Gas-Sensing Membrane Electrode

A number of membrane electrodes respond to the concentration of a dissolved gas. The basic design of a gas-sensing electrode consists of a thin membrane that separates the sample from an inner solution containing an ion-selective electrode. The membrane is permeable to the gaseous analyte, but impermeable to nonvolatile components in the sample’s matrix. The gaseous analyte passes through the membrane where it reacts with the inner solution, producing a species whose concentration is monitored by the ion-selective electrode. For example, in a CO₂ electrode, shown here, CO₂ diffuses across the membrane where it reacts in the inner solution to produce H₃O^₊. The change in the activity of H₃O₊ in the inner solution is monitored with a pH electrode, and is proportional to the concentration of CO₂ in the sample.