Clark electrode
The Clark electrode is an electrochemical sensor for determining the oxygen concentration in a solution (or in gases). It was developed in 1962 by Leland Clark in Yellow Springs , Ohio .
construction
The Clark electrode is usually made of a platinum - cathode and a silver - anode , the a electrolytic solution are connected. But other combinations, such as gold against silver or gold against lead, also form functional arrangements. These metal electrodes are usually separated from the material to be measured by an oxygen- permeable membrane , usually made of Teflon. But there are also membrane-free arrangements in which the material to be measured also serves as an electrolyte.
In the case of the Pt / Ag combination, the platinum cathode has a polarization voltage of −0.8 V against the silver anode. (Since the electrolyte solution used would be electrolyzed from approx. 1.6 V , the polarization voltage must not be of this order of magnitude). Other combinations, for example those with lead, do not require polarization.
If the membrane of the measuring chamber is now immersed in the material to be measured (e.g. in water or an arterial blood sample) to determine the oxygen content, O 2 diffuses through the membrane into the measuring chamber according to its partial pressure and is reduced there at the cathode. The cathode releases electrons to the oxygen, whereby hydroxide ions (OH - ) are formed. For example, silver is oxidized to silver ions at the anode and, in the presence of chloride, is deposited on the electrode as an insoluble AgCl crust. The deposits of the oxidized anode metal must be removed regularly in order to maintain the unimpeded flow of current in the measuring arrangement.
- Cathode: O 2 + 2 e - + 2 H 2 O → H 2 O 2 + 2 OH - and H 2 O 2 + 2 e - → 2 OH -
- Anode: 4 Ag → 4 Ag + + 4e - and 4 Ag + + 4Cl - → 4 AgCl
This results in a current I which is directly proportional to the partial pressure p (O 2 ) of the oxygen O 2 .
- p (O 2 ) ~ I.
In the further evaluation of this resulting measurement current, it must be taken into account that both the diffusion rate of the oxygen through the membrane and the oxygen solubility in the water are temperature-dependent.
Applications
It is mostly used in blood gas analyzers in intensive care units for ventilated patients or as adhesive electrodes for transcutaneous (through the skin) measurement of blood gases , especially in newborns.
Membrane-covered Clark electrodes are sold commercially as oxygen measuring devices. They are used in water monitoring, in aquaristics and in bioreactors . But there are also other applications, for example for determining the biochemical oxygen demand of wastewater and in food technology.
literature
- Peter Schuler: Oxi primer. Introduction to dissolved oxygen measurement technology. WTW, Scientific and Technical Workshops, Weilheim 1987.
swell
- ^ Leland C. Clark, Richard Wolf, Donald Granger, Zena Taylor: Continuous recording of blood oxygen tensions by polarography. In: Journal of Applied Physiology . Vol. 10. No. 6, ISSN 8750-7587 , pp. 189-193, PMID 13096460 .
- ↑ John W. Severinghaus, Poul B. Astrup: History of blood gas analysis. IV. Leland Clark's oxygen electrode. In: Journal of Clinical Monitoring. Vol. 2, No. 2, ISSN 0748-1977 , pp. 125-139, PMID 3519875 , doi : 10.1007 / BF01637680 .