COD
COD, also known as Chemical Oxygen Demand, is a chemical method used to measure the amount of reducing substances that need to be oxidized in water samples. The oxygen equivalent of substances (usually organic matter) that can be oxidized by strong oxidants in wastewater, wastewater treatment plant effluent, and polluted water. In the study of river pollution and industrial wastewater properties, as well as the operation and management of wastewater treatment plants, it is an important and quickly measurable organic pollution parameter, often represented by the symbol COD.
COD measurement method
Generally, the oxidant used to measure Chemical oxygen demand is Potassium permanganate or Potassium dichromate, and the values obtained by using different oxidants are different, so the detection method needs to be noted. In order to achieve uniformity and comparability, countries have certain monitoring standards. According to the different oxidants strengthened, they are respectively called Potassium dichromate oxygen consumption (customarily called Chemical oxygen demand) and Potassium permanganate oxygen consumption (customarily called oxygen consumption, OC for short, also called Permanganate index).
Chemical oxygen demand can also be compared with Biochemical oxygen demand (BOD). The ratio of BOD/COD reflects the biodegradability of sewage. Biochemical oxygen demand analysis takes a long time. Generally, only after more than 20 days can the living things in the water be basically consumed completely. For convenience, about 95% of the oxygen consumed in five days is taken as environmental monitoring data, which is marked as BOD5.
Determination method
Spectrophotometry
Based on the classical standard method, Potassium dichromate oxidizes organic matter, Hexavalent chromium generates trivalent chromium, and the COD value of water sample is determined through the relationship established between the absorbance value of Hexavalent chromium or trivalent chromium and the COD value of water sample. Using the above principles, the most representative foreign methods are EPA. Method 0410.4 Automatic Manual Colorimetry of the U.S. Environmental Protection Agency, ASTM: D1252-2000 Determination of Chemical oxygen demand of Water B-Sealed Digestion Spectrophotometry, and the international standard ISO15705-2002 Determination of Chemical oxygen demand of Water Quality Small Sealed Tube Method. China is a unified method of the State Environmental Protection Administration, which is the “Rapid Closed Catalytic Digestion Method (Including Spectrometric Method)”.
Rapid digestion method
The classic standard method is the reflux 2h method, and various fast analysis methods have been proposed to improve the analysis speed. There are two main methods: one is to increase the concentration of oxidants in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, and add co catalysts to improve the reaction speed. The domestic methods are represented by GB/T14420-1993 Analytical Method for Boiler Water and Cooling Water Determination of Chemical oxygen demand – Potassium dichromate Rapid Method, and the unified methods recommended by the State Environmental Protection Administration – Coulometric Method and Rapid Closed Catalytic Digestion Method (including photometric method). The German standard method DIN38049 T.43, Rapid Determination of Chemical oxygen demand of Water, is represented abroad.
Compared with the classical standard method, the above method increases the sulfuric acid acidity of the digestion system from 9.0 mg/L to 10.2 mg/L, the reaction temperature from 150 ℃ to 165 ℃, and the digestion time from 2 hours to 10-15 minutes. The second is to change the traditional method of thermal radiation heating and digestion, and adopt microwave digestion technology to improve the digestion reaction speed. Due to the wide variety and varying power of microwave ovens, it is difficult to experiment with a unified power and time in order to achieve the best digestion effect. The price of microwave ovens is also high, making it difficult to develop a unified standard method.
Potassium permanganate method
Potassium permanganate is used as oxidant to determine COD, and the measured value is called Potassium permanganate index.
Chromate and dichromate process
The standard method for determination of Chemical oxygen demand is represented by the national standard GB 11914 Determination of Chemical oxygen demand in Water Quality – Dichromate Method and the international standard ISO6060 Determination of Chemical oxygen demand in Water Quality. This method has high oxidation rate, good reproducibility, accuracy and reliability, and has become a classic standard method widely recognized by the international community.
The determination principle is as follows: in sulfuric acid acid medium, Potassium dichromate is used as oxidant, Silver sulfate is used as catalyst, Mercury(II) sulfate is used as masking agent for chloride ion, the sulfuric acid degree of digestion reaction solution is 9 mol/L, heating makes the digestion reaction solution boil, and the boiling point temperature of 148 ℃± 2 ℃ is the digestion temperature. Heat the reaction with water cooling reflux for 2h, add water to dilute the digestion solution to about 140ml after natural cooling, titrate the remaining Potassium dichromate with Ammonium iron(II) sulfate solution with Ferroin as indicator, and calculate the COD value of the water sample according to the consumption of Ammonium iron(II) sulfate solution. The oxidant used is Potassium dichromate, and Hexavalent chromium has oxidation performance, so it is called the double Chromate and dichromate process.
However, this classic standard method still has shortcomings: the reflux device occupies a large experimental space, consumes a large amount of water and electricity, uses a large amount of reagents, and is inconvenient to operate, making it difficult to quickly measure in large quantities.