Classification by principle
COD: refers to the amount of oxidants consumed by substances in water that are easily oxidized by oxidants under specified conditions. It is an important indicator for evaluating the relative content of organic matter in water.
Oxidant types: potassium dichromate, potassium permanganate.
Manganese method COD
COD using potassium permanganate as an oxidant is called permanganate index (manganese method COD).
Permanganate index can be divided into acid method and alkaline method.
1. Acid method (applicable to water samples with chloride ion concentration not exceeding 300mgL, applicable to a wide range of water quality.)
Method principle: After adding sulfuric acid to the water sample to make it acidic, add a certain amount of potassium permanganate solution and heat it in a boiling water bath for a certain period of time. The remaining potassium permanganate is reduced by adding excess sodium oxalate solution, and then the excess sodium oxalate is back-titrated with potassium permanganate solution, and the permanganate index value is calculated.
2. Alkaline method (applicable to water samples with chloride ion concentration higher than 300mgL, mostly applicable to water samples with high chloride content such as seawater.)
Method principle: Compared with the acid method, the alkaline method is to add alkali to the water sample to provide an alkaline environment, and the test steps are the same as the acid method.
Chromium COD (reflux method)
COD using potassium dichromate as an oxidant is called chromium COD.
Chromium COD can be divided into two types: reflux method and rapid method.
1. The reflux method refers to the oxidation of reducing substances in water samples with a certain amount of potassium dichromate in a strong acid solution, and the excess potassium dichromate is back-titrated with ammonium ferrous sulfate solution using ferrous ferrochlore as an indicator. The amount of oxygen consumed by reducing substances in the water sample is calculated based on the amount of ammonium ferrous sulfate used.
2. The rapid method is to use a specified amount of potassium dichromate to oxidize water under certain conditions, so that hexavalent chromium is quantitatively converted into trivalent chromium, and the COD content is determined by photometry using the absorption peak of trivalent chromium at 610 nm or the absorption peak of hexavalent chromium at 420 nm.
Chromium COD (fast method)
1. Coulometric method
Method principle: The water sample is oxidized with potassium dichromate as an oxidant. After reflux oxidation in a 10.2 mol/L sulfuric acid medium, the excess potassium dichromate is used as a coulometric titrant to produce ferrous ions by electrolysis. The amount of electricity consumed by electrolysis to produce ferrous ions is calculated according to Faraday's law. It is rarely used in practice.
2. Energy-saving heating method (similar to the reflux method, the heater replaces the electric furnace)
3. Chlorine correction method (similar to the reflux method, suitable for high-chlorine wastewater, COD-chloride ion correction value = actual COD value of the water sample)
4. Oven method
5. UV-visible spectroscopy (mostly used for online COD instruments)
6. Full spectrum method
7. Biological method
8. Chemiluminescence method
9. Electrochemical method (generally, the error of testing sewage is large, and the results of testing different water samples are unstable)
10.......
There are many methods for determining water quality COD. At present, the permanganate index is mainly used to test relatively clean water, and the rapid method of chromium COD and the reflux method and UV-visible spectroscopy are mainly used to test sewage. These methods can basically solve the current water quality COD detection needs. The other methods mentioned above also have their own advantages and disadvantages, so they should be used reasonably.
