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

Little is known about the effect of oxygen diffusion on the accuracy of sol ution oxygen measurements. The effect of O-2 diffusion on the measurement o f dissolved O-2 With a polarographic Clark-type membrane-covered electrode was studied during enzymatic and abiotic oxidation of catechol catalyzed by tyrosinase or delta-MnO2. The depletion of dissolved O-2 during the oxidat ion of catechol was followed as a function of time in (a) air- or O-2-satur ated systems in a vessel completely filled with solution and sealed and (b) air- or O-2-saturated systems with a headspace (the same volume of solutio n but in a larger vessel). The depletion of dissolved O-2 was also followed after flushing the headspace with N-2. The presence of the headspace had a substantial effect on the measurement of the O-2 consumed by the reaction in air- or O-2-saturated systems. The diffusion of O-2 from the headspace t o the solution resulted in an underestimation of the O-2 consumed. In contr ast, flushing the headspace with N-2 triggered the diffusion of O-2 from th e solution to the headspace, resulting in a very significant overestimation of the O-2 consumed by the reaction. The data indicate that O-2 diffusion- induced problems in solution oxygen measurements by oxygen sensors merit cl ose attention.