Effect of diffusion on measuring oxygen consumption in oxidation reactions with polarographic membrane-covered probe

Abstract

Little is known about the effect of oxygen diffusion on the accuracy of solution oxygen measurements. The effect of O₂ diffusion on the measurement of dissolved O₂ with a polarographic Clark-type membrane-covered electrode was studied during enzymatic and abiotic oxidation of catechol catalyzed by tyrosinase or δ-MnO₂. The depletion of dissolved O₂ during the oxidation of catechol was followed as a function of time in (a) air- or O₂-saturated systems in a vessel completely filled with solution and sealed and (b) air- or O₂-saturated systems with a headspace (the same volume of solution but in a larger vessel). The depletion of dissolved O₂ was also followed after flushing the headspace with N₂. The presence of the headspace had a substantial effect on the measurement of the O₂ consumed by the reaction in air- or O₂-saturated systems. The diffusion of O₂ from the headspace to the solution resulted in an underestimation of the O₂ consumed. In contrast, flushing the headspace with N₂ triggered the diffusion of O₂ from the solution to the headspace, resulting in a very significant overestimation of the O₂ consumed by the reaction. The data indicate that O₂ diffusion-induced problems in solution oxygen measurements by oxygen sensors merit close attention.