Monitoring water quality is essential for sustaining healthy ecosystems, ensuring public safety and managing water resources. Various characteristics such as pH, turbidity and dissolved oxygen can be measured to establish an overall profile of water quality and assess the viability of water for a particular use.
Many industries use dissolved oxygen as an indicator of the health of a water system. For example, monitoring dissolved oxygen is critical in aquaculture due to the varying oxygen demands of aquatic life. In water distribution systems, dissolved oxygen is monitored due to its correlation with accelerated plumbing corrosion. In the wastewater treatment industry, dissolved oxygen must be measured to ensure sufficient conversion of organic waste into inorganic byproducts. Lastly, beverage companies may measure dissolved oxygen to verify the shelf life of their bottled products.
Oxygen can be dissolved in water in numerous ways. The aeration of water through normal churning or the release of oxygen via photosynthesis are just a few examples. Oxygen also may diffuse into water from air based on a chemistry principle called Henry’s Law, which correlates the pressure of surrounding air to the amount of gas dissolved in the water. Other factors, such as the temperature and salinity of the water, also affect its ability to hold dissolved oxygen.
The Winkler Titration
Multiple techniques exist to measure dissolved oxygen, ranging from scientifically basic to technologically advanced. One of the most basic but widely used methods is known as the Winkler titration. This method was developed in 1888 by Ludwig Winkler and uses iodine to indirectly determine the amount of dissolved oxygen in a sample. A manganese solution is added to a water sample, followed by a potassium iodide solution. The dissolved oxygen in the sample oxidizes the manganese ions to form a brown precipitate. Sulfuric acid then is added to convert the iodide-containing precipitate into iodine. This iodine then is titrated with sodium thiosulfate to determine the dissolved oxygen level. The endpoint can be determined using a starch color indicator, or potentiometrically, with an oxidation reduction potential electrode.
The Winkler titration is considered a highly accurate method against which all other dissolved oxygen measurement methods are compared. However, it requires a high level of technical training and expensive chemicals, and is not field-friendly. In addition, it is subject to human error if the titration is performed manually. Multiple variations of the Winkler method exist to overcome sample matrix interference and oxidizing/reducing agent interference. Methods for the colorimetric determination of dissolved oxygen also utilize Winkler-based principles.
Electrochemical Sensors
A more advanced solution for measuring dissolved oxygen arises through the use of electrochemical sensors. These consist of an anode and cathode that are enclosed in an electrolyte solution by an oxygen-permeable membrane. Dissolved oxygen in the water travels through the membrane and is reduced at the cathode. This produces an electrical signal, which travels from the cathode to the anode and then to the meter, where it is converted into a user measurement unit.
The two main types of electrochemical sensors used for dissolved oxygen measurement are galvanic sensors and polarographic sensors. In a galvanic sensor, the cathode and anode typically are made of silver and zinc, respectively. When placed in an electrolyte solution, the cathode and anode self-polarize and a voltage is spontaneously generated. This voltage allows the reduction of oxygen to occur at the cathode. The reduction of oxygen to hydroxyl ions generates a current that is measured by the meter and converted into a user measurement unit.
Because galvanic probes are self-polarizing, they require no “warm-up” time and are ready to take measurements immediately. In addition, the electrolyte does not deplete, nor does it require frequent replacement. Routine maintenance for electrochemical sensors includes regular replacement of membranes and electrodes. When measuring with any electrochemical sensor, it is also important to constantly stir the sample to supply the probe with fresh sources of dissolved oxygen, as it is constantly being consumed by the redox reaction.
Polarographic probes work on the same principle as galvanic probes. The most common type of polarographic probe, the Clark type, contains a cathode made of platinum and an anode made of silver. However, unlike galvanic probes, it does not generate its own voltage to reduce dissolved oxygen. Instead, outside voltage must be applied to the system. Because the cathode and anode are not continuously reacting, polarographic probes require a five- to 15-minute warm-up time to allow for sensor stability before use. In addition, the electrolyte must be replaced periodically due to the depletion of chloride ions in the solution.
A second type of polarographic probe, the Ross type, generates oxygen at the same rate it is consumed, reducing the need to replace the electrolyte and decreasing the rate of anode consumption.
One critical consideration when measuring dissolved oxygen with electrochemical sensors is the use of temperature compensation. As temperature increases, gas permeability through the sensor membrane increases, and the solubility of oxygen decreases. Therefore, it is important that all dissolved oxygen measurements are temperature compensated to maintain high accuracy.
Optical Oxygen Sensors
One of the most recent developments in dissolved oxygen technology is the optical oxygen sensor. This technology contains an internal blue LED, which pulses and excites an oxygen-sensitive luminescent dye on the sensor tip. When the dye glows, it emits red light that is monitored by an internal photo sensor. When the sensor tip is in contact with a sample, dissolved oxygen present in the sample inhibits the dye’s glow. By measuring the intensity of the glow, the meter can interpret it as a dissolved oxygen measurement. An alternate method using optical sensors measures the glow’s lifetime—the amount of time the dye takes to return to its relaxed state in between blue light pulses—instead of intensity.
Optical dissolved oxygen sensors eliminate many problems associated with electrochemical sensors. First, the samples do not need to be stirred because no dissolved oxygen is being consumed by the reaction. In addition, optical sensors do not require warm-up time and only need calibration once every six months.
Optical sensors also feature higher accuracy when measuring low concentrations of dissolved oxygen (zero to 20 mg/L), but have a much slower response time. Some optical sensors take twice as long to achieve a stable measurement as electrochemical sensors. Furthermore, optical sensors have a higher power consumption rate than electrochemical sensors, resulting in a shorter meter battery life. This can be disadvantageous in field measurement applications. Finally, compared with electrochemical sensors, optical sensors require a higher initial purchase cost.
Accurately measuring dissolved oxygen plays an important role in the success of many industries. The advancement of instrumentation technology has given science-minded consumers more options than ever to measure dissolved oxygen. Portable options such as electrochemical and optical sensors allow convenient measurement in the field, while the tried-and-true Winkler titration provides a gold standard of accuracy when performed properly. Determining which technology is best suited for a customer’s application is crucial to understanding a user’s needs.
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