Measuring dissolved oxygen levels involves optical or electrochemical sensors. These sensors are affixed to a process monitor, data logger, or transmitter for process control. If you’re a manufacturer and need any such services, you can monitor dissolved oxygen measurements with Professional Process Systems (PPS) devices and instrumentation.
Other dissolved oxygen measurement methods like colorimetry involve estimating the oxygen concentration in a particular sample.
Winkler titration has been the gold standard for measuring dissolved oxygen levels for many years. However, it is prone to human error because of the considerable difficulty involved in performing it.
The following are the methods for measuring dissolved oxygen levels:
1)Sensor Method
The sensor method is a very common way of measuring dissolved oxygen (DO) levels. Optical and electrochemical sensors are the two main categories here. However, electrochemical sensors can further be subdivided into polarographic, galvanic, and pulsed polarographic sensors.
Most of these sensors utilize analog methods of operations. However, technological advances mean that more of them are being modified to use digital input. Dissolved oxygen sensors can be used for spot sampling or long-term monitoring of certain phenomena, both inside and outside a laboratory setting. One of its well-known uses is in biochemical oxygen demand (BOD) tests, where they measure the oxygen levels consumed by microorganisms while acting on organic matter.
A DO sensor provides data for recording by water quality sonde, a data logging system, or an oxygen meter. Temperature, pressure, and salinity are factors that affect dissolved oxygen concentrations. As such, any methods for measuring DO need to account for these factors. A logging software of the DO meter can help to take these measurements accurately. Most sensors have a thermistor for measuring temperature. Conductivity sensors provide data on salinity while an external barometer can be used to measure air pressure.
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2) Optical Dissolved Oxygen Sensors
The interaction between oxygen and certain dyes that emit light without heat can be the basis for measuring DO using the optical method. When exposed to blue light, the electrons in such dyes gain energy, thereby releasing light as they revert to their initial energy state. The presence of dissolved oxygen affects the frequency of light emissions (wavelength), usually limiting it. This is due to the interaction of oxygen and dye molecules. While some DO sensors are referred to as fluorescent sensors, this is a misnomer because they don’t emit ultraviolet (UV) rays.
Optical sensors are perfect for long-term monitoring of phenomena due to low maintenance requirements.*
3) Electrochemical Method
The sensors that utilize this method are also known as Clark-type or amperometric sensors. They’re divided into two:
i)Polarographic DO Sensors
They are made of a noble metal cathode and a silver anode. The cathode can be made of platinum or gold ( silver can also be used, though rarely). This sensor is usually kept in a potassium chloride solution. When the sensor is switched on, it may take anywhere from a few minutes to an hour to polarize both the cathode and anode before any calibration or measurement. This is known as the warm-up period. The electrodes are polarized by a constant voltage. The anode becomes positively polarized while the cathode becomes negatively polarized as a result of electrons traveling counter to the current. This causes oxygen diffusion across the membrane, leading to the reduction of molecules at the cathode and increasing electrical signals. The polarizing potential is kept constant as the sensors detect variations in the current due to dissolved oxygen concentrations.
The polarographic DO sensor displays a greater electrical current reading as more oxygen is reduced at the membrane.
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- ii) Galvanic DO Sensor
It is an electrochemical DO sensor that utilizes metals with different electro potentials (how readily they accept electrons). When placed in an electrolyte like a potassium chloride solution, self-polarization occurs due to the different electropotentials of the electrodes. Self-polarization ensures that galvanic DO sensors don’t require warm-up time like the polarographic ones. Here, the reduction of oxygen requires a minimum of 0.5 volts if the application of an external potential is to be avoided.
Galvanic DO sensors usually consist of a lead or zinc anode while the cathode can be silver. Sodium chloride or sodium hydroxide is usually the ideal electrolyte even though inert ones can also be used. The major difference between reactions in galvanic DO sensors and polarographic ones is that the former doesn’t require a constant, separate potential.
After self-polarization, the electrons travel from the anode to the cathode. The cathode doesn’t participate in the chemical reaction, acting only as a channel for the electrons.
Ultimately, choosing the right method for measuring dissolved oxygen largely depends on the processes involved.