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Product literature - Brochure |
| Historical development of the PCO2 Electrode In 1952, Stow described the direct reading PCO2 electrode. Early direct reading electrodes have a silver/silver chloride reference cell and a latex membrane separating the sample from a film of distilled water on the surface of a glass membrane. The carbon dioxide diffused through the membrane until equilibrium occurred. The research of Snell and Severinghaus, shortly thereafter, facilitated the rapid advancement of the PCO2 electrode. Theory of PCO2 Electrode operation The direct measurement of PCO2 using the PCO2 Electrode is an adaptation of a pH measurement. A combination of pH measuring and reference electrodes makes contact with a solution behind a gas permeable membrane. Carbon dioxide diffuses across the membrane in either direction in response to a partial pressure difference, equilibrating the inner electrolyte with the external gas pressure. Hydration of PCO2 in the electrolyte water produces carbonic acid causing a change in hydrogen ion activity:
The pH electrode senses the changein PCO2 concentration as a change in pH of the electrolyte and develops a voltage exponentially related to PCO2. Thus a ten-fold increase in PCO2 is nearly equivalent to a decrease of one pH unit. Description of the IDRONAUT PCO2 Electrode The PCO2 electrode consists of a measuring half-cell and a reference half-cell both part of one common or combination electrode. The measuring half-cell with the pH sensitive glass at the tip is sealed within the glass electrode’s body. Inside the electrode, the constant pH buffer and Ag/ AgCl electrode are located. The reference electrode (Ag/AgCl wire) is a band on the exterior of the electrode. The PCO2 electrode inserts into the membrane cap containing PCO2 electrolyte, allows the reference electrode to make electrical contact with the measuring electrode. The components of the electrode are described below. The outer silastic membrane The gas permeable outer membrane of the IDRONAUT PCO2 electrode is a 150 μ silastic film. This membrane acts as a selective barrier in the sense that it is permeable to gas (CO2) but not to ions. Thus, ions of the sample do not diffuse across the membrane to contribute to the pH change of the PCO2 measuring system. The silastic membrane represents a break-through in the improvement of the response time of the PCO2 systems. The inner nylon spacer Since diffusion rates are slow relative to the desired response time, the measuring pH glass electrode system must be placed as close as is practical to the gas permeable membrane. However, the PCO2 electrode requires a thin film of aqueous PCO2 electrolyte between the glass tip and the reference band of the electrode. A nylon mesh spacer covers the tip of the electrode and maintains a layer of electrolyte between the glass tip and the silastic membrane. Both the spacer and the membrane material with O-rings and membrane tools are supplied with the PCO2 membrane kit. |
