Oxygen electrodes

Much of our knowledge of electron transport in mitochondria and chloroplasts comes from oxygen electrode recordings. The oxygen concentration in a sealed incubation chamber is continuously monitored, and the effects of making various additions to the chamber can be observed. A cross section through a typical apparatus is shown below:

The upper section containing a transparent, thermostatted sample chamber is secured to the lower electrode assembly with a screwed ring. A thin teflon membrane is trapped between the two sections and separates the isotonic incubation medium from the strong KCl electrolyte in the electrode compartment. The adjustable stopper is used to seal the incubation chamber and prevent room air dissolving during the experiments. The small hole in the centre of the stopper permits the expulsion of air bubbles and allows small volumes of reagents to be added with a microlitre syringe. The contents of the chamber are stirred continuously with a magnetic flea.

A small polarising voltage (ca. 0.6 volt) is applied between the silver anode (+) and the platinum cathode (-). Oxygen diffuses through the teflon membrane and is reduced to water at the platinum cathode:

O2 + 4H+ + 4e-1 = 2H2O

The circuit is completed at the silver anode, which is slowly corroded by the KCl electrolyte:

Ag + Cl- = Ag Cl + e-1

The resulting current is proportional to the oxygen concentration in the sample chamber. This signal can be amplified and recorded.

The whole process is totally dependent on the supply of oxygen. The rate of oxygen diffusion to the cathode (and hence the current) depends on the oxygen concentration in the main incubation chamber. It also depends on several other factors: temperature, membrane thickness and permeability, sample viscosity and stirring speed. In contrast to pH electrodes which measure an equilibrium position, oxygen electrodes measure the velocity of a physico-chemical process that is far from equilibrium. pH electrodes have an intrinsic thermodynamic response which is relatively insensitive to temperature and sample composition, but there is no intrinsic calibration to an oxygen electrode - at regular intervals, or if the instrument is dis-assembled, it must be re-calibrated against a known standard, usually air. It is particularly important to control the temperature.

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