​How to test Dissolved Oxygen (DO)?

There are a number of different methods that can be used to measure dissolved oxygen (DO) in water. First, there are wet chemical techniques, where a water sample is collected and then subject to a chemical reaction used to determine the DO level. Second, traditional membrane DO sensors are available, where a probe operating on electrochemical principles is inserted into the water to read the DO level. Finally, newer optical sensors are available that allow for fast, continuous measurement without many of the shortcomings of traditional 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 contained without coming into contact with air. Chemical reagents are then added to the water, including a titrant that is added until a reaction involving oxygen is complete (indicated by a color change). The concentration of DO is proportional to the volume of titrant added, which allows for a quantitative determination of DO. This technique can be used with low-precision kits for field use or for high-precision analysis using laboratory equipment.

This method has a number of limitations. First of all, sampling must be conducted very carefully. Not only must care be taken not to agitate the sample or expose it to gases, but special techniques or equipment may be needed for sampling water at depths where pressure is greater than at the surface, such as the use of Kemmerer water samplers at depths greater than 2 m[1].

Second, because biological activity consumes oxygen, there is only a limited time available between sampling and when the analysis must be completed. Samples containing appreciable amounts of biodegradable material must be tested immediately, and other samples may be stored for a few hours after preservatives are added to temporarily stop biological activity[1].

Membrane sensors

With sensor techniques, a probe can be inserted directly into the water, so a sample does not necessarily need to be collected. Traditional DO sensors employ electrochemical cells separated from the water by membranes. There are two different types of these sensors: galvanic and polarographic, the difference being that a polarographic system requires that a voltage 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 passes through the membrane, it interacts with the electrodes, causing a current to flow through the meter, which is used to determine the DO concentration.

Polarographic sensors require that the electrodes become polarized before measurement can take place. This warm-up period can take several minutes.

The reaction in the sensor consumes oxygen, so the signal detected by the meter depends on the transfer of oxygen across the membrane. Because of this, the method requires that the water be either flowing or stirred. One consequence of this is that the measurement may 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 filling solution be replaced prior to each study[2], which adds to the operating cost of these devices.

Optical (fluorescent) sensors

This newer type of sensor operates on a very different set of principles than galvanic or polarographic probes. In this method, oxygen in the water interacts with a fluorescent material, which in turn affects how it interacts with certain wavelengths of light. Blue light from within the probe excites the fluorescence of the material, but this effect is quenched by the presence of oxygen. The higher the DO concentration, the smaller the amount of fluorescence that is seen by the detector.

optical_sensor_illustration

This type of sensor provides some important advantages over traditional membrane sensors. They require less maintenance as there is no membrane or filling solution to replace. Additionally, because the measurement does not consume oxygen, the measurement is not affected by the flow of water, and stirring is not necessary. Unlike polarographic sensors, optical sensors do not need to polarize, so the sensor is ready for measurement immediately.

Calibration of Sensors

Both traditional membrane sensors and optical sensors can be calibrated using air as a source of oxygen. This can be accomplished because the concentration of oxygen in the atmosphere is a constant, known value (20.9%). A cap containing water-saturated air is often used for calibration. Alternatively, water saturated with air or standards with known DO concentrations (determined using the Winkler method) can be used for calibration[1,2].

Conclusion

The most precise measurements of DO are Winkler titrations conducted using laboratory equipment. However, this requires careful sample collection and preservation, as well as transportation to a laboratory within a short time frame. Field test kits using wet chemical techniques do not offer the same level of precision.

Sensors, including traditional membrane sensors and newer optical sensors, are more convenient to use because the DO can be measured in place without sample collection, and they allow for continuous and even remote monitoring. Traditional sensors require replacement of membranes and filling solutions, stirring, and may require a warmup period before use (for polarographic sensors). Newer optical sensors are even more convenient 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.