What Is Dissolved Oxygen?
Dissolved oxygen is closely related to the partial pressure of oxygen in the air, atmospheric pressure, water temperature, and water quality. At 20°C and 100 kPa, pure water contains approximately 9 mg/L of dissolved oxygen. Some organic compounds undergo biodegradation under the action of aerobic bacteria, consuming the dissolved oxygen in the water. If organic matter is calculated as carbon, according to C+O₂=CO₂, every 12 g of carbon consumes 32 g of oxygen. When the dissolved oxygen value drops to 5 mg/L, some fish species experience difficulty breathing.
Dissolved oxygen usually has two sources: one is the infiltration of oxygen from the atmosphere into the water body when the dissolved oxygen in the water is unsaturated; the other is the oxygen released by aquatic plants through photosynthesis. Therefore, the dissolved oxygen in water is continuously replenished through the dissolution of oxygen from the air and the photosynthesis of green aquatic plants. But when the water body is polluted by organic matter and oxygen consumption is severe, dissolved oxygen cannot be replenished in time, and the anaerobic bacteria in the water body will multiply rapidly, and the organic matter will turn the water black and foul due to putrefaction.
The dissolved oxygen value is a basis for studying the self-purification capacity of water. If the dissolved oxygen in the water is consumed and the time required to recover to the initial state is short, it indicates that the water body has a strong self-purification capacity, or in other words, the water pollution is not serious. Otherwise, it indicates serious water pollution, weak self-purification capacity, or even loss of self-purification ability.
1. Principle: Add manganese sulfate and alkaline potassium iodide to the water sample; the dissolved oxygen in the water oxidizes the low-valent manganese to high-valent manganese, generating a brown precipitate of tetravalent manganese hydroxide. After adding acid, the hydroxide precipitate dissolves and reacts with iodide ions to release free iodine. Using starch as an indicator, titrate the released iodine with a standard sodium thiosulfate solution; calculate the dissolved oxygen content from the consumption of the titrant.
① The manganese sulfate solution must not produce a blue color with starch.
② Alkaline potassium iodide solution: Weigh 480 g of manganese sulfate (MnSO₄·4H₂O), dissolve in water, and dilute with water to 1000 mL.
② Note the timing of adding the starch indicator. The starch indicator should be added only after the solution is titrated from brown to pale yellow; otherwise the endpoint will oscillate and be difficult to judge.
③ (1+5) sulfuric acid solution: 1 part concentrated sulfuric acid + 5 parts water; mix and shake well.
④ 1% starch solution: Weigh 1 g of soluble starch, make into a paste with a small amount of water, then dilute with freshly boiled water to 100 mL. After cooling, add 0.1 g of salicylic acid or 0.4 g of zinc chloride as a preservative.
⑤ 0.02500 mol/L potassium dichromate standard solution: Weigh 1.2258 g of potassium dichromate dried at 105–110°C for 2 h and cooled, dissolve in water, transfer to a 1000 mL volumetric flask, dilute with water to the mark, and mix well.
⑥ Sodium thiosulfate solution: Weigh 3.2 g of sodium thiosulfate (Na₂S₂O₃·5H₂O), dissolve in boiled and cooled water, add 0.2 g of sodium carbonate, dilute with water to 1000 mL, store in a brown bottle, and standardize with 0.02500 mol/L potassium dichromate standard solution before use.
⑦ Sulfuric acid, pH=1.84.
① Fixation of dissolved oxygen: Insert a pipette below the liquid surface of the dissolved-oxygen bottle, add 1 mL of manganese sulfate solution and 2 mL of alkaline potassium iodide solution, stopper tightly, invert and mix several times, and let stand. Fixation is generally performed on-site at the sampling point.
② Open the stopper, immediately insert a pipette below the liquid surface and add 2.0 mL of sulfuric acid. Stopper tightly, invert and mix until the precipitate is completely dissolved, then place in the dark and let stand for 5 min.
③ Pipette 100.00 mL of the above solution into a 250 mL conical flask, titrate with the standard sodium thiosulfate solution to a pale yellow color, add 1 mL of starch solution, continue titrating until the blue color just disappears, and record the volume of sodium thiosulfate solution used.
Dissolved oxygen (O₂, mg/L) = M×V×8000/100
In the above formula: M — concentration of the standard sodium thiosulfate solution (mol/L);
V — volume of standard sodium thiosulfate solution consumed by titration (mL).
① When adding reagents, care should be taken not to contact the air, so as not to bring oxygen from the air into the sample and affect the determination.
This solution is added to the acidified potassium iodide solution: Weigh 500 g of sodium hydroxide and dissolve in 300–400 mL of water; separately weigh 150 g of potassium iodide and dissolve in 200 mL of water; after the sodium hydroxide solution cools, combine the two solutions, mix well, dilute with water to 1000 mL. If there is a precipitate, let stand overnight, decant the supernatant, store in a brown bottle, and seal tightly with a rubber stopper, protected from light. After acidification, this solution should not turn blue when it encounters starch.
③ Suspended matter in the sample will adsorb and precipitate iodine, causing low results. In this case, it is necessary to first hydrolyze with alum under alkaline conditions, and after the precipitate precipitates, measure the dissolved oxygen in the supernatant.
④ When the water sample contains nitrite, it will interfere with the determination; sodium azide can be added to decompose the nitrite in the water and eliminate the interference. The method of addition is to pre-add sodium azide to the alkaline potassium iodide solution.
⑤ If the water sample contains Fe²⁺ at 100–200 mg/L, add 1 mL of 40% potassium fluoride solution to eliminate the interference.
⑥ If the water sample contains oxidizing substances (such as free chlorine, etc.), an equivalent amount of sodium thiosulfate should be added in advance to remove them. [1]
The dissolved oxygen meter consists of two parts: a sensor and a display instrument. The sensing part of the dissolved oxygen analyzer consists of a gold electrode (cathode) and a silver electrode (anode) and potassium chloride or potassium hydroxide electrolyte; oxygen diffuses through a membrane into the electrolyte and forms a measurement circuit with the gold and silver electrodes. When a polarization voltage of 0.6–0.8 V is applied to the dissolved-oxygen analyzer electrodes, oxygen diffuses through the membrane, the cathode releases electrons, the anode accepts electrons, and a current is generated. The overall reaction process is:
Anode: Ag+Cl→AgCl+2e⁻
Cathode: O₂+2H₂O+4e⁻→4OH⁻
According to Faraday's law: the current flowing through the dissolved-oxygen analyzer electrodes is proportional to the oxygen partial pressure; at constant temperature, there is a linear relationship between current and oxygen concentration. This method requires no reagents and is easy to operate, and the color and turbidity of the sample do not affect the determination. The dissolved oxygen meter should be calibrated before each measurement to reduce instrument error. Since oxygen in the water is consumed at the cathode during measurement, the water sample around the electrode must be kept agitated to replenish oxygen; if measured statically, the result will be low. At the same time, temperature has a great influence on the measurement results, so the water temperature should be measured simultaneously when determining the dissolved oxygen of the water sample. [2]
The amount of dissolved oxygen consumed by microorganisms in the process of decomposing organic matter in surface water bodies is called biochemical oxygen demand, usually denoted as BOD, with the common unit being milligrams per liter. Generally, the degradation of organic matter by microorganisms can be divided into two stages: the first stage is the process of organic matter being converted into carbon dioxide, ammonia, and water; the second stage is the further conversion of ammonia under the action of nitrosating and nitrifying bacteria into nitrite and nitrate, the so-called nitrification process. BOD generally refers to the oxygen consumption of the first-stage biochemical reaction.
The speed and degree of microbial decomposition of organic matter are related to temperature and time; the most suitable temperature is 15–30°C. Theoretically, the complete biological oxidation of organic matter requires an infinite amount of time, but for practical applications, the reaction can be considered complete within 20 days, called BOD20. According to practical experience, the BOD measured after 5 days of culture accounts for about 70–80% of the total BOD and can represent the oxygen consumption of organic matter in water.
To make the BOD value comparable, the standard method adopts the determination of dissolved oxygen consumption after 5 days of culture at 20°C, called the five-day biochemical oxygen demand, denoted as BOD5. BOD reflects the total amount of organic matter in the water body that can be decomposed by microorganisms, expressed as the milligrams of dissolved oxygen consumed per liter of water. BOD < 1 mg/L indicates clean water; > 3–4 mg/L indicates organic pollution. However, the BOD determination takes a long time; for highly toxic wastewater, the microbial activity is inhibited, making accurate determination difficult.
The amount of oxidant consumed during the chemical oxidation of oxidizable substances in water under specified conditions, expressed as the milligrams of oxygen consumed per liter of water sample, is usually denoted as Chemical Oxygen Demand (COD). During COD determination, organic matter is oxidized to carbon dioxide and water. The ease of chemical oxidation reactions of various organic substances in water differs, so COD only indicates the total oxygen demand of oxidizable substances in water under specified conditions. The commonly used methods for determining COD are the KMnO₄ and K₂Cr₂O₇ methods; the former is used for relatively clean water samples, and the latter for heavily polluted water samples and industrial wastewater. The results of the two methods on the same water sample are different, so the determination method should be noted when reporting COD results.
Compared with COD, the determination of COD is not limited by water quality conditions and takes a short time. However, COD cannot distinguish between organics that can be biologically oxidized and those that are difficult to biodegrade, nor can it indicate the amount of organics that microorganisms can oxidize; moreover, chemical oxidants not only fail to oxidize all organic matter but also oxidize some reducing inorganic substances. Therefore, BOD is more appropriate as an indicator of the degree of organic pollution; when water quality conditions prevent BOD determination, COD can be used as a substitute.
A phenomenon in which dissolved oxygen becomes significantly increased due to violent aeration and other reasons causing molecular oxygen from the air to dissolve in water, resulting in supersaturation of dissolved oxygen in the water body. The dissolved oxygen content in water is closely related to the partial pressure of oxygen in the air and the water temperature. Under natural conditions, the oxygen content in the air changes little, so water temperature is the main factor. The lower the water temperature, the higher the dissolved oxygen content in the water.
However, water conservancy projects can cause supersaturation of dissolved oxygen. For example, under the conditions of high dams and large reservoirs, when the discharge structure releases flow or the dam releases flow through flood discharge holes, the water flow is accompanied by violent air-water exchange during the falling process, often causing a significant increase in dissolved gas content in the downstream water body, causing adverse effects and harm to aquatic organisms, especially fish, over a wider downstream range.
