base-forming reactions

Enzyme-potentiometric sensors in which the chemical conversion of the analyte leads to the formation of an acid and/or a base can often display complex response characteristics. In these sensors, the, electrochemically monitored species is usually the H(+) ion (pH-based sensors). However, in some cases the conjugate-ion of the H(+) (or OH(-)) ion of the acid (or base) produced-can also be monitored-a specific example being the urease-NH(4) (+) sensor. The response of both types of sensors is strongly affected by: (1) the degree of dissociation of the products and their transport properties in the enzymic film, (2) the amounts of pH-buffers present in the test solution, (3) the test solution's pH, and (4) the diffusion coefficients of the various species. In this article, a previously developed theoretical model for pH-based sensors-in which the differences in diffusivities of the various species were ignored-is generalized to accommodate for such differences, and extended to the latter of the above two types of sensors. It is shown that when the sensor operates under analyte diffusion-controlled conditions, the response of either type of sensor can be predicted by a simple algebraic equation which is independent of the actual kinetics of the enzymic reaction.