Three methods of COD water quality detector

The research on COD water quality detection instrument in China started relatively early. In recent years, the research on COD water quality detection instrument has become more active, and the testing methods are constantly developing and updating. The methods are interpenetrating, and although some methods such as colorimetric method have not been widely used, they have been recognized by the US Environmental Protection Agency. At present, there are three mature COD water quality detection methods: reflux titration, microwave method, and colorimetric method.

COD water quality detector reflux titration method:

This method is currently widely used in laboratories,

It is also a standard method. This method is applicable to various types of water samples with COD values greater than 30mg/L, and the detection limit for undiluted water samples is 700mg/L. The specific method is to add a known amount of potassium dichromate solution to the water sample, and use silver sulfate as a catalyst in a strong acid medium. After boiling and refluxing, use ferrous sulfate as an indicator to titrate the unreduced potassium dichromate in the water sample with ferrous ammonium sulfate. The amount of ferrous ammonium sulfate consumed is converted to the mass concentration of oxygen consumed. The disadvantage of this method is that the operation process is relatively complicated, and there are many reasons for errors, such as reflux collection, which may cause significant errors. After calculation, each drop of ferrous ammonium sulfate can bring about a COD value change of about 8mg/L, which may cause significant errors; Not applicable to water with high chlorine and low COD. The advantage of this method is that it is relatively mature and has relatively low requirements for instruments.

COD water quality detector microwave method:

The microwave method is a COD testing method developed in recent years. Like the reflux method, this method uses a sulfuric acid potassium dichromate digestion system. After the water sample is digested by microwave heating, excess potassium dichromate is titrated with ferrous ammonium sulfate as an indicator to calculate the COD value. A major feature of this method is that the heating of the reaction solution is carried out using electromagnetic wave energy with a frequency of 2450M Hz. Under the action of high-frequency microwaves, the molecules of the reaction solution will undergo frictional motion. In addition, a sealed digestion method is used to rapidly increase the pressure of the digestion tank to 203k Pa. Therefore, the advantage of this method is that the reaction time is short and it can achieve the determination of high chlorine water. The disadvantage of this method is also the limitation of titration analysis, which will not be elaborated here.

COD water quality detector colorimetric method:

Colorimetric method is currently adopted by a few laboratories

One testing method used is to use bottled digestion solution as the digestion reagent. During testing, add a water sample and heat it on the COD reactor for 2 hours. After cooling to room temperature, perform colorimetric analysis using a spectrophotometer and directly read the COD value in the water sample. Due to the partial conversion of Cr2O 72- to Cr3+under the action of a reducing agent, its absorbance was measured at a wavelength of 620nm, and then the COD value was converted from the absorbance. The advantage of this method is that it overcomes the errors that may be caused by titration analysis, and the operation is simple and convenient. Although this method has obvious advantages, it also has certain drawbacks: due to the use of specially bottled digestion solution, the cost per bottle of the bottled digestion solution is relatively high.