Design of enzyme‐pH electrodes

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Abstract
A theoretical framework useful for predicting the steady state response of enzyme‐pH electrodes is developed. The model takes into account that the catalytic activity of the enzymes as well as the degree of dissociation of the product acids and bases is strongly dependent upon the local pH, and that buffering salts present in solution facilitate the transport of H+ ions. It is shown that the electrode should operate under substrate diffusion‐limited conditions and the concentration of H+ ions should not have steep variations within the enzymic membrane. This condition can be fulfilled by adjusting the buffer concentration in the bulk solution. In the latter conditions the electrode response does not depend on the actual kinetics of the enzymic reaction, and can be predicted by an algebraic equation. The predictions are in excellent agreement with the experiments on penicillinase‐pH and urease‐pH electrodes.