Chemical oxygen demand is also called chemical oxygen demand (chemical oxygen demand), referred to as COD. It is the use of chemical oxidants (such as potassium permanganate) to oxidize and decompose oxidizable substances in water (such as organic matter, nitrite, ferrous salt, sulfide, etc.), and then calculate the oxygen consumption based on the amount of residual oxidant. Like biochemical oxygen demand (BOD), it is an important indicator of water pollution. The unit of COD is ppm or mg/L. The smaller the value, the lighter the water pollution.
The reducing substances in water include various organic matter, nitrite, sulfide, ferrous salt, etc. But the main one is organic matter. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the amount of organic matter in water. The larger the chemical oxygen demand, the more serious the water pollution by organic matter. The determination of chemical oxygen demand (COD) varies with the determination of reducing substances in water samples and the determination method. The most commonly used methods at present are the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method. The potassium permanganate (KMnO4) method has a low oxidation rate, but is relatively simple. It can be used to determine the relative comparative value of the organic content in water samples and clean surface water and groundwater samples. The potassium dichromate (K2Cr2O7) method has a high oxidation rate and good reproducibility. It is suitable for determining the total amount of organic matter in water samples in wastewater monitoring.
Organic matter is very harmful to industrial water systems. Water containing a large amount of organic matter will contaminate ion exchange resins when passing through the desalination system, especially anion exchange resins, which will reduce the exchange capacity of the resin. Organic matter can be reduced by about 50% after pretreatment (coagulation, clarification and filtration), but it cannot be removed in the desalination system, so it is often brought into the boiler through the feed water, which reduces the pH value of the boiler water. Sometimes organic matter may also be brought into the steam system and condensate water, which will reduce the pH and cause system corrosion. High organic matter content in the circulating water system will promote microbial reproduction. Therefore, whether for desalination, boiler water or circulating water system, the lower the COD, the better, but there is no unified limiting index. When COD (KMnO4 method) > 5mg/L in the circulating cooling water system, the water quality has begun to deteriorate.
Chemical oxygen demand (COD) is a measurement indicator of the degree to which water is rich in organic matter, and it is also one of the important indicators for measuring the degree of water pollution. With the development of industrialization and the increase in population, water bodies are becoming more and more polluted, and the development of COD detection has gradually improved.
The origin of COD detection can be traced back to the 1850s, when water pollution problems had attracted people’s attention. Initially, COD was used as an indicator of acidic beverages to measure the concentration of organic matter in beverages. However, since a complete measurement method had not been established at that time, there was a large error in the determination results of COD.
In the early 20th century, with the advancement of modern chemical analysis methods, the detection method of COD was gradually improved. In 1918, German chemist Hasse defined COD as the total amount of organic matter consumed by oxidation in an acidic solution. Subsequently, he proposed a new COD determination method, which is to use a high-concentration chromium dioxide solution as an oxidant. This method can effectively oxidize organic matter into carbon dioxide and water, and measure the consumption of oxidants in the solution before and after oxidation to determine the COD value.
However, the shortcomings of this method have gradually emerged. First, the preparation and operation of the reagents are relatively complicated, which increases the difficulty and time-consuming of the experiment. Second, high-concentration chromium dioxide solutions are harmful to the environment and are not conducive to practical applications. Therefore, subsequent studies have gradually sought a simpler and more accurate COD determination method.
In the 1950s, Dutch chemist Friis invented a new COD determination method, which uses high-concentration persulfuric acid as an oxidant. This method is simple to operate and has high accuracy, which greatly improves the efficiency of COD detection. However, the use of persulfuric acid also has certain safety hazards, so it is still necessary to pay attention to the safety of operation.
Subsequently, with the rapid development of instrumentation technology, the COD determination method has gradually achieved automation and intelligence. In the 1970s, the first COD automatic analyzer appeared, which can realize the fully automatic processing and detection of water samples. This instrument not only improves the accuracy and stability of COD determination, but also greatly improves work efficiency.
With the enhancement of environmental awareness and the improvement of regulatory requirements, the detection method of COD is also being continuously optimized. In recent years, the development of photoelectric technology, electrochemical methods and biosensor technology has promoted the innovation of COD detection technology. For example, photoelectric technology can determine the COD content in water samples by the change of photoelectric signals, with shorter detection time and simpler operation. The electrochemical method uses electrochemical sensors to measure COD values, which has the advantages of high sensitivity, rapid response and no need for reagents. Biosensor technology uses biological materials to specifically detect organic matter, which improves the accuracy and specificity of COD determination.
COD detection methods have undergone a development process from traditional chemical analysis to modern instrumentation, photoelectric technology, electrochemical methods and biosensor technology in the past few decades. With the advancement of science and technology and the increase in demand, COD detection technology is still being improved and innovated. In the future, it can be foreseen that as people pay more attention to environmental pollution issues, COD detection technology will develop further and become a faster, more accurate and reliable water quality detection method.
At present, laboratories mainly use the following two methods to detect COD.
1. COD determination method
Potassium dichromate standard method, also known as reflux method (National Standard of the People’s Republic of China)
(I) Principle
Add a certain amount of potassium dichromate and catalyst silver sulfate to the water sample, heat and reflux for a certain period of time in a strong acidic medium, part of the potassium dichromate is reduced by the oxidizable substances in the water sample, and the remaining potassium dichromate is titrated with ammonium ferrous sulfate. The COD value is calculated based on the amount of potassium dichromate consumed.
Since this standard was formulated in 1989, there are many disadvantages in measuring it with the current standard:
1. It takes too much time, and each sample needs to be refluxed for 2 hours;
2. The reflux equipment occupies a large space, making batch determination difficult;
3. The analysis cost is high, especially for silver sulfate;
4. During the determination process, the waste of reflux water is amazing;
5. Toxic mercury salts are prone to secondary pollution;
6. The amount of reagents used is large, and the cost of consumables is high;
7. The test process is complicated and not suitable for promotion.
(II) Equipment
1. 250mL all-glass reflux device
2. Heating device (electric furnace)
3. 25mL or 50mL acid burette, conical flask, pipette, volumetric flask, etc.
(III) Reagents
1. Potassium dichromate standard solution (c1/6K2Cr2O7=0.2500mol/L)
2. Ferrocyanate indicator solution
3. Ammonium ferrous sulfate standard solution [c(NH4)2Fe(SO4)2·6H2O≈0.1mol/L] (calibrate before use)
4. Sulfuric acid-silver sulfate solution
Potassium dichromate standard method
(IV) Determination steps
Ammonium ferrous sulfate calibration: Accurately pipette 10.00mL of potassium dichromate standard solution into a 500mL conical flask, dilute to about 110mL with water, slowly add 30mL of concentrated sulfuric acid, and shake well. After cooling, add 3 drops of ferrocyanate indicator solution (about 0.15mL) and titrate with ammonium ferrous sulfate solution. The end point is when the color of the solution changes from yellow to blue-green to reddish brown.
(V) Determination
Take 20mL of water sample (if necessary, take less and add water to 20 or dilute before taking), add 10mL of potassium dichromate, plug in the reflux device, and then add 30mL of sulfuric acid and silver sulfate, heat and reflux for 2h. After cooling, rinse the condenser tube wall with 90.00mL of water and remove the conical flask. After the solution is cooled again, add 3 drops of ferrous acid indicator solution and titrate with ammonium ferrous sulfate standard solution. The color of the solution changes from yellow to blue-green to reddish brown, which is the end point. Record the amount of ammonium ferrous sulfate standard solution. While measuring the water sample, take 20.00mL of redistilled water and perform a blank experiment according to the same operating steps. Record the amount of ammonium ferrous sulfate standard solution used in the blank titration.
Potassium dichromate standard method
(VI) Calculation
CODCr(O2, mg/L)=[8×1000(V0-V1)·C]/V
(VII) Precautions
1. The maximum amount of chloride ion complexed with 0.4g mercuric sulfate can reach 40mg. If 20.00mL water sample is taken, the maximum chloride ion concentration of 2000mg/L can be complexed. If the concentration of chloride ions is low, a small amount of mercuric sulfate can be added to keep mercuric sulfate: chloride ions = 10:1 (W/W). If a small amount of mercuric chloride precipitates, it does not affect the determination.
2. The range of COD determined by this method is 50-500mg/L. For water samples with chemical oxygen demand less than 50mg/L, 0.0250mol/L potassium dichromate standard solution should be used instead. 0.01mol/L ammonium ferrous sulfate standard solution should be used for back titration. For water samples with COD greater than 500mg/L, dilute them before determination.
3. After the water sample is heated and refluxed, the remaining amount of potassium dichromate in the solution should be 1/5-4/5 of the added amount.
4. When using potassium hydrogen phthalate standard solution to check the quality and operation technology of the reagent, since the theoretical CODCr of each gram of potassium hydrogen phthalate is 1.176g, 0.4251g of potassium hydrogen phthalate (HOOCC6H4COOK) is dissolved in redistilled water, transferred to a 1000mL volumetric flask, and diluted to the mark with redistilled water to make it a 500mg/L CODcr standard solution. Prepare it fresh when used.
5. The CODCr determination result should retain four significant digits.
6. During each experiment, the ammonium ferrous sulfate standard titration solution should be calibrated, and the concentration change should be paid special attention to when the room temperature is high. (You can also add 10.0ml of potassium dichromate standard solution to the blank after titration and titrate with ammonium ferrous sulfate to the end point.)
7. The water sample should be kept fresh and measured as soon as possible.
Advantages:
High accuracy: Reflux titration is a classic COD determination method. After a long period of development and verification, its accuracy has been widely recognized. It can more accurately reflect the actual content of organic matter in water.
Wide application: This method is suitable for various types of water samples, including high-concentration and low-concentration organic wastewater.
Operation specifications: There are detailed operation standards and processes, which are convenient for operators to master and implement.
Disadvantages:
Time-consuming: Reflux titration usually takes several hours to complete the determination of a sample, which is obviously not conducive to the situation where results need to be obtained quickly.
High reagent consumption: This method requires the use of more chemical reagents, which is not only costly, but also pollutes the environment to a certain extent.
Complex operation: The operator needs to have certain chemical knowledge and experimental skills, otherwise it may affect the accuracy of the determination results.
2. Rapid digestion spectrophotometry
(I) Principle
The sample is added with a known amount of potassium dichromate solution, in a strong sulfuric acid medium, with silver sulfate as a catalyst, and after high-temperature digestion, the COD value is determined by photometric equipment. Since this method has a short determination time, small secondary pollution, small reagent volume and low cost, most laboratories currently use this method. However, this method has a high instrument cost and a low use cost, which is suitable for long-term use of COD units.
(II) Equipment
Foreign equipment was developed earlier, but the price is very high, and the determination time is long. The reagent price is generally unaffordable for users, and the accuracy is not very high, because the monitoring standards of foreign instruments are different from those of my country, mainly because the water treatment level and management system of foreign countries are different from those of my country; the rapid digestion spectrophotometry method is mainly based on the common methods of domestic instruments. The catalytic rapid determination of COD method is the formulation standard of this method. It was invented as early as the early 1980s. After more than 30 years of application, it has become the standard of the environmental protection industry. The domestic 5B instrument has been widely used in scientific research and official monitoring. Domestic instruments have been widely used due to their price advantages and timely after-sales service.
(III) Determination steps
Take 2.5ml sample—–add reagent—–digest for 10 minutes—–cool for 2 minutes—–pour into the colorimetric dish—–the equipment display directly displays the COD concentration of the sample.
(IV) Precautions
1. High-chlorine water samples should use high-chlorine reagent.
2. The waste liquid is about 10ml, but it is highly acidic and should be collected and processed.
3. Ensure that the light-transmitting surface of the cuvette is clean.
Advantages:
Fast speed: The rapid method usually only takes a few minutes to more than ten minutes to complete the determination of a sample, which is very suitable for situations where results need to be obtained quickly.
Less reagent consumption: Compared with the reflux titration method, the rapid method uses fewer chemical reagents, has lower costs, and has less impact on the environment.
Easy operation: The operation steps of the rapid method are relatively simple, and the operator does not need to have too high chemical knowledge and experimental skills.
Disadvantages:
Slightly lower accuracy: Since the rapid method usually uses some simplified chemical reactions and measurement methods, its accuracy may be slightly lower than the reflux titration method.
Limited scope of application: The rapid method is mainly suitable for the determination of low-concentration organic wastewater. For high-concentration wastewater, its determination results may be greatly affected.
Affected by interference factors: The rapid method may produce large errors in some special cases, such as when there are certain interfering substances in the water sample.
In summary, the reflux titration method and the rapid method each have their own advantages and disadvantages. Which method to choose depends on the specific application scenario and needs. When high precision and wide applicability are required, reflux titration can be selected; when quick results are required or a large number of water samples are processed, the rapid method is a good choice.
Lianhua, as a manufacturer of water quality testing instruments for 42 years, has developed a 20-minute COD rapid digestion spectrophotometry method. After a large number of experimental comparisons, it has been able to achieve an error of less than 5%, and has the advantages of simple operation, quick results, low cost and short time.
Post time: Jun-07-2024
