35
Polarographic Oxygen Electrode
The polarographic (amperometric) oxygen electrode or Clark-type electrode constituted the first practical electronic means of measuring DO concentration and
advanced models remain in use today. The enabling reaction is the electrochemical
reduction of DO to hydroxide as follows:
O
HO
OH
2
2
4
2
4
−
−
−
+
+
→
e
(2.6)
The theoretical redox potential of natural oxygenated waters versus the standard
hydrogen electrode, largely dominated by the oxygen reduction reaction, is a little
over 700 mV and varies little despite wide variations in DO concentration.
Application of a potential in excess of this value will result in the above reaction and
measurement of the current flowing through the cell provides a measure of oxygen
concentration. In practice an overpotential of about 800 mV is applied across an
electrode pair (usually a platinum or gold cathode and a silver anode). Electrons
dispensed to the aqueous medium at the cathode are provided through oxidation of
the silver anode with the subsequent deposition of silver chloride (a largely insoluble salt) upon its surface. The reaction cell is confined in a small volume with the
electrodes immersed in a potassium chloride (KCl) electrolyte. A gas-permeable
membrane separates the cell from the seawater. Cellophane was once widely used
as the membrane material; Teflon® is currently preferred. Consumption of DO
within the cell creates a gradient and prompts diffusion of DO from the seawater
medium to the cell. Hence, the cell current is dictated by the rate of diffusion of DO
through the membrane to the cathode site and this in turn is dictated by the outside
concentration. The resulting measurement is known as the diffusion current. In
addition to the raw current reading, calibration requires knowledge of temperature
and local pressure.
Initial instruments proved to be unsuitable for high precision oceanographic
work requiring that they be continuously calibrated against Winkler analyses of
simultaneously collected samples (Atwood et al. 1977). This drawback is in large
part a result of the continuous change in chemical properties of the electrode known
as electrochemical drift and is dependent upon the amount of DO in the sample and
the time transpired in operation. Moreover, poorly designed cells suffer significant
pressure deformation inducing hysteresis, in which the instrument reading becomes
a function of the past state of the cell. Thus in vertical instrument casts at sea the
downcast profile differs significantly from the ensuing upcast profile. Membrane
fouling is a further cause for concern. Refinements in instrument design, such as
reducing cell volume and pressure-induced cell volume changes, reducing membrane thickness to facilitate oxygen diffusion and thus increase response rate, and in
operation strategies, such as decreasing operation time to reduce electrochemical
drift, have largely minimized (but not eliminated) these drawbacks. Oxygen depletion due to consumption by the cell has been largely overcome by actively pumping
seawater across the cell.
2.3 Electrochemical Sensors for Coastal Ocean Observing
Polarographic Oxygen Electrode
The polarographic (amperometric) oxygen electrode or Clark-type electrode constituted the first practical electronic means of measuring DO concentration and
advanced models remain in use today. The enabling reaction is the electrochemical
reduction of DO to hydroxide as follows:
O
HO
OH
2
2
4
2
4
−
−
−
+
+
→
e
(2.6)
The theoretical redox potential of natural oxygenated waters versus the standard
hydrogen electrode, largely dominated by the oxygen reduction reaction, is a little
over 700 mV and varies little despite wide variations in DO concentration.
Application of a potential in excess of this value will result in the above reaction and
measurement of the current flowing through the cell provides a measure of oxygen
concentration. In practice an overpotential of about 800 mV is applied across an
electrode pair (usually a platinum or gold cathode and a silver anode). Electrons
dispensed to the aqueous medium at the cathode are provided through oxidation of
the silver anode with the subsequent deposition of silver chloride (a largely insoluble salt) upon its surface. The reaction cell is confined in a small volume with the
electrodes immersed in a potassium chloride (KCl) electrolyte. A gas-permeable
membrane separates the cell from the seawater. Cellophane was once widely used
as the membrane material; Teflon® is currently preferred. Consumption of DO
within the cell creates a gradient and prompts diffusion of DO from the seawater
medium to the cell. Hence, the cell current is dictated by the rate of diffusion of DO
through the membrane to the cathode site and this in turn is dictated by the outside
concentration. The resulting measurement is known as the diffusion current. In
addition to the raw current reading, calibration requires knowledge of temperature
and local pressure.
Initial instruments proved to be unsuitable for high precision oceanographic
work requiring that they be continuously calibrated against Winkler analyses of
simultaneously collected samples (Atwood et al. 1977). This drawback is in large
part a result of the continuous change in chemical properties of the electrode known
as electrochemical drift and is dependent upon the amount of DO in the sample and
the time transpired in operation. Moreover, poorly designed cells suffer significant
pressure deformation inducing hysteresis, in which the instrument reading becomes
a function of the past state of the cell. Thus in vertical instrument casts at sea the
downcast profile differs significantly from the ensuing upcast profile. Membrane
fouling is a further cause for concern. Refinements in instrument design, such as
reducing cell volume and pressure-induced cell volume changes, reducing membrane thickness to facilitate oxygen diffusion and thus increase response rate, and in
operation strategies, such as decreasing operation time to reduce electrochemical
drift, have largely minimized (but not eliminated) these drawbacks. Oxygen depletion due to consumption by the cell has been largely overcome by actively pumping
seawater across the cell.
2.3 Electrochemical Sensors for Coastal Ocean Observing