Error in oxygen measurements in open systems owing to oxygen consumption in unstirred layer

Abstract

Abstract When a steady‐state oxygen concentration is measured with a membrane‐covered probe in an open system, the oxygen consumption in the unstirred layer gives rise to an error of measurement whose seriousness depends on the kinetics of the oxygen‐consuming process. First‐order oxygen consumption in the sample causes a proportional reduction in the signal so that the calibration in curve remains linear. A zeroth‐order process causes the calibration curve to be offset from the origin, but it remains linear. Oxygen consumption according to the Michaelis–Menten equation causes the calibration curve to become nonlinear with the maximum deviation at the lower end of the scale. The error determines a lower limit for usefulness of membrane‐covered probes. Steady‐state kinetics at oxygen concentrations in the order of K M cannot be determined with a membrane‐covered probe for enzyes with K M for oxygen lower than 0.01μ M . In a dense culture of respiring microorganisms, no oxygen will reach the probe when the bulk concentration of oxygen is in the order of K M .