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Electrochemical DO electrodes are divided into two separate types: polarographic and galvanic. These electrodes are constructed with an anode and a cathode submerged in an electrolyte solution. An oxygen-permeable membrane is used to confine the cathode. When the cathode is polarized with a constant voltage, dissolved oxygen molecules diffusing through the membrane is reduced at the cathode. Then, an electrical signal produced by the cathode travels to the anode and then to the instrument. The oxygen tension versus the electrode current can be calibrated since the diffusive flux is a function of the partial pressure of oxygen in the flow. Instead of measuring the DO concentration, the electrode senses DO activity (or tension).
The oxygen-reduction Reduction reaction at the cathode can be presented as:
O2 + 2H2O + 2e- → H2O2 + 2OH-
H2O2 + 2e- → 2OH-
4e + 4H+ → 2H2OAs in the case for the polarographic electrodes, a voltage is applied externally while an internal potential is generated as in the galvanic electrodes.
Amperometry
Figure 6. An illustration of an electrochemical sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Amperometry
Amperometry is a technique used to Amperometry is a technique used to detect ions in a solution based on electrical current produced by electrochemical reaction of an electro-active species.
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A concentration gradient of ox caused by its depletion at the electrode surface leads to mass transport by diffusion. This leads to a flux of ox, Jox (mol/m2s) that related to the reduction current, ired, through the electrode with an area A according to Faraday’s law:
ired = - nFAJn×F×A×Jox ↔ Jox = - ired / nFAn×F×A
Where:
ired = reducing current (A)
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The electrical current is now proportional to the amount of DO transported by the electrode.
Fick's law of diffusion: ......
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The driving force for Jox is the concentration gradient (∂C/∂x) of ox near the electrode. Fick's first law of diffusion is used to relate the original concentration of ox to the measured current:
ired = - n×Jox×A×F = n×F×A×D×∂C/∂x
Where:
D = diffusion coefficient (m2/s)
∂C/∂x = concentration gradient ( x = 0 => C = 0, means the concentration of ox at the electrode drops to 0 )
A few assumptions are made in order to obtain a simple expression for the gradient.
- The slope of the gradient is linear,
- The thickness of the layer (s) is fixed.
- The concentration of ox drops to zero. x = 0, C = 0.
The equation above becomes:
ired = n×F×A×D×C / s
Polarographic Electrode
A typical polarographic electrode consists of a silver anode, a gold or platinum cathode and an electrolyte solution (KCl or AgCl). In order to create a sensor, a constant voltage of 0.8 volts is applied to the probe, and a digital meter is installed to read the DO response measured by the sensor.(being edited)
Figure 6. A simplified diagram of a polarographic sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Limitations
- Response time is described as the time required for the electrode to reach >90% of the output. Typical response time for polarographic sensors are is 30 sec, which makes them not compatible to be used for dynamic measurements.
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Galvanic Electrode
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Measuring dissolved oxygen with either sensor type
Variables that affect DO measurements
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