Dissolved oxygen (do): the molecular oxygen dissolved in water is called dissolved oxygen. Dissolved oxygen in the atmosphere and Photosynthesis of aquatic organisms such as aquatic algae are the sources of dissolved oxygen in water. The content of dissolved oxygen in water is related to atmospheric pressure, water temperature and salt content. The decrease of atmospheric pressure, the increase of water temperature and the increase of salt content will lead to the decrease of dissolved oxygen content. Dissolved oxygen mainly has the following changes: (1) diurnal variation. The oxygen content is high in the daytime, and the dissolved oxygen in the water is often oversaturated at 2-4 PM. The dissolved oxygen in the water is low at night and reaches the lowest value before dawn（ 2) Vertical variation. In general, the dissolved oxygen in the upper water is much higher than that in the lower water in the daytime. At night, due to the convection of pool water, the dissolved oxygen difference between the upper and lower water gradually decreases, and the oxygen difference is the largest in the afternoon of the whole day（ 3) Level change. Generally, due to the effect of wind, the dissolved oxygen in the downwind is higher than that in the upwind during the day, but the change of dissolved oxygen in the morning is opposite, that is, the dissolved oxygen in the upwind is higher than that in the downwind（ 4) Seasonal change. Generally, low dissolved oxygen occurs in summer and autumn, especially in rainy weather. The dissolved oxygen of clean surface water is nearly saturated. When a large number of algae propagate, do may be oversaturated; When the water body is polluted by organic and inorganic reducing substances, the dissolved oxygen content will decrease or even tend to zero. At this time, the anaerobic bacteria are active and the water quality will deteriorate. When dissolved oxygen in water is lower than 3-4mg / L, many fish have difficulty breathing; If it continues to decrease, it will suffocate and die. It is generally stipulated that the dissolved oxygen in water should be at least 4 mg / L. The content of dissolved oxygen in water can be used as an indirect indicator of organic pollution and self purification. The dissolved oxygen content of rivers, lakes and reservoirs in China is more than 4 mg / L. the dissolved oxygen content of some rivers south of the Yangtze River is generally higher, up to 6-8 mg / L.   Dissolved oxygen is also an important control index in the process of wastewater biochemical treatment. Because the content of dissolved oxygen has a great relationship with the atmosphere, temperature and other factors, the sample collection of dissolved oxygen should use special sampling bottles, such as double oxygen bottle and dissolving bottle. When sampling, pay attention not to make the water sample contact with the air, and the sampling action should be gentle to minimize the disturbance. When sampling, the sampling bottle must be filled, and then the bottle stopper must be tightly closed. At the same time, pay attention not to leave bubbles. When collecting water samples from pipes and water taps, a rubber tube or other hose can be used to guide the water to flow along the bottle wall to overflow, and the water can be collected continuously for a few minutes, and then the water can be plugged tightly without bubbles. In order to prevent the change of dissolved oxygen in water samples, the collected water samples must be fixed on site (adding manganese sulfate and basic potassium iodide) or determined directly on site with oxygen electrode. The methods to determine dissolved oxygen in water include iodometry and its correction method (gb7489-87) and oxygen electrode method (gb11913-89). Clean water can be measured by iodometry; The modified iodometric method or oxygen electrode method must be used for polluted surface water and industrial wastewater. In addition, in order to realize the automatic monitoring of dissolved oxygen, the National Environmental Protection Bureau formulated the technical requirements for dissolved oxygen (do) water quality automatic analyzer (HJ / t99-2003). (1)    Iodimetry Iodometry is the standard method for the determination of dissolved oxygen in water. In the absence of interference, this method is suitable for all kinds of water samples with dissolved oxygen concentration greater than 0.2mg/l and less than twice of oxygen saturation concentration (about 20mg / L). Easily oxidized organic compounds, such as tannic acid, humic acid and lignin, will interfere with the determination; Oxidable sulfides, such as thiourea, can also cause interference. When the water sample contains the above substances, the oxygen electrode method should be used. The principle of iodimetry is: manganese sulfate and basic potassium iodide are added to the water sample, and the dissolved oxygen in the water will oxidize the divalent manganese to the tetravalent manganese, and generate hydroxide precipitation. After adding acid, the precipitate is dissolved, and tetravalent manganese can oxidize iodide ion and release free iodine equivalent to dissolved oxygen. With starch as indicator, the content of dissolved oxygen can be calculated by titrating the released iodine with sodium thiosulfate standard solution. The reaction equation is as follows MnSO4＋2NaOH＝Na2SO4＋Mn(OH)2↓ (white precipitate) 2Mn(OH)2+O2＝2MnO(OH)2↓ (brown precipitate) MnO(OH)2＋2H2SO4＝Mn(SO4)2＋3H2O Mn(SO4)2＋2KI＝MnSO4＋K2SO4＋I2 2Na2S2O3＋I2＝Na2S4O6＋2Nal （2） Modified iodometry In the determination of dissolved oxygen in water samples by iodometry, if there are some reducing substances in water samples, it will be interfered. At this time, some reagents can be added for correction. The common correction methods are sodium azide correction method and potassium permanganate correction method. one   Sodium azide correction method Nitrite in water sample will interfere with the determination of dissolved oxygen by iodometry. Sodium azide can be used to decompose nitrite and then determine it by iodometry. The decomposition reaction of nitrite is as follows 2NaN3＋H2SO4＝2HN3+Na2SO4 HNO2＋NH3＝N2O＋N2＋H2O Nitrite mainly exists in wastewater and river water after biochemical treatment. It can react with potassium iodide to release free iodine and produce positive interference 2HNO2＋2KI＋H2SO4＝K2SO4＋2H2O＋N2O2＋I2 If the reaction is up to this point, the introduction error is not big; However, when the water sample comes into contact with air, the newly dissolved oxygen reacts with N2O2 to form nitrite 2N2O2＋2H2O＋O2＝4HNO2 Such a cycle, continuous release of iodine, will introduce considerable error. When the content of ferric ion in water sample is high, the interference can be eliminated by adding potassium fluoride or acidifying with phosphoric acid instead of sulfuric acid. The measurement results are calculated as follows: Where: m -- concentration of sodium thiosulfate standard solution, mol / L; V -- volume of sodium thiosulfate standard solution consumed in titration, ml; V water -- volume of water sample, ml; 8 -- oxygen conversion value, G. It should be noted that sodium azide is a highly toxic and explosive reagent. It is not allowed to acidify the alkaline potassium iodide sodium azide solution directly to avoid toxic azide mist. two   Potassium permanganate correction method The method is suitable for water samples containing a large amount of ferrous ions and without other reducing agents and organic compounds. Potassium permanganate is used to oxidize ferrous ion to eliminate interference. Excess potassium permanganate is removed by sodium oxalate solution, and the generated high valent iron ion is masked by potassium fluoride. Others are the same as iodometry. （3） Oxygen electrode method The widely used dissolved oxygen electrode is polytetrafluoroethylene film electrode, which is a typical oxygen electrode. According to its working principle, it can be divided into polarographic type and galvanic cell type. The structure of polarographic oxygen electrode is shown in Fig. 3-27. It is composed of gold cathode, silver silver chloride anode, polytetrafluoroethylene film and shell. The electrode cavity is filled with potassium chloride solution, the PTFE film separates the electrolyte from the water sample, and the dissolved oxygen diffuses through the film. When a fixed polarization voltage of 0.5-0.8V is applied between the two electrodes, the dissolved oxygen in the water sample diffuses through the film and is reduced on the cathode, resulting in a diffusion current proportional to the oxygen concentration. The electrode reaction is as follows Figure 3-27 dissolved oxygen electrode structure 1. Gold cathode; 2. Silver wire anode; 3. Thin film; 4. KCl solution; 5. Shell Cathode: O2 + 2H2O + 4E = 4OH- Anode: 4ag + 4CL - = 4agcl + 4e The reduction current I can also be expressed as: Where: K - proportional constant; N -- number of electrons gained and lost in electrode reaction; F -- Faraday constant; A -- cathode area; PM -- permeability coefficient of membrane; L -- thickness of film; C0 -- partial pressure or concentration of dissolved oxygen. Therefore, as long as the reduction current is measured, the concentration of dissolved oxygen in water sample can be calculated. All kinds of dissolved oxygen meters work according to this principle (see Figure 3-28). In the process of determination, the zero point is first corrected with the anaerobic water sample, and then the calibration value of the instrument is calibrated with the chemical method. Finally, the dissolved oxygen concentration of the water sample can be directly displayed. The instrument is equipped with automatic or manual temperature compensation device to compensate the measurement error caused by temperature change. 1. Polarized voltage source; 2. Dissolved oxygen electrode and measuring cell; 3. Operational amplifier; 4. Indicator The determination of do by do electrode method is not affected by the color and turbidity of water sample and the interfering substances in chemical titration; The method is rapid, simple and suitable for on-site determination; It is easy to realize automatic continuous measurement. However, if the water sample contains algae, sulfide, carbonate, oil and other substances, the film will be blocked or damaged, so the film should be replaced in time.