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Overview

Dissolved oxygen is a parameter that characterizes the concentration of oxygen in an aqueous solution. It is free oxygen dissolved in water. The unit of dissolved oxygen is mg/L, expressed in milligrams of oxygen per liter of water. The amount of dissolved oxygen in water is an indicator of the self-purification ability of water. High dissolved oxygen is conducive to the degradation of various pollutants in the water body, so that the water body can be purified faster; conversely, low dissolved oxygen, the degradation of pollutants in the water body is slower. So what are the current methods for measuring dissolved oxygen in water? Commonly used methods include iodometric method, electrochemical probe method and fluorescence method. Next, we will explain the principles of these dissolved oxygen measurement methods in detail.

Method for measuring dissolved oxygen in water quality:
1. Iodometric method: Iodometric method is the benchmark method for measuring dissolved oxygen in water. In the absence of interference, this method is applicable to various water samples with a dissolved oxygen concentration greater than 0.2 mg/L and less than twice the saturated concentration of oxygen (about 20 mg/L). Easily oxidized organic matter such as tannins, humic acid and lignin will interfere with the determination. Oxidizable sulfur compounds such as sulfide and thiourea will also interfere with the respiratory system that is prone to oxygen consumption. When containing such When detecting substances, the electrochemical probe method is suitable. There will be no interference when the nitrite concentration is no higher than 15 mg/L, because they will be destroyed by the added sodium azide. If there are oxidizing substances or reducing substances, pretreatment is required, and the modified iodine method is used. .
2. Electrochemical probe method: The dissolved oxygen electrochemical probe is a small chamber sealed with a selective film. There are two metal electrodes in the chamber and are filled with electrolyte. Oxygen and a certain amount of other gases and lyophilic substances can pass through this film, but ions of water and soluble substances can hardly pass through this film. When the probe is put into the water to measure dissolved oxygen, a potential difference is generated between the two electrodes due to the action of the battery or the applied voltage, causing the metal ions to enter the solution at the anode. At the same time, the oxygen diffuses through the film and obtains electrons at the cathode to be reduced, and the current generated is the same as The rate of oxygen transfer through the membrane and electrolyte layer is proportional, that is, at a certain temperature, the current is proportional to the partial pressure of oxygen in the water.

3. Fluorescence method:
The sensor is covered with a fluorescent material, and blue light emitted from the LED light source is transmitted to the sensor surface. The blue light excites the fluorescent material, causing it to emit red light. The time from the emission of blue light to the emission of red light was recorded. The more oxygen is present, the shorter this time is. From this, this period of time is recorded and correlated to the oxygen content.

Comparison of the advantages and disadvantages of the three methods
The traditional iodometric method is the most classic, but it requires chemical reagents and is time-consuming. The sampling requirements are high, they need to be fixed on site, and the storage conditions are strict. Interfering substances have the greatest impact. Easily oxidized organic matter such as tannins, humic acid and lignin will interfere with the measurement. Oxidizable sulfur compounds such as sulfide and thiourea will also interfere with the respiratory system, which is prone to oxygen consumption. If the nitrite concentration is not higher than 15 mg/L, there will be no interference, because they will be destroyed by the added sodium azide. If there are oxidizing substances or reducing substances, pretreatment is required, and the corrected iodine content is used.

The electrochemical probe method is currently the most commonly used method. It is simple to operate, fast and efficient. It does not require reagents and can be quickly measured and read on site. However, when there are some gases and vapors in the water, such as chlorine, chlorine dioxide, hydrogen sulfide, amines, ammonia, carbon dioxide, bromine and iodine, they will affect the measured current through membrane diffusion and interfere with the measurement. Other substances in the water sample, such as solvents, oils, sulfides, carbonates and algae, may block the membrane, cause membrane damage and electrode corrosion, affect the measured current and interfere with the measurement. The probe requires maintenance, and the membrane and electrolyte are consumables and need to be replaced regularly.

The fluorescence method makes up for many shortcomings of the membrane method. It does not need to replace the electrolyte, is simple to maintain, does not consume oxygen during measurement, so there is no flow rate and stirring requirements, and is not affected by some gases and steam in the water, such as chlorine, sulfur dioxide, sulfide, etc. Substances such as hydrogen, amines, ammonia, carbon dioxide, bromine and iodine, solvents, oils, sulfides, carbonates and algae diffuse through the membrane to affect the measured current interference, and the sensor has a long life. However, as a new method, there is still no national standard, and there is little relevant information and research. Some problems may occur during the use of reagents. There is no theoretical basis and no reason can be found. This method needs to be studied. And the turbidity and salinity in the water have a greater impact on the measurement results.

Daruifuno currently provides online dissolved oxygen sensors with electrochemical probe methods and fluorescence methods. If you want to know more, please contact us. Delfino focuses on the R&D and production of water quality analysis instruments. Our measurement parameters include pH, ORP, conductivity, TDS, salinity, dissolved oxygen, COD, ammonia nitrogen, turbidity, chlorophyll, blue-green algae, residual chlorine, and chlorine dioxide. wait. Our main products include pH ORP analyzer, conductivity analyzer, turbidity analyzer, COD analyzer, dissolved oxygen analyzer, ammonium analyzer, chlorine analyzer, etc.

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