There are several methods by which dissolved oxygen (DO) in water can be measured. First, there are wet chemical techniques in which a water sample is collected and then subjected to a chemical reaction used to determine oxygen content. Second, conventional membrane DO sensors are available in which a probe operating on electrochemical principles is inserted into the water to read the DO level. Finally, newer optical sensors are available that provide fast, continuous measurement without many of the shortcomings of conventional membrane sensors.
Wet chemical techniques
The most common wet chemical technique for DO measurement is titration by the Winkler method. In this technique, a sample is collected in a special bottle that allows the water to be absorbed without coming into contact with air. Chemical reagents are then added to the water, including a titrant, which is added until a reaction with oxygen is complete (indicated by a color change). The concentration of DO is proportional to the volume of titrant added, which allows quantitative determination of DO. This technique can be used with low precision kits for field use or for high precision analysis with laboratory equipment.
This method has a number of limitations. First of all, sampling must be done very carefully. Not only must care be taken not to move or expose the sample to gases, but special techniques or equipment may be required to sample water at depths where the pressure is higher than at the surface, such as using Kemmerer water samplers at depths greater than 2 m [1].
Second, because biological activity consumes oxygen, there is limited time between sampling and completion of analysis. Samples containing appreciable amounts of biodegradable material must be tested immediately, and other samples can be stored for a few hours after adding preservatives to temporarily stop biological activity [1].
Membrane sensors
Sensor techniques allow a probe to be inserted directly into the water, eliminating the need to collect a sample. Conventional DO sensors use electrochemical cells separated from water by membranes. There are two different types of these sensors: galvanic and polarographic. The difference is that a polarographic system requires a voltage to be applied to polarize the electrodes and the galvanic system does not. In both types, the electrochemical cell contains two electrodes and a filling solution (containing potassium chloride or potassium hydroxide). This cell is separated from the water by a membrane that is highly permeable to oxygen but otherwise separates the water from the filling solution. As oxygen flows through the membrane, it interacts with the electrodes, causing a current to flow through the meter that is used to determine the oxygen concentration.
Polarographic sensors require that the electrodes be polarized before the measurement can take place. This warm-up phase may take a few minutes.
The reaction in the sensor consumes oxygen, so the signal detected by the meter depends on the transfer of oxygen across the membrane. For this reason, the process requires that the water either flows or is agitated. A consequence of this is that the measurement can be affected by the flow rate of the water. These types of sensors also require occasional cleaning of the electrodes and replacement of the membrane and filling solution. The U.S. Environmental Protection Agency recommends that the membrane and fill solution be replaced prior to each study [2], which increases the operating costs of these devices.
Optical (fluorescent) sensors
This newer type of sensor operates on completely different principles than galvanic or polarographic probes. In this method, oxygen in water interacts with a fluorescent material, which in turn affects the interaction with certain wavelengths of light. Blue light from the probe excites the fluorescence of the material, but this effect is quenched by the presence of oxygen. The higher the oxygen concentration, the lower the amount of fluorescence seen by the detector.
This type of sensor offers several important advantages over conventional membrane sensors. They require less maintenance because no membrane or filling solution needs to be replaced. Since the measurement does not consume oxygen, the measurement is not affected by the water flow and no stirring is required. Unlike polarographic sensors, optical sensors do not need to polarize, so the sensor is ready to measure immediately.
Calibration of sensors
Both conventional membrane sensors and optical sensors can be calibrated using air as the oxygen source. This can be achieved because the oxygen concentration in the atmosphere is a constant, known value (20.9%). A cap with water-saturated air is often used for calibration. Alternatively, water saturated with air or standards with known oxygen concentrations (determined by the Winkler method) can be used for calibration [1,2].
Conclusion
The most accurate DO measurements are Winkler titrations performed with laboratory equipment. However, this requires careful sample collection and preservation and transport to a laboratory within a short time. Field test kits using wet chemical techniques do not provide the same precision.
Sensors, including conventional membrane sensors and newer optical sensors, are more convenient to use because oxygen levels can be measured in situ without sampling, allowing continuous and even remote monitoring. Conventional sensors require the replacement of membranes and filling solutions with stirring and may require a warm-up period before use (for polarographic sensors). Newer optical sensors are even more practical because they do not have these limitations.
References
[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.
[2] U.S. Environmental Protection Agency (2017) Field Measurement of Dissolved Oxygen. SESD Operating Procedure SESDPROC-106-R4.