Choice of polarisation voltage in polarographic oxygen tension measurement: influence ofpCO2 and pH of the medium

The influence of variations in the pH and the pCO₂ of the medium surrounding an oxygen electrode on the polarograms of two types of electrodes was investigated. Most measurements were made with polystyrene-covered monopolar needle electrodes with a 50 μm Pt cathode as used for subcutaneous pO₂ measurements in the foetal scalp during labour. Some of the measurements were made with conventional Clark electrodes with 3 or 25 μm PTFE membranes. It appeared that in an oxygen-free solution the polarisation voltage at the beginning of the hydrogen wave shifted to a less negative value when the pH of the solution was decreased by adding HCl or by increasing the pCO₂. Lowering the pH diminished the flat part of the polarogram. In a fluid with an appreciably high pO₂ the influence of the pH of the solution when changed by adding HCl was negligible, whereas there was a considerable influence of the pC₂ on the polarisation voltage at which the hydrogen wave starts. This may be explained by OH⁻ production through reduction of O₂, which increases pH at the electrode surface. This effect may be undone by CO₂ diffusing rapidly through the membrane covering the Pt surface, but not by the slow influx of H⁺, for which the membrane is much less permeable. Knowledge of these phenomena may be important in choosing the right level of polarisation voltage so that the oxygen electrodes are independent of changes in pCO₂.