Clark electrode



The Clark electrode is an electrode that measures oxygen on a catalytic platinum surface using the reaction:


 * O2 + 2 e− + 2 H2O &rarr; H2O2 + 2 OH−

History
Leland Clark (Professor of Chemistry, Antioch College, Yellow Springs, Ohio, and Fels Research Institute, Yellow Springs, Ohio) had developed the first bubble oxygenator for use in cardiac surgery. However, when he came to publish his results, his article was refused by the editor since the oxygen tension in the blood coming out from the device could not be measured. Necessity is the mother of invention, and after a couple of trials, Clark presented his Oxygen Electrode.

The electrode consumed oxygen, and hence had to be stirred in order to maintain an equilibrium with the environment. Severinghaus improved the design by adding a stirred cuvette in a thermostat. Because blood and gas of the same Po2 still yielded different readings, it had to be calibrated with blood of known Po2, so a microtonometer was added to the water thermostat.

Application
Electron flow to oxygen as a result of oxidative phosphorylation can be demonstrated using an oxygen electrode. The electrode compartment is isolated from the reaction chamber by a thin Teflon membrane; the membrane is permeable to molecular oxygen and allows this gas to reach the cathode, where it is electrolytically reduced.

The reduction allows a current to flow; this creates a potential difference which is recorded on a flatbed chart recorder. The trace is thus a measure of the oxygen activity of the reaction mixture. The current flowing is proportional to the activity of oxygen provided the solution is stirred constantly (stir bar) to minimize the formation of an unstirred layer next to the membrane.

In Practice
- A voltage of around 0.7V is used to allow linearity between the measurements of the current and oxygen concentration. - Lifespan is limited to around 3 years due to the teflon membrane, which becomes encoated with protein - Requires power source - Requires constant temperature - Calibration can be performed using 21% oxygen (air) and 100% oxygen