What is dissolved oxygen?

The molecular oxygen in the air dissolved in water is called dissolved oxygen, and the content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of 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 dissolved oxygen content 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 index to measure the self-purification ability of water.

Which factors are related to the value of dissolved oxygen in water?

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 100kPa, the dissolved oxygen in pure water is about 9mg/L. Some organic compounds biodegrade under the action of aerobic bacteria, consuming dissolved oxygen in water. If the organic matter is calculated by carbon, according to C+O=CO, it can be known that every 12g of carbon consumes 32g of oxygen. When the dissolved oxygen value in the water drops to 5 mg/L, some fish have difficulty breathing.

There are usually two sources of dissolved oxygen: one source is when the dissolved oxygen in water is not saturated, the oxygen in the atmosphere seeps into the water body; the other source is the oxygen released by plants in the water through photosynthesis. Therefore, the dissolved oxygen in the water will be continuously replenished due to the incorporation of oxygen in the air and the photosynthesis of green aquatic plants. However, when the water body is polluted by organic matter, the oxygen consumption is serious, and the dissolved oxygen cannot be replenished in time, the anaerobic bacteria in the water body will multiply quickly, and the organic matter will cause the water body to become black and smelly due to corruption.

The dissolved oxygen value is a basis for studying the self-purification ability of water. The dissolved oxygen in the water is consumed, and it takes a short time to return to the original state, indicating that the water body has a strong self-purification ability, or the water body is not seriously polluted. Otherwise, it means that the water body is seriously polluted, the self-purification ability is weak, or even loses the self-purification ability.

Determination method of dissolved oxygen

Iodometric method

1. Principle: Manganese sulfate and basic potassium iodide are added to the water sample, and the dissolved oxygen in the water will oxidize low-valent manganese to a high-valent manganese, forming a brown precipitate of tetravalent manganese hydroxide. After acid addition, the hydroxide precipitate dissolves and reacts with iodide ions to release free iodine. Using starch as indicator, titrate the released iodine with sodium thiosulfate standard solution, and calculate the dissolved oxygen content according to the consumption of the titration solution.

2. Reagents:

① In manganese sulfate solution, no blue color should be produced when it encounters starch.

② Alkaline potassium iodide solution: Weigh 480g of manganese sulfate (MnSO·4HO) and dissolve it in water, dilute to 1000mL with water.

Add this solution to the acidified potassium iodide solution: weigh 500g of sodium hydroxide and dissolve it in 300 to 400mL of water, and weigh another 150g of potassium iodide and dissolve it in 200mL of water. Dilute to 1000mL. If there is precipitation, leave it overnight, pour out the supernatant, store it in a brown bottle, stop it tightly with a rubber stopper, and keep it away from light. After the solution is acidified, it should not turn blue when it encounters starch.

③ (1+5) sulfuric acid solution: 1 part of concentrated sulfuric acid + 5 parts of water, mix and shake well.

④ 1% starch solution: Weigh 1g of soluble starch, make a paste with a small amount of water, and then dilute to 100mL with freshly boiled water. After cooling, add 0.1g salicylic acid or 0.4g zinc chloride for corrosion protection.

⑤ 0.02500mol/L Potassium dichromate standard solution: Weigh 1.2258g of potassium dichromate dried at 105~110°C for 2 hours and cooled, dissolve in water, transfer to a 1000mL volumetric flask, dilute with water to the marked line, shake uniform.

⑥Sodium thiosulfate solution: Weigh 3.2g of sodium thiosulfate (NaSO·5HO) and dissolve it in boiled and cooled water, add 0.2g of sodium carbonate, dilute to 1000mL with water, store in a brown bottle, and use 0.02500mol before use /L Potassium dichromate standard solution for calibration.

⑦Sulfuric acid, pH=1.84.

3. Determination steps:

①Fixation of dissolved oxygen: Insert a pipette under the liquid surface of the dissolved oxygen bottle, add 1mL of manganese sulfate solution and 2mL of alkaline potassium iodide solution, cap the bottle, invert and mix several times, and let stand. Usually fixed at the sampling site.

②Open the bottle stopper and immediately insert 2.0mL of sulfuric acid into the liquid with a straw. Cover the cork well, mix and shake upside down until the sediment is completely dissolved, and put it in a dark place for 5 minutes.

③Pipe 100.00mL of the above solution into a 250mL Erlenmeyer flask, titrate with sodium thiosulfate standard solution until the solution turns light yellow, add 1mL of starch solution, continue titrating until the blue just fades, and record the amount of sodium thiosulfate solution.

4. Calculation:

Dissolved oxygen (O2, mg/L)=M*V*8000/100 In the above formula: M—concentration of sodium thiosulfate standard solution (mol/L);

5. Matters needing attention:

①When the reagent is added, care should be taken not to contact it with the air, so as not to bring oxygen in the air into the sample and affect the measurement.

②Pay attention to the time of adding starch indicator. The solution should be titrated from brown to light yellow before adding starch indicator, otherwise the end point will appear repeatedly and it is difficult to judge.

③The suspended matter in the sample will adsorb and precipitate iodine, which will lower the result. At this time, it is necessary to use alum to hydrolyze under alkaline conditions in advance, and then measure the dissolved oxygen in the supernatant after the precipitation.

④ When the water sample contains nitrite, it will interfere with the determination. Sodium acid can be added to decompose the nitrite in the water to eliminate the interference. The method of adding it is to add sodium acid to an alkaline potassium iodide solution in advance.

⑤ If the Fe content in the water sample reaches 100-200 mg/L, add 1 ml of 40% potassium fluoride solution to eliminate the interference.

⑥If the water sample contains oxidizing substances (such as free chlorine, etc.), a considerable amount of sodium thiosulfate should be added in advance to remove it.

Dissolved oxygen meter method

The dissolved oxygen meter consists of two parts: a sensor and a display instrument. The sensing part of dissolved oxygen analyzer is composed of a gold electrode (cathode) and silver electrode (anode) and potassium chloride or potassium hydroxide electrolyte. Oxygen diffuses into the electrolyte through the membrane and forms a measurement circuit with gold electrode and silver electrode. When a polarization voltage of 0.6~0.8V is applied to the electrode of the dissolved oxygen analyzer, oxygen diffuses through the membrane, the cathode releases electrons, and the anode accepts electrons to generate current. The whole reaction process is:

Anode: Ag+Cl→AgCl+2e

Cathode: O+2HO+4e→4OH

According to Faraday's law: the current flowing through the electrode of the dissolved oxygen analyzer is proportional to the oxygen partial pressure, and there is a linear relationship between the current and the oxygen concentration when the temperature is constant. The method does not require reagents and is easy to operate, and the color and turbidity of the sample do not affect the determination. The dissolved oxygen analyzer should be calibrated before each measurement to reduce instrument errors. Since the oxygen in the water reacts on the cathode and is consumed during the measurement, the water sample around the electrode must be kept stirred to supplement the oxygen. If the measurement is static, the result will be low. At the same time, the temperature has a great influence on the measurement results, so the water temperature should be measured while measuring the dissolved oxygen in the water sample.

3 related extensions

What is biochemical oxygen demand?

The amount of dissolved oxygen in water consumed by microorganisms decomposing organic matter in surface water is called biochemical oxygen demand, usually recorded as BOD, and the common unit is mg/L. Generally, under the action of microorganisms, the degradation process of organic matter can be divided into two stages. The first stage is the process of converting organic matter into carbon dioxide, ammonia and water, and the second stage is that ammonia is further under the action of nitrifying bacteria and nitrifying bacteria. Converted to nitrite and nitrate, the so-called nitrification process. BOD generally refers to the oxygen consumption in the first stage of biochemical reactions.

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, it takes an infinite time to complete the biological oxidation of organic matter, but for practical applications, it can be considered that the reaction can be It is 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 values comparable, the dissolved oxygen consumption was measured after culturing for five days at 20°C as a standard method, which is called the five-day biochemical oxygen demand and expressed as BOD5. BOD reflects the total amount of organic matter in water that can be decomposed by microorganisms, expressed in milligrams of dissolved oxygen consumed per liter of water. BOD less than 1mg/L means the water body is clean; greater than 3-4mg/L means polluted by organic matter. However, the measurement time of BOD is long; it is difficult to accurately measure the highly toxic wastewater due to the inhibition of microbial activities.

What is chemical oxygen demand?

The amount of oxidant consumed in the process of chemical oxidation of oxidizable substances in water under specified conditions is expressed in milligrams of oxygen consumed per liter of water sample, and is usually recorded as Chemical Oxygen Demand (COD). During COD determination, organic matter is oxidized to carbon dioxide and water. The difficulty of chemical oxidation reaction of various organic substances in water is different, so chemical oxygen demand only means the sum of the oxygen demand of oxidizable substances in water under specified conditions.

Currently, the commonly used methods for measuring COD are KMnO and KCrO methods. The former is used to measure clean water samples, and the latter is used to measure heavily polluted water samples and industrial wastewater. The results of the same water sample measured by the above two methods are different, so the determination method should be indicated when reporting the measurement results of chemical oxygen demand.

Compared with BOD, the measurement of COD is not limited by water quality conditions, and the measurement time is short. However, COD cannot distinguish between biooxidizable and difficult to biooxidize organic matter, and cannot indicate the amount of organic matter that microorganisms can oxidize, and chemical oxidants not only cannot oxidize all organic matter, but also oxidize some reductive inorganic matter. Therefore, it is more appropriate to use BOD as an indicator of the degree of organic pollution. When BOD cannot be measured 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 water to become dissolved oxygen increases significantly, making the dissolved oxygen in the water body supersaturated. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of 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 dissolved oxygen content in the water.

However, water conservancy projects will cause supersaturation of dissolved oxygen. For example, under the conditions of high dams and large reservoirs, when the discharge structure overflows or the dam discharges through the flood discharge hole, the process of water falling is accompanied by violent water-air exchange, and the dissolved gas content in the downstream water body is often reduced due to severe aeration. Significantly increased, resulting in a farther downstream range, thereby causing adverse effects and damage to aquatic organisms, especially fish.

What are the common application fields of dissolved oxygen sensors?

1. Aquaculture

In aquaculture there are freshwater (currently factory farming) and offshore mariculture methods. Dissolved oxygen is a parameter that must be measured in aquaculture. When the dissolved oxygen concentration in the water body is less than (3~4) mg/L, the fish will have difficulty breathing; if it continues to decrease, it will suffocate and die. At this time, it is necessary to turn on the aerator to supplement the oxygen in the water body. When measuring dissolved oxygen in mariculture, salinity compensation is required to ensure the accuracy of the measurement. In the instrument without automatic salinity compensation, the salinity of seawater is generally measured with a hydrometer first, and the data is manually input for measurement.

2. Water source monitoring, sewage treatment and other industries

In order to ensure the quality of the water value, the waterworks detect the dissolved oxygen in the water entering the waterworks. The sewage treatment plant also measures the dissolved oxygen of the sewage entering the plant for treatment. During the sewage treatment process of the sewage treatment plant, in order to ensure the normal life of microorganisms/bacteria in the water body and the effect of water treatment, the dissolved oxygen content of the sewage pool must be controlled within a certain range. One of the parameters to be measured. In industries such as sewage treatment, it is necessary to measure the biochemical oxygen demand BOD5 of the water body to understand the effect of sewage treatment. Dissolved oxygen instrument is one of the methods to measure the change of dissolved oxygen before and after water treatment. Therefore, dissolved oxygen analyzers are often used as a detection tool for measuring changes in dissolved oxygen in BOD5.

3. Environmental monitoring

Environmental Water Dissolved oxygen in rivers, rivers, lakes, and sea water is one of the important indicators to measure the quality of water, and it is a parameter that must be measured. The amount of dissolved oxygen in the water body reflects the quality of the water body, and the reduction of dissolved oxygen in the water body is the main reason for the water body to stink. Therefore, water quality testing stations at the national, city, prefecture, county, and district levels, as well as the testing stations of important rivers, rivers, and lakes, must be equipped with dissolved oxygen measuring instruments.

4. Application in the fields of education and scientific research

For example, add a layer of immobilized glucose oxidase membrane that is sensitive to changes in glucose concentration outside the oxygen sensor, and use the oxygen sensor to manufacture a glucose content analyzer.

An instrument for measuring biochemical oxygen demand can be manufactured by adding a special layer of bacteria/microbial film that is sensitive to changes in BOD concentration outside the oxygen sensor. Adding membranes made of different kinds of bacteria/microorganisms on the oxygen electrode can be used for BOD measurement in different industries.

To sum up, dissolved oxygen sensors are essential automated process detection tools in aquaculture, water source monitoring, sewage treatment, and environmental monitoring industries. Supmea is a leading manufacturer and supplier of dissolved oxygen meters, located in Hangzhou, China, with offices around in more than 50 cities. The dissolved oxygen meters manufactured by Supmea are of high-accuracy and high-quality. Supmea's dissolved oxygen meters are exported to 127 countries and regions such as Germany, Russia, Malaysia, Singapore, Vietnam, and Pakistan, and are well-received by users. If you want to know more about the product requirements of dissolved oxygen meters and other process control instruments, please do not hesitate to contact us and we will reply to you in a timely manner.

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