Severinghaus electrode

The Severinghaus electrode is an electrode that measures carbon dioxide (CO2). It was developed by Dr. John W. Severinghaus and his technician A. Freeman Bradley in 1958.
It utilizes a CO2-sensitive glass electrode in a surrounding film of bicarbonate solution covered by a thin plastic carbon dioxide permeable membrane, but impermeable to water and electrolytic solutes. The carbon dioxide pressure of a sample gas or liquid equilibrates through the membrane and the glass electrode measures the resulting pH of the bicarbonate solution. 
Clark, galvanic, and paramagnetic electrodes measure oxygen.  Severinghaus electrode measures CO2.  Sanz electrode measures pH.