I am in dire need of a COD detection method

Rapid digestion spectrophotometry.

The chemical oxygen demand (COD) determination method, whether it is reflux volume method, rapid method or photometric method, is based on the determination of COD digestion system under acidic conditions of sulfuric acid with potassium dichromate as oxidant, silver sulfate as catalyst and mercury sulfate as masking agent for chloride ions. On this basis, a lot of research work has been carried out for the purpose of saving reagents, reducing energy consumption, and making operation simple, fast, accurate and reliable. Rapid digestion spectrophotometry combines the advantages of the above methods, which refers to the use of a sealed tube as a digestion tube, taking a small amount of water samples and reagents in a sealed tube, putting them into a small constant temperature heating dish, heating and digestion at a constant temperature, and measuring the COD value by spectrophotometry; The size of the sealing tube is 16mm, the length is 100mm, the wall thickness is mm, the opening is a screw mouth, and the screw sealing cap is added.

The sealing pipe has acid resistance, high temperature resistance, compression resistance and anti-burst performance. A sealed tube can be used for digestion and is called a digestion tube. Another type of sealed tube can be used for digestion, and can also be used as a colorimetric tube for colorimetry, which is called a digestion cuveterine.

The small heating digester uses aluminum blocks as the heating body, and the heating holes are evenly distributed. Pore diameter, hole depth 50mm 100mm, set heating temperature for digestion reaction temperature. At the same time, due to the appropriate size of the sealed tube, the digestion reaction liquid occupies an appropriate proportion of the space of the sealed tube.

A part of the sealing tube containing the digestion reaction solution is inserted into the heating hole of the heater, and the bottom of the sealing tube is heated at a constant temperature of 165; The upper part of the sealing pipe is exposed to space by raising the heating hole, and the top of the nozzle is lowered to about 85 under the natural cooling of the air; The difference in temperature ensures that the reaction liquid in the small sealed tube is in a state of micro-boiling reflux at this constant temperature. The compact COD reactor can accommodate up to 25 sealed tubes. After the digestion reaction with a sealed tube, the digestion solution is transferred to a cuvette and can be determined on a general photometer, and can be directly determined on a special photometer for COD with a sealed tube.

Specimens with COD values of 100 mg L and 1000 mg L can be measured at 600 nm wavelength, and samples with COD values of 15 mg L and 250 mg L can be determined at 440 nm wavelength. This method has the characteristics of small space occupation, low energy consumption, small amount of reagent, minimized waste liquid, low energy consumption, easy operation, safe and stable, accurate and reliable, and suitable for large-scale measurement, which makes up for the shortcomings of the classical standard method.

It's good to buy a portable COD detector, which is not expensive about 1W, and it only takes more than 10 minutes to detect.

The method for determining COD is as follows:

1. Dichromate method.

The standard method for the determination of chemical oxygen demand is represented by the Chinese standard GB11914 "Determination of Chemical Oxygen Demand of Water Quality" and the international standard ISO6060 "Determination of Chemical Oxygen Demand of Water Quality".

The determination principle is as follows: in the acidic medium of sulfuric acid, potassium dichromate is used as the oxidant, silver sulfate is used as the catalyst, mercury sulfate is the masking agent of chloride ions, the acidity of sulfuric acid in the digestion reaction solution is 9mol l, the digestion reaction solution is boiled by heating, and the boiling point temperature of 148 2 is the digestion temperature.

2. Potassium permanganate method.

Potassium permanganate is used as an oxidant to determine COD, and the measured is called potassium permanganate index.

3. Spectrophotometry.

Based on the classical standard method, potassium dichromate oxidizes organic substances, hexavalent chromium produces trivalent chromium, and the COD value of the water sample is determined by the relationship between the absorbance value of hexavalent chromium or trivalent chromium and the COD value of the water sample.

4. Rapid digestion method.

The classic standard method is the reflux 2h method, and various rapid analysis methods are proposed to improve the speed of analysis. There are two main methods: one is to increase the concentration of oxidant in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, increase the cocatalyst and other conditions to improve the reaction speed.

5. Spectrophotometry.

The chemical oxygen demand (COD) determination method, whether it is reflux volume method, rapid method or photometric method, is based on potassium dichromate as the oxidant, silver sulfate as the catalyst, and mercury sulfate as the masking agent for chloride ions, and the determination of the COD digestion system under acidic conditions of sulfuric acid.

The COD national standard detection method is as follows.

National standard detection method: potassium dichromate reflux digestion, ferrous titration method. Methodology:

Under a strong acidic medium, the reducing substances in the water were oxidized with excess potassium dichromate, and the excess potassium dichromate was titrated with ferrous standard solution, and the arrival of the endpoint was indicated by the test of ferrous chromate. Then, according to the total amount of potassium dichromate and the amount of ferrous iron, the oxygen consumption of the reducing substances reacted with potassium dichromate is calculated. This method is the method of arbitration analysis.

The disadvantage is that the method is cumbersome and time-consuming.

Under certain conditions, the mass concentration of oxygen corresponding to the potassium dichromate salt consumed by the dissolved substances and suspended solids in the water sample when treated with potassium dichromate oxidation.

A known amount of potassium dichromate solution was added to the water sample, and silver salt was used as a catalyst under a strong acid medium, and after boiling and reflux, the amount of potassium dichromate in the water sample that was not reduced was titrated with ammonium ferrous sulfate as an indicator, and the amount of ammonium ferrous sulfate that was not reduced in the water sample was converted into the mass concentration of oxygen consumption.

Under the condition of acidic potassium dichromate, aromatic hydrocarbons and pyridine are difficult to be oxidized, and their oxidation rate is low. Linear aliphatic compounds can be effectively oxidized by sulfuric acid catalysis.

The national standard for COD detection is GB11914-89.

China has a good mass foundation in COD determination, in the late seventies to early eighties of the last century, a number of laboratories were organized nationwide to conduct comparative verification tests on the accuracy of the method on standard samples and wastewater samples, and refer to the International Organization for Standardization standard ISO DIS6060.

The national standards GB11914-89 "Determination of Chemical Oxygen Demand of Water Quality" and GB11892-89 "Determination of Permanganate Index of Water Quality" have been formulated, and there are HJ T70-2001 "Chlorine Calibration Method for Determination of Chemical Oxygen Demand of High Chlorine Wastewater" (suitable for high chlorine wastewater with chloride ion content less than 20000mg L), HJ T132-2003 "Determination of Chemical Oxygen Demand of High Chlorine Xiaofan Wastewater Potassium Iodide Alkaline Potassium Permanganate Method" (It is suitable for the determination of chloride ion content of oil and gas fields and refining enterprises as high as tens of thousands, hundreds of thousands of milligrams, and the chemical oxygen demand of pure water of chlorine waste.) The lowest detection limit of the method, the upper limit of the assay is: ）

Classification of COD:

COD stands for chemical oxygen demand, which refers to the oxidation dose that is carefully consumed when a certain strong oxidant is used to treat a water sample under certain conditions. COD is a metric that characterizes the amount of reducing substances, especially organic matter, in water. Therefore, the larger the COD, the more polluted the water body is by organic matter.

There are two types of oxidants commonly used in COD determination, namely potassium dichromate and potassium permanganate. When potassium dichromate is used as an oxidant, the measured value is called CODCR; The value measured with potassium permanganate as an oxidant is CODMN, also known as potassium permanganate index. The potassium dichromate method has high oxidation rate, good reproducibility, accuracy and reliability, and is a classic standard method widely recognized by the international community.

In China's water environment standards, only the value measured by the acid potassium dichromate method is called chemical oxygen demand. <>

CR or MN, and then search for it on the Internet, see "Water and Wastewater Monitoring and Analysis Methods" fourth edition for details).

The commonly used potassium dichromate method is the specific measurement method can be checked on the Internet, and there are many of them!

Chemical oxygen demand (COD) refers to the amount of oxidant consumed by reducing substances that are easily oxidized by strong oxidants in the water body, and the result is converted into oxygen, which is measured in mg L. It is a comprehensive index to characterize reducing substances in water bodies.

In the cycle of nature, organic compounds continuously consume dissolved oxygen in water during the biodegradation process, resulting in oxygen loss, thereby destroying the ecological balance of the water environment and biological community, and bringing adverse effects, thus determining the status of COD in water environment detection.

The principle of COD detection is briefly described.

The analysis principle of COD is based on the oxidation method, and its quantitative method has different results due to different conditions such as the type and concentration of oxidant, oxidative acidity, reaction temperature and reaction time, so COD is the measurement result under conditional test. On the other hand, under the same conditions, there will be different degrees of oxidation due to the different types and concentrations of reducing substances in the water. Therefore, for COD, the measured values vary depending on the assay method.

Differences in COD assay methods.

The determination methods of COD are mainly classified by the type of oxidant, the most common are potassium dichromate method and potassium permanganate method.

Since the oxidizing capacity of potassium dichromate is greater than that of potassium permanganate under the specified determination conditions, CODCR > CODMNGenerally speaking, the oxidation rate of CODCR is about 90%, while the oxidation rate of CODMN is less than 50%, and the CODCR method is commonly used for various types of wastewater; The CODMN method is only suitable for the determination of surface water, drinking water.

1. Dichromate reflux method.

Determination principle: in the acidic medium of sulfuric acid, potassium dichromate is used as the oxidant, silver sulfate is used as the catalyst, mercury sulfate is the masking agent of chloride ions, the acidity of sulfuric acid in the digestion reaction solution is 9mol l, the heating makes the digestion reaction solution boil, and the boiling point temperature of 148 2 is the digestion temperature. The reaction was cooled with water and refluxed to heat the reaction for 2h, and after the natural cooling of the digestion solution, the remaining potassium dichromate was titrated with ammonium ferrous sulfate solution with ferrous ammonium sulfate solution, and the COD value of the water sample was calculated according to the consumption of ferrous sulfate according to the solution.

2. Potassium permanganate method.

Determination principle: Potassium permanganate is used as an oxidant to determine COD, and the measured COD is called permanganate index (CODMN). After the water sample was added to sulfur and the saurryl acid was acidic, a certain amount of potassium permanganate solution was added, and the reaction was heated in a boiling water bath for 30min.

The remaining potassium permanganate was reduced by adding excess sodium oxalate solution, and then the excess sodium oxalate was dripped back with potassium permanganate solution to calculate the permanganate index.