Dissolved oxygen (DO)

The molecular oxygen dissolved in water is called dissolved oxygen. The dissolution of oxygen in the atmosphere and the photosynthesis process of aquatic organisms such as algae are both sources of dissolved oxygen in water. The content of dissolved oxygen in water is related to factors such as atmospheric pressure, water temperature, and salt content. A decrease in atmospheric pressure, an increase in water temperature, and an increase in salt content can all lead to a decrease in dissolved oxygen content.

There are several main changes in dissolved oxygen: (1) diurnal variation. The oxygen content is high during the day, and the dissolved oxygen in the water is often supersaturated from 2-4 pm. At night, the dissolved oxygen is low and reaches its lowest value before dawn. (2) Vertical variation. Generally, the dissolved oxygen in the upper layer of water is much higher than that in the lower layer during the day. At night, due to the convection of the pool water, the difference in dissolved oxygen between the upper and lower layers gradually decreases, and the oxygen difference is the largest in the middle and afternoon of the day. (3) Horizontal changes. Generally, due to the influence of wind, the dissolved oxygen in the downwind area is higher than that in the upwind area during the day, but the change in dissolved oxygen level in the early morning is opposite, with the upwind area having higher dissolved oxygen than the downwind area. (4) Seasonal changes. Low dissolved oxygen levels generally occur in summer and autumn seasons, especially during rainy and cloudy weather, with lower dissolved oxygen levels.

The dissolved oxygen in clean surface water is close to saturation. When there is a large proliferation of algae, dissolved oxygen may be supersaturated; When water is polluted by organic and inorganic reducing substances, the dissolved oxygen content will decrease or even approach zero. At this time, anaerobic bacteria will proliferate actively and water quality will deteriorate. When the dissolved oxygen in water is below 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 4mg/L. The content of dissolved oxygen in water can be used as an indirect indicator of organic pollution and its degree of self purification. The dissolved oxygen content in rivers, lakes, and reservoirs in China is mostly above 4mg/L, and some rivers south of the Yangtze River are generally higher, reaching 6-8 mg/L. In the process of wastewater biochemical treatment, dissolved oxygen is also an important control indicator.

Due to the significant relationship between the content of dissolved oxygen and factors such as atmosphere and temperature, specialized sampling bottles such as hydrogen peroxide bottles and dissolution bottles are required for the collection of dissolved oxygen samples. When sampling, be careful not to let the water sample come into contact with the air, and the sampling action should be gentle to minimize disturbance. When sampling, the sampling bottle must be filled and then the bottle stopper must be tightly closed, while being careful not to leave any air bubbles. Collect water samples from pipelines and water faucets, and use rubber hoses or other flexible hoses to guide the water along the bottle wall until it overflows. Continue collecting for a few minutes, then plug and seal tightly without leaving any bubbles. To prevent changes in dissolved oxygen in the water sample, the collected water sample must be fixed on site (by adding manganese sulfate and alkaline potassium iodide) or directly measured on site using an oxygen electrode.

The methods for determining dissolved oxygen in water include iodometric method and its modified method (GB7489-87) and oxygen electrode method (GB11913-89). Clean water can be measured using the iodine method; Polluted surface water and industrial wastewater must be analyzed using the modified iodometric method or oxygen electrode method. In addition, in order to achieve automatic monitoring of dissolved oxygen, the National Environmental Protection Administration has formulated technical requirements for dissolved oxygen (DO) water quality automatic analyzers (HJ/T99-2003).

（1） Iodometric method

The iodometric method is a benchmark method for determining dissolved oxygen in water. Without interference, this method is applicable to water samples with dissolved oxygen concentrations greater than 0.2mg/L and less than twice the saturation concentration of oxygen (about 20mg/L). Organic compounds that are prone to oxidation, such as tannic acid, humic acid, and lignin, can interfere with the measurement; Oxidizable sulfides such as thiourea can also cause interference, and when the water sample contains these substances, the oxygen electrode method should be used.

The principle of iodometric method is to add manganese sulfate and alkaline potassium iodide to the water sample. The dissolved oxygen in the water oxidizes divalent manganese to tetravalent manganese and generates hydroxide precipitation. After adding acid, the precipitate dissolves, and tetravalent manganese can oxidize iodide ions to release free iodine equivalent to the dissolved oxygen. Using starch as an indicator, the dissolved oxygen content can be calculated by titrating the released iodine with a standard solution of sodium thiosulfate. 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） Revised iodometric method

When using the iodometric method to determine dissolved oxygen in water samples, interference may occur if there are some reducing substances present in the water sample. At this point, some reagents can be added for correction, and commonly used methods include sodium azide correction and potassium permanganate correction.

1. Sodium azide correction method

The presence of nitrite in water samples can interfere with the determination of dissolved oxygen by iodometric method. Sodium azide can be used to decompose nitrite and then determine it by iodometric method. The reaction for decomposing nitrite is as follows:

2NaN3＋H2SO4＝2HN3+Na2SO4

HNO2＋NH3＝N2O＋N2＋H2O

Nitrite mainly exists in biochemically treated wastewater and river water. It can react with potassium iodide to release free iodine and produce positive interference

2HNO2＋2KI＋H2SO4＝K2SO4＋2H2O＋N2O2＋I2

If the reaction stops here, the introduced error is not significant; But when the water sample comes into contact with air, the newly dissolved oxygen will react with N2O2 to form nitrite:

2N2O2＋2H2O＋O2＝4HNO2

This cycle, constantly releasing iodine, will introduce significant errors.

When the content of trivalent iron ions in the water sample is high, interference with the measurement can be eliminated by adding potassium fluoride or using phosphoric acid instead of sulfuric acid for acidification. The measurement results are calculated according to the following formula:

In the formula: M - concentration of sodium thiosulfate standard solution, mol/L；

V - titration consumes the volume of standard solution of sodium thiosulfate, 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, and alkaline potassium iodide sodium azide solution should not be directly acidified to avoid the production of toxic azide acid mist.

2. Potassium permanganate correction method

This method is suitable for water samples containing a large amount of ferrous ions, without other reducing agents or organic matter. Use potassium permanganate to oxidize ferrous ions, eliminate interference, remove excess potassium permanganate with sodium oxalate solution, and mask the generated high valent iron ions with potassium fluoride. Other iodine measurement methods.

（3） Oxygen electrode method

The widely used dissolved oxygen electrode is a polytetrafluoroethylene film electrode, which is a typical oxygen electrode. According to its working principle, it can be divided into two types: polarographic type and primary battery type. The structure of the polarographic oxygen electrode is shown in Figure 3-27. It consists of a gold cathode, a silver chloride anode, a polytetrafluoroethylene film, a shell, and other components. Potassium chloride solution is filled into the electrode chamber, and a polytetrafluoroethylene film separates the electrolyte from the measured water sample. Dissolved oxygen diffuses through the film. When a fixed polarization voltage of 0.5-0.8V is applied between the two poles, dissolved oxygen in the water sample diffuses through the membrane and is reduced on the cathode, producing a diffusion current proportional to the oxygen concentration. The electrode reaction is as follows:

Figure 3-27 Structure of Dissolved Oxygen Electrode

１. Golden cathode; 2. Silver wire anode; 3. Thin film; 4. Kcl solution; 5. Shell

Cathode: O2＋2H2O＋4e＝4OH-

Anode: 4Ag+4Cl -=4AgCl+4e

The generated reduction current i can also be expressed as:

In the formula: K - proportionality constant;

N - number of electrons gained or lost in electrode reaction;

F - Faraday constant;

A - cathode area;

PM - permeability coefficient of the film;

L - thickness of the film;

C0- Partial pressure or concentration of dissolved oxygen.

It can be seen that when the experimental conditions are fixed, all other terms in the above equation except for c0 are constant values. Therefore, as long as the reduction current is measured, the concentration of dissolved oxygen in the water sample can be determined. Various dissolved oxygen meters work based on this principle (see Figure 3-28). When measuring, first calibrate the zero point with an anaerobic water sample, then calibrate the instrument scale value using chemical methods, and finally measure the water sample to directly display its dissolved oxygen concentration. The instrument is equipped with automatic or manual temperature compensation devices to compensate for measurement errors caused by temperature changes.

1. Polarized voltage source; 2. Dissolved oxygen electrode and measuring cell; 3. Operational amplifier; 4. Indicator table

The dissolved oxygen electrode method for measuring dissolved oxygen is not affected by the chromaticity, turbidity, and interfering substances in chemical titration of water samples; Quick and convenient, suitable for on-site measurement; Easy to achieve automatic continuous measurement. However, when the water sample contains substances such as algae, sulfides, carbonates, oil, etc., it can cause the film to clog or be damaged, and the film should be replaced in a timely manner.