Development of a Oxygen Sensor for Marine ... - DTU Nanotech

3.1. CHEMISTRY OF THE CLARK SENSOR 15
face. When the negative bias voltage is further increased, the current output
of the electrode increases rapidly due to other reactions. If a fixed voltage in
the plateau region (for instance, -0.7V) is applied to the cathode, the current
output of the electrode can be linearly calibrated to the dissolved oxygen
(short termed DO). It should be noted that the current is proportional not
to the actual concentration but to the activity or equivalent partial pressure
of dissolved oxygen, which is often referred to as oxygen tension. A fixed
voltage between -0.6 and -0.8 V (due to the cell potential, and the onset of
other processes) is usually selected as the polarization voltage when using
Ag/AgCl as the reference electrode.
Figure 3.2: Polarogram: Current to voltage at different oxygen tensions and
calibration obtained at a fixed polarization voltage. Reproduced
from [24]. The shape of the curves indicates the nature of the
substance in the sample that is being reduced; where the height
is equivalent to the concentrations of oxygen. Also note that
convention usually have -V along the x-axis and not V, as is
otherwise the norm.
The cathode, the reference electrode, and the electrolyte are separated
from the measurement medium by a membrane, which is permeable to the
dissolved gas but not to most of the ions and other species. When at the
same time most of the mass transfer resistance is confined in the membrane,
the electrode system can measure oxygen tension in various liquids. This is
the basic operating principle of the membrane covered DO probe[25].
Electrode Reactions: The reaction can proceed as follows (a 4e − reaction
with a Ag/AgCl electrode is used as an example)
Cathodic reaction: