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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 [3].
The oxygen-reduction reaction at the cathode can be presented as:
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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 [5]:
ired = - n×F×A×Jox ↔ Jox = - ired / n×F×A
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The electrical current is now proportional to the amount of DO transported by the electrode. 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 [5]:
ired = - n×Jox×A×F = n×F×A×D×∂C/∂x
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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 [6].
Figure 7. A simplified diagram of a polarographic sensor. Source: [6]
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Figure 8. A simplified diagram of a galvanic sensor and circuit. Source: [6]
Limitations
- The sensor continuously consumes the anode, even when truned off. Therefore the lifetime of the sensor is much sorther than of the polarographic sensor and the warranty is usually for 6 months only [2].
- Since the electrode consumes oxygen, readings are affected by flow across the sensor tip. Thus enough flow rate at the membrane (or sample renewal rate) must be ensured for accurate results .
Advantages
- No warm up time, it can be sed immediately after turn-off
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