What is chemical oxygen demand, how to measure

Chemical Oxygen Demand COD (Chemical Oxygen Demand) is a chemical method to measure the amount of reducing substances that need to be oxidized in a water sample. Wastewater, wastewater treatment plant effluent and contaminated water, can be oxidized by strong oxidants (generally organic matter) of the oxygen equivalent. In the river pollution and industrial wastewater nature of the study and the operation and management of wastewater treatment plants, it is an important and can be more quickly measured organic pollution parameters, often expressed in the symbol COD.

Determination methods: dichromate method, potassium permanganate method, spectrophotometric method, rapid elimination method, rapid elimination spectrophotometric method in accordance with the national standard HJ-T399-2007 water quality determination of chemical oxygen demand.

Dichromate method

Chemical oxygen demand determination of the standard method to China's standard GB/T11914 "Determination of chemical oxygen demand of water quality dichromate method" and the international standard ISO6060 "Determination of Chemical Oxygen Demand of Water Quality" as a representative of the method of oxidation rate is high, reproducible, accurate and reliable, and has become the international community generally recognized as a classic standard method.

The principle of its determination is: in the sulfuric acid acid medium, potassium dichromate as an oxidizing agent, silver sulfate as a catalyst, mercuric sulfate as a masking agent for chloride ions, the elimination of the reaction solution of sulfuric acid acid acidity of 9 mol / L, heating so that the elimination of the reaction solution boiling, 148 ° C ± 2 ° C of the boiling point temperature for the elimination temperature. The water-cooled reflux heating reaction reaction for 2h, after the natural cooling of the digestion solution, the remaining potassium dichromate was titrated with ammonium ferrous sulfate solution using test ferrous spirit as indicator, and the COD value of the water samples was calculated according to the consumption of ammonium ferrous sulfate solution. The oxidizing agent used is potassium dichromate, and it is hexavalent chromium that has oxidizing properties, so it is called the dichromate method.

However, there are still shortcomings in this classic standard method: the reflux device occupies a large experimental space, water, electricity consumption, reagent consumption, operation is inconvenient, difficult to large quantities of rapid determination.

Potassium permanganate method

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

Spectrophotometric method

Based on the classical standard method, potassium dichromate oxidizes the organic substances, hexavalent chromium generates trivalent chromium, and the COD value of water samples is determined by the relationship established between the absorbance value of hexavalent chromium or trivalent chromium and the COD value of the water samples. Adopting the above principle, the most representative foreign methods are the U.S. Environmental Protection Agency EPA.Method 0410.4 "automatic manual colorimetric method", the American Society for Testing and Materials ASTM: D1252-2000 "Determination of Chemical Oxygen Demand in Water Method B - Sealed Elimination Spectrophotometric Method" and international standards. ISO15705-2002 "water quality chemical oxygen demand (COD) determination of small sealed tube method". China is the State Environmental Protection Administration unified method "fast

fast sealed catalytic digestion method (including spectrophotometric method)".

Fast digestion method

Classical standard method is the reflux 2h method, people in order to improve the speed of analysis, put forward a variety of rapid analysis methods. There are two main methods: one is to increase the concentration of oxidizing agent in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, increase the co-catalyst and other conditions to improve the reaction speed of the method. Domestic methods to GB/T14420-1993 "boiler water and cooling water analysis methods for the determination of chemical oxygen demand potassium dichromate rapid method" and the State Environmental Protection Administration recommended a unified method "Coulomb method" and "rapid closed catalytic digestion method (including photometric method)" as the representative of the method. The German standard method DIN38049 T.43 "Rapid Method for the Determination of Chemical Oxygen Demand in Water" is the representative of the foreign countries.

Compared with the classical standard method, the acidity of sulfuric acid in the degradation system has been increased from 9.0mg/l to 10.2mg/l, the reaction temperature has been increased from 150℃ to 165℃, and the degradation time has been reduced from 2h to 10min~15min. 2) The traditional way of degradation is changed by heat-conducting radiation heating, and the microwave degradation technology is adopted to improve the speed of the degradation reaction. Because there are many kinds of microwave ovens with different power, it is difficult to test out the unified power and time in order to achieve the best dissolution effect. The price of microwave oven is also very high, it is more difficult to develop a unified standard method.

Rapid elimination spectrophotometric method

Chemical oxygen demand (COD) determination method, whether reflux volumetric method, rapid method or photometric method, are based on is potassium dichromate as an oxidizing agent, silver sulfate as a catalyst, mercuric sulfate as a masking agent for chlorine ions, in the sulfuric acidic conditions for the determination of COD elimination system for the determination of the basis of the method. On this basis, a lot of research work has been carried out for the purpose of saving reagents, reducing energy consumption, easy operation, rapidity, accuracy and reliability. Rapid digestion spectrophotometric method combines the advantages of the above methods, refers to the use of sealed tubes as a digestion tube, take a small measurement of water samples and reagents in the sealed tube, into a small thermostatic heating dish, thermostatic heating digestion, and spectrophotometric determination of COD value; sealed tube specifications for φ16mm length 100mm ~ 150 mm wall thickness of 1.0mm ~ 1.2 mm opening for the helical mouth, and with a spiral sealing cap. The sealing tube has a spiral opening and a spiral sealing cap. The sealing tube is acid-resistant, high temperature-resistant, pressure-resistant and burst-resistant. One type of sealing tube can be used for digestion, called digestion tube. Another type of sealed tube can be used as a digestion, but also can be used as a colorimetric tube for colorimetric use, known as the elimination of colorimetric tube. The small-sized heated dissolver is heated by an aluminum block, and the heating holes are uniformly distributed. The aperture diameter is φ16.1mm, the depth of the aperture is 50mm to 100mm, and the heating temperature is set as the temperature of the digestion reaction. At the same time, due to the appropriate size of the sealing tube, the ablation reaction liquid occupies an appropriate proportion of space in the sealing tube. Part of the tube containing the reaction solution is inserted into the heater's heating hole, and the bottom of the tube is heated at a constant temperature of 165°C; the top of the tube is exposed to the space above the heating hole, and the top of the tube is cooled naturally by the air so that the top of the tube is reduced to about 85°C; the difference in temperature ensures that the reaction solution in the compact tube is in a state of micro-boiling reflux at this constant temperature. The compact COD reactor can hold up to 25 sealed tubes. After digestion in the sealed tubes, the solution can be transferred to a cuvette for measurement on a normal photometer, or directly in the sealed tubes for measurement on a COD-specific photometer. The COD value of 100mg/L~1000mg/L can be measured at 600nm, and the COD value of 15mg/L~250mg/L can be measured at 440nm. The method is characterized by small space occupation, small energy consumption, small reagent dosage, minimized waste liquid, small energy consumption, easy operation, safe and stable, accurate and reliable, suitable for large-volume determination, and so on, which makes up for the shortcomings of the classical standard method.

Method for removing chemical oxygen demand from water

A method for cleaning a water system and a single potassium persulfate compound for use in the method are provided. Potassium monopersulfate containing a low concentration (<0.5%) of potassium oxydisulfate byproduct is used in the method. Due to the low potassium oxo-disulfate content, the compound is not subject to the stringent limitations on the use of current potassium monopersulfate compounds. Also provided herein is a polysaccharide coating for controlling the decomposition rate of potassium monopersulfate. With this coating, potassium monopersulfate is capable of continuous use rather than periodic shock treatments. The inclusion of a low oxygenated potassium disulfate compound makes the use of this method independent of whether or not a water system is used.