Molecular oxygen in the air dissolved in water is called dissolved oxygen. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of the water. Under natural conditions, the oxygen content in the air does not change much, so the water temperature is the main factor. The lower the water temperature, the higher the content of dissolved oxygen in the water. Molecular oxygen dissolved in water is called dissolved oxygen, usually recorded as DO, 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 bodies.
Product Introduction
Dissolved oxygen is closely related to the partial pressure of oxygen in the air, atmospheric pressure, water temperature and water quality. At 20℃ and 100kPa, there is about 9mg/L dissolved oxygen in pure water. Some organic compounds are biodegraded by aerobic bacteria, which consumes dissolved oxygen in the water. If organic matter is calculated as carbon, according to C+O2=CO2, 32g of oxygen is consumed for every 12g of carbon. When the dissolved oxygen value in the water drops to 5mg/L, some fish have difficulty breathing.
Dissolved oxygen usually comes from two sources: one is when the dissolved oxygen in the water is not saturated, the oxygen in the atmosphere penetrates into the water body; the other is the oxygen released by aquatic plants through photosynthesis. Therefore, the dissolved oxygen in the water will be continuously replenished due to the dissolution of oxygen in the air and the photosynthesis of green aquatic plants. However, when the water body is polluted by organic matter, oxygen consumption is serious, and dissolved oxygen cannot be replenished in time, anaerobic bacteria in the water body will multiply quickly, and the organic matter will turn the water body black and smelly due to corruption.
The dissolved oxygen value is a basis for studying the self-purification ability of water. If the dissolved oxygen in the water is consumed and it takes a short time to return to its initial state, it means that the water has a strong self-purification ability, or the water pollution is not serious. Otherwise, it means that the water is seriously polluted, the self-purification ability is weak, or even lost.
Determination method
Iodometric method
1. Principle: Manganese sulfate and alkaline potassium iodide are added to the water sample. The dissolved oxygen in the water oxidizes low-valent manganese into 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, the released iodine is titrated with sodium thiosulfate standard solution, and the dissolved oxygen content is calculated based on the consumption of the titration solution.
2. Reagents:
① In the manganese sulfate solution, no blue color should be produced when it comes into contact with starch.
② Alkaline potassium iodide solution: weigh 480g manganese sulfate (MnSO4·4H2O) and dissolve it in water, and dilute it to 1000mL with water.
Add this solution to the acidified potassium iodide solution: weigh 500g sodium hydroxide and dissolve it in 300 to 400mL water, and weigh 150g potassium iodide and dissolve it in 200mL water. After the sodium hydroxide solution cools down, combine the two solutions, mix them well, and dilute them to 1000mL with water. If there is precipitation, after leaving it overnight, pour out the supernatant, store it in a brown bottle, plug it with a rubber stopper, and keep it away from light. After this solution is acidified, it should not turn blue when it comes into contact with starch.
③ (1+5) sulfuric acid solution: 1 part concentrated sulfuric acid + 5 parts water, mix and shake well.
④ 1% starch solution: weigh 1g soluble starch, make it into a paste with a small amount of water, and then dilute it to 100mL with freshly boiled water. After cooling, add 0.1g salicylic acid or 0.4g zinc chloride for corrosion protection.
⑤0.02500mol/L potassium dichromate standard solution: weigh 1.2258g of potassium dichromate dried at 105~110℃ for 2h and cooled, dissolve in water, transfer to a 1000mL volumetric flask, dilute with water to the mark, and shake well.
⑥Sodium thiosulfate solution: weigh 3.2g of sodium thiosulfate (Na2S2O3·5H2O) and dissolve in boiled and cooled water, add 0.2g of sodium carbonate, dilute with water to 1000mL, store in a brown bottle, and calibrate with 0.02500mol/L potassium dichromate standard solution before use.
⑦Sulfuric acid, pH=1.84.
3. Determination steps:
① Fixation of dissolved oxygen: insert a pipette under the liquid surface of the dissolved oxygen bottle, add 1mL of manganese sulfate solution, 2mL of alkaline potassium iodide solution, cover the bottle stopper, invert and mix several times, and let it stand. It is usually fixed at the sampling site.
② Open the bottle stopper, immediately insert a pipette under the liquid surface and add 2.0mL of sulfuric acid. Cover the bottle stopper, invert and mix and shake until the precipitate is completely dissolved, and let it stand in the dark for 5 minutes.
③ Pipette 100.00mL of the above solution into a 250mL conical flask, titrate with sodium thiosulfate standard solution until the solution is light yellow, add 1mL of starch solution, continue to titrate until the blue color just fades, and record the amount of sodium thiosulfate solution used.
4. Calculation:
Dissolved oxygen (O2, mg/L) = M*V*8000/100
In the above formula: M——Concentration of sodium thiosulfate standard solution (mol/L);
V——Volume of sodium thiosulfate standard solution consumed in titration (mL).
5. Precautions:
①When adding the reagent, be careful not to contact with the air to avoid bringing oxygen in the air into the sample and affecting the determination.
②Pay attention to the time of adding the starch indicator. The starch indicator should be added when the solution is titrated from brown to light yellow, otherwise the end point will be repeated and difficult to judge.
③The suspended matter in the sample will adsorb and precipitate iodine, making the result low. At this time, alum should be used for hydrolysis under alkaline conditions in advance, and the dissolved oxygen in the upper clear liquid should be measured after the precipitation is precipitated.
④When the water sample contains nitrite, it will interfere with the determination. Sodium azide can be added to decompose the nitrite in the water to eliminate the interference. The method of adding is to add sodium azide to the alkaline potassium iodide solution in advance.
⑤ If the water sample contains Fe2+ up to 100-200 mg/L, 1 mL of 40% potassium fluoride solution can be added to eliminate interference.
⑥ If the water sample contains oxidizing substances (such as free chlorine, etc.), a considerable amount of sodium thiosulfate should be added in advance to remove them.
Dissolved oxygen meter method
The dissolved oxygen meter consists of two parts: the sensor and the display instrument. The sensor part of the dissolved oxygen analyzer is composed of a gold electrode (cathode) and a silver electrode (anode) and a potassium chloride or potassium hydroxide electrolyte. Oxygen diffuses through the membrane into the electrolyte and forms a measurement circuit with the gold electrode and the silver electrode. When a polarization voltage of 0.6~0.8V is applied to the dissolved oxygen analyzer electrode, oxygen diffuses through the membrane, the cathode releases electrons, and the anode accepts electrons to generate current. The entire 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 electrode is proportional to the oxygen partial pressure, and the current and oxygen concentration are linearly related when the temperature remains unchanged. This method does not require reagents and is easy to operate, and the sample color and turbidity do not affect the measurement. The dissolved oxygen meter should be calibrated before each measurement to reduce instrument errors. Since oxygen in the water is consumed by the reaction on the cathode during the measurement, the water sample around the electrode must be stirred to replenish oxygen. If it is measured statically, the result will be low. At the same time, temperature has a greater impact on the measurement result, so the water temperature should be measured while measuring the dissolved oxygen in the water sample.
Related extensions
Biochemical oxygen demand
The amount of dissolved oxygen consumed by microorganisms in surface water during the decomposition of organic matter is called biochemical oxygen demand, usually recorded as BOD, and the common unit is mg/L. Generally, the degradation process of organic matter under the action of microorganisms can be divided into two stages. The first stage is the process of organic matter being converted into carbon dioxide, ammonia and water, and the second stage is the further conversion of ammonia into nitrite and nitrate under the action of nitrite bacteria and nitrifying bacteria, which is the so-called nitrification process. BOD generally refers to the oxygen consumption of the first stage of 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℃. Theoretically, it takes an infinite amount of time to complete the biological oxidation of organic matter. However, for practical applications, it can be considered that the reaction can be completed within 20 days, which is called BOD20. According to practical experience, the BOD measured after 5 days of cultivation accounts for about 70-80% of the total BOD, which can represent the oxygen consumption of organic matter in water.
In order to make the BOD value comparable, the dissolved oxygen consumption after five days of cultivation at 20℃ is used as the standard method, which is called five-day biochemical oxygen demand and expressed as BOD5. BOD reflects the total amount of organic matter that can be decomposed by microorganisms in the water body, expressed in milligrams of dissolved oxygen consumed per liter of water. BOD less than 1mg/L indicates that the water body is clean; greater than 3-4mg/L indicates that it is polluted by organic matter. However, the determination time of BOD is long; it is difficult to accurately determine the toxic wastewater because the microbial activity is inhibited.
Chemical Oxygen Demand
The amount of oxidant consumed by oxidizable substances in water under specified conditions during chemical oxidation, expressed as milligrams of oxygen consumed per liter of water sample, is usually recorded as Chemical Oxygen Demand (COD). During the COD determination process, organic matter is oxidized into carbon dioxide and water. The difficulty of chemical oxidation reactions of various organic matter in water is different, so the chemical oxygen demand only represents the sum of the oxygen demand of oxidizable substances in water under specified conditions. The commonly used methods for determining chemical oxygen demand are KMnO4 and K2Cr2O7. The former is used to determine cleaner water samples, and the latter is used for heavily polluted water samples and industrial wastewater. The results of the same water sample determined by the above two methods are different, so the determination method should be noted when reporting the results of chemical oxygen demand.
Compared with BOD, the determination of COD is not limited by water quality conditions and the determination time is short. However, COD cannot distinguish between organic matter that can be oxidized by organisms and organic matter that is difficult to be oxidized by organisms, and cannot indicate the amount of organic matter that can be oxidized by microorganisms. Moreover, chemical oxidants cannot oxidize all organic matter, but will also oxidize some reducing inorganic matter. Therefore, it is more appropriate to use BOD as an indicator of the degree of organic pollution. When BOD determination cannot be performed due to water quality conditions, COD can be used instead.
Dissolved oxygen supersaturation
Due to violent aeration and other reasons, the amount of molecular oxygen in the air dissolved in the water to become dissolved oxygen increases significantly, making the dissolved oxygen in the water supersaturated. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of the water. Under natural conditions, the oxygen content in the air does not change much, 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, when there is overflow in a water release building under a high dam or large reservoir, or when a dam releases water through a flood discharge hole, the falling process of the water flow is accompanied by severe water and gas exchange, which often causes the dissolved gas content in the downstream water to increase due to severe aeration. Significant increase, causing further downstream range, thus causing adverse effects and harm to aquatic life, especially fish.
