Is the rapid digestion method accurate for COD?

Experiments are always based on error, you have to analyze the main error, the rapid digestion determination of COD will affect the oxidation of organic matter, but it should be able to carry out 99% under that condition, so it is accurate.

Chemical Oxygen Demand (COD) is a chemical measurement of the amount of reducing substances that need to be oxidized in a water sample. The oxygen equivalent of substances (usually organics) that can be oxidized by strong oxidants in wastewater, wastewater treatment plant effluent, and contaminated water. It is an important and rapidly determined organic pollution parameter in the study of river pollution and the properties of industrial wastewater, as well as in the operation and management of wastewater treatment plants, and is often represented by the symbol COD.

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" are used as the standard methods for determination.

COD Determination Method:

Dichromate method:

Classical standard method, good reproducibility, accuracy and reliability. Disadvantages: The reflux device occupies a large experimental space, the consumption of water and electricity is large, the amount of reagent is large, the operation is inconvenient, and it is difficult to quickly measure in large quantities.

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 for 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 ammonium ferrous sulfate solution.

The oxidant used is potassium dichromate, and the oxidizing performance is hexavalent chromium, so it is called dichromate method.

Potassium permanganate method:

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

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.

Rapid digestion method:

The advantages are that the acidity of sulfuric acid in the digestion system is increased to 150, the reaction temperature is increased from 150 to 165, and the digestion time is reduced from 2h to 10min and 15min. The disadvantage is that the power and time of the test are different depending on the type of microwave oven.

The classic standard method is the reflux 2h method, and various rapid analysis methods are proposed to improve the speed of analysis. It is mainly a method to increase the concentration of oxidant in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, and increase the co-catalyst to improve the reaction speed.

Rapid digestion spectrophotometry:

It includes the advantages 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, suitable for large-scale measurement, etc.

Rapid digestion spectrophotometry refers to the use of a sealed tube as the digestion tube, taking a small amount of water samples and reagents in the 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.

It is enough for system monitoring, and the difference between the national standard is not very large, of course, the accuracy is not as good as the national standard; Generally, when it comes to the inspection of the first class, the national standard law will be used for the law enforcement inspection.

The steps for the determination of COD by rapid digestion are as follows:

1. Prepare several reaction tubes and place them on the air cooling tank of the cooling rack.

2. Accurately measure the distilled water into the "0" reaction tube in turn, and measure the water samples for each test into other reaction tubes.

3. Add consumables D reagent to each reaction tube in turn, and it is recommended to use the manufacturer's quantifier).

4. Add consumables and E reagents to each reaction tube in turn, and it is recommended to use the manufacturer's quantifier).

5. Mix the samples well and put them into a 5B-1 COD digester preheated to 165 in advance. Cover with a pop filter and press the "Dissolve" button. At the same time, turn on the COD tester switch for preheating.

6. After the digestion is completed, put each sample on the air cooling tank of the cooling rack in turn, and then press the "cooling" button of the COD digester for air cooling.

7. After the air cooling is completed, distilled water is added to each reaction tube in turn and mixed.

8. Put the mixed reaction tube into the water cooling tank of the cooling rack (tap water is added in advance). Press the "Cool" button for water cooling.

9. After the water cooling is completed, take out the reaction tube from the water cooling tank and wipe the water droplets on the outer wall. The solution is sequentially poured into the corresponding numbered cuvettes (mix thoroughly).

10. Put the "0" cuvette (solution) into the cuvette of the instrument and close the lid.

11. Observe for 3-4 seconds and wait for the value to be stable, press the key on the keyboard of the COD tester, the instrument screen will display 0000, and then take out the "0" cuvette (solution) from the cuvette after pretending.

12. Put the No. 1 cuvette into the cuvette and close the lid.

13. After observing that the value is stable for 3-4 seconds, the result displayed on the screen at this time is the COD concentration of sample No. 1.

14. Then take the "No. 1" cuvette from the calendar instrument, and then put the "No. 2" cuvette into the cuvette slot, and close the upper lid. Observe for 3-4 seconds and wait for the values to stabilize, at which point the result displayed on the screen is the COD concentration of sample No. 2. The process is similar for other sample testers.

15. The solution after the colorimetric completion cannot be dumped at will, but should be collected uniformly and processed centrally.

What is COD:

Chemical oxygen demand is also known as chemical oxygen demand, abbreviated as COD. It is the use of chemical oxidants (such as potassium permanganate) to oxidize and decompose oxidizable substances (such as organic matter, nitrite, ferrous salts, sulfides, etc.) in water, and then calculate the oxygen consumption according to the amount of oxidant retained in the residual limb. COD is an important indicator of water pollution.

It reflects the degree of contamination of the water with substances, and the higher the chemical oxygen demand, the more serious the contamination of the water with organic matter.

COD is expressed in mg L, the COD value detected by water quality monitoring instruments, the water quality can be divided into five categories, of which the first and second class COD 15mg L, basically can meet the drinking water standard, the value is greater than the second class of water can not be used as drinking water, of which the third class of COD 20mg L, the fourth class of COD 30mg L, the fifth class of COD 40mg L belongs to polluted water quality, the higher the COD value, the more serious the pollution.

The digestion temperature and timing time can be selected in the range of minutes to 96 hours.

The intelligent digestion device of COD digestion instrument has a new intelligent function design and is easy to operate. It can be used for the digestion of COD, total nitrogen, total phosphorus and other water samples, and the digestion temperature and timing time can be selected in the range of minutes to 96 hours.

It adopts digital setting, display heating temperature, automatic control of heating temperature, and heating time. The heating speed is fast, the temperature is constant and uniform, the power consumption is small, the operation is simple, the performance is stable and reliable, and it is widely used in water quality testing in environmental protection, medical treatment, health, food, tap water, papermaking, sewage treatment, printing and dyeing, petrochemical, metallurgy, colleges and universities and other industries.

Determination of chemical oxygen demand (UV absorption).

Chemical oxygen demand (COD).

Indicates the amount of oxidant consumed after the reducing substances in the water can be oxidized by the oxidant, which is converted into the amount of oxygen. The reducing substances in the water include organic matter, nitrite, ferrous salt, sulfide, etc." This index is an important indicator of water quality monitoring and an important basis for water quality evaluation, which can be used to comprehensively evaluate the degree of water pollution.

The potassium dichromate method in the national standard is usually used for the determination of COD, and the results of this method are accurate, reliable and reproducible. However, due to the long analysis cycle of 2 hours for heating and refluxing samples, it is not suitable for batch analysis. The composition and concentration of organic matter in water are the main factors influencing its COD value, and the researchers have concluded through a large number of experiments that most organic matter has strong absorption in the ultraviolet spectral region.

Under certain conditions, the absorbance of organic matter has a good correlation with COD, using this correlation, CODA can be directly determined by ultraviolet method, the relationship between COD value and absorbance of water sample conforms to Lambert-Beer's law, compared with potassium heavy acid method, ultraviolet absorption method is fast, does not need to add chemical reagents, and avoids environmental pollution.

Compared with the chemical method, the Yushan UV COD sensor has the following advantages: no reagent, no secondary pollution, low cost: simple operation and maintenance, low failure rate, short test time, response time is generally tens of seconds, and the instrument stability is good.

High reliability and no interference from chloride ions. These obvious advantages of the UV method make it a technology that can be widely used in the field of continuous monitoring.

COD sensors.

Water quality monitoring system.

The national standard method is 2 hours, and the temperature is 165 degrees Celsius, but it is generally set at 170 degrees Celsius. The temperature of HJ-101B rapid digester is still 165, and the time is 15min

The digestion time of the COD digestion instrument is 2 hours, and the temperature is roughly maintained at about 160-170 technical specifications.

Measurement range: 0 1000mg l, 0 10000mg l (aqueous sample dilution) Measurement error:

a Potassium hydrogen phthalate standard solution (500mg L), relative to the standard is not greater than bIndustrial organic wastewater (500mg l), relative to the standard is not greater than the ambient temperature: 5 40

Heating power: 1500W (AC 220V 22V, 50Hz) with reference to the national standard analysis method GB11914-89

COD was determined by microwave digestion

Microwave digestion COD tester: sulfuric acid-potassium dichromate is used.

The digestion system uses the heat generated by microwaves acting on the inside of the reaction to cause high friction between molecules to dissolve the product.

This instrument is powered by the main machine.

Sealed digestion tanks constitute state suspicions.

Sealing Measuring Range:

COD(CR): 10 800 mg L, COD(CR)>800 mg L (dilution assay);

Reagent. 1) Potassium dichromate solution.

Potassium dichromate digest (for seal digestion):

Potassium dichromate standard for unsealed microreflux and calibration):

2) Try ferrous spirit indicator solution:

3) Ammonium ferrous sulfate.

Standard solution, approx.

Digestion test steps.

This instrument adopts an intelligent integrated control system, if a sealed digestion tank is used to digest the sample, the "method".

Select "Dial to "0"; Digest the sample using a non-sealed microreflux digestion flask and set the "Method Selection" to "1". The bridge of this instrument can digest 3 9 samples at the same time, and the "number of samples" should be dialed to the corresponding number position according to the number of samples in the furnace cavity. Then select the time

The knob is turned to the "1 15" (sealing method) and the instrument automatically completes the entire digestion process.