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Chemical Oxygen Demand (I) - Potassium Cobaltous Sulfate Method
- Oct 27, 2017 -

Recently, there are several people suddenly asked me the same question, COD is what, all of a sudden do not know how to answer, so prepare and we simply understand the next COD in the end what is the ghost, and the precautions to detect.

COD (Chemical Oxygen Demand) Chemical Oxygen Demand is a chemical method to measure the amount of reducing substances in the water sample that need to be oxidized. Waste water, waste water treatment plant effluent and contaminated water, can be strong oxidant oxidation of substances (usually organic) oxygen equivalent. It is an important and rapidly measurable organic pollutant parameter in the study of river pollution and the nature of industrial wastewater and the operation and management of wastewater treatment plants.

Determination of potassium Cobaltous Sulfate method, potassium permanganate method, spectrophotometry, rapid digestion method

Potassium Cobaltous Sulfate method is that we often CODcr, usually directly that COD is potassium potassium Cobaltous Sulfate method, but we also have a potassium permanganate method, that is, CODMn ,, also known as permanganate index.

Today, even the main that is potassium Cobaltous Sulfate method. Principle In a strong acid solution, a certain amount of potassium Cobaltous Sulfate oxidation of water in the reduction of substances, excessive potassium potassium Cobaltous Sulfate to test the iron as the indicator, with ammonium ferrous sulfate solution back to the water, according to the amount of water The amount of oxygen consumed by the reducing substance.

Titration end point change:

Acidic potassium Cobaltous Sulfate oxidation is strong, can oxidize most of the organic matter, adding silver sulfate as a catalyst, the linear aliphatic compounds can be completely oxidized, and aromatic organic matter is not easy to be oxidized, pyridine is not oxidized, volatile straight Chain aliphatic compounds, benzene and other organic matter in the vapor phase, can not contact with the oxidant liquid, oxidation is not obvious. Chloride can be oxidized by dichromate, and can produce precipitation with silver sulfate, affecting the determination of the results, so before the return of water to the sample by adding mercury sulfate, so that the complex to eliminate interference, chloride ion content higher than 2000mg / L of the sample should be done quantitative dilution, the content reduced to 2000mg / L below, and then measured. The COD value of 50 ~

The precautions for measuring are as follows:

 1, the use of 0.4g mercury chloride complex chloride ion up to the maximum amount of up to 40mg, such as access to 20.00mL water samples, that is, the highest can be complexed 2000mg / L chloride ion concentration of water samples. If the chloride ion concentration is low, but also less mercury sulfate, so that to maintain mercury sulfate: chloride ion = 10: 1 (mass fraction). If a small amount of mercury chloride precipitation, does not affect the determination.

2, the water sample can be taken in the range of 10.00 ~ 50.00mL range, but the amount and concentration of reagents according to the following table to adjust accordingly.

3 for chemical oxygen demand less than 50mg / L of water samples should be switched to 0.0250mol / L potassium Cobaltous Sulfate standard solution, drop back with 0.01mol / L ammonium ferrous sulfate standard solution.

4, water samples after heating reflux, the solution of potassium Cobaltous Sulfate remaining amount should be added to the amount of 1/5 ~ 4/5 is appropriate.

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