The acceptance of electrons by O2 causes a current–carried by the surrounding electrolyte–to flow between the anode and cathode. This current increases with a higher concentration of oxygen and the change is measured by the sensor.
The electrochemical method consumes oxygen, which poses an obstacle for measurement. While this does not affect the overall concentration of dissolved oxygen in the sample, it does affect the concentration in solution surrounding the tip of the electrode and causes artificially low readings. To overcome this, there must be constant mixing, or flow, of the sample. This means that the sensor is flow dependent and is important to be aware of when choosing what type of sensor is right for your application. This flow dependence is affected by the membrane materials and thickness, but those details are outside of the scope of this article. Manufacturers always state what the required flow rate is.
The electrochemical method is separated into two main types based on the metals used for the anode and cathode: galvanic and polarographic.
Galvanic Dissolved Oxygen Electrodes
Galvanic electrodes have anodes and cathodes with a large cell potential between them, meaning that one metal wants to give up electrons while the other wants to gain electrons. This potential is strong enough to cause electrons to flow from the anode to the cathode, in a process called self-polarization. In a sample with dissolved oxygen, the polarized cathode gives up electrons to any oxygen present and loses some polarization. As stated previously, electrons flow from the anode to replace ones lost to oxygen, resulting in current flow.
Oxidation of the anode results in formation of hydroxide precipitate, which eventually blocks the membrane and interferes with measurements. Be aware that, while presence of precipitate can affect performance, it does not automatically mean the electrode requires replacement. Many electrodes continue to function properly for months after visible build-up occurs.
Metals most commonly used in galvanic cells are lead or zinc for the anode and gold or platinum for the cathode.
Polarographic Dissolved Oxygen Electrodes
Polarographic electrodes have silver anodes and gold cathodes, resulting in a smaller cell potential that requires additional voltage to drive the transfer of electrons. Therefore, they are not self-polarizing like galvanic electrodes. This results in a warm-up time of 5-15 minutes before a polarographic electrode can be used.
Oxidation of the anode causes formation of silver chloride which coats the anode over time. Like the precipitate in galvanic electrodes, this coating eventually interferes with measurements. Because polarographic electrodes are not self-polarizing, this reaction does not occur when the electrode is not in use. As a result, polarographic electrodes can be stored for long periods of time without loss of life, whereas galvanic electrodes continue to be oxidized if exposed to oxygen.
The electrochemical electrodes described so far are known as steady-state electrodes, but there is one other variation worth mentioning. Polarographic electrodes can also be “pulsed”, meaning that a voltage is applied periodically instead of constantly. When the voltage is not being applied, dissolved oxygen from the sample can replace what was used up when the electrode was on. This means that pulsed polarographic measurements are not dependent on sample flow like steady-state electrodes are.
Galvanic vs. Polarographic Comparison