COD detector reflux titration method: This method is widely used in the laboratory and is also a national standard method. This method is suitable for all kinds of water samples with COD value greater than 30mg/L. The detection limit of undiluted water samples is 700mg/L. The specific method is: add a known amount of potassium dichromate solution to the water sample. The medium uses silver sulfate as a catalyst, after boiling and refluxing, using ammonium ferrous sulfate as an indicator, titrate the unreduced potassium dichromate in the water sample with ferrous ammonium sulfate, and convert the consumption of ferrous ammonium sulfate into oxygen consumption The amount of mass concentration. The disadvantage of this method is that the operation process is cumbersome and there are many reasons for the error. For example, backflow collection may cause large errors. After calculation, each drop of ferrous ammonium sulfate will change the COD value by about 8mg/L, which has a large error and is not suitable for water with high chlorine and low COD. The advantage of this method is that the method is relatively mature and the requirements on the instrument are relatively low.
COD detector HACH colorimetric method: HACH colorimetric method is a test method used by a few advanced laboratories. This method uses the bottled digestion solution specially made by American HACH company as the digestion reagent. During the test, add a water sample, heat for 2h on the COD reactor made by HACH, and after cooling to room temperature, use a spectrophotometer produced by HACH to perform colorimetric analysis, and directly read the COD value in the water sample. Since Cr2O72- is partially converted to Cr3+ under the action of reducing agent, the absorbance is measured at a wavelength of 620nm, and then the COD value is converted from the absorbance. The advantage of this method is that it can overcome the error caused by titration analysis, and the operation is simple and convenient. Although this method has obvious advantages, it also has certain defects: because the digestion solution uses the bottled digestion solution specially made by HACH Company, the bottled digestion solution costs more per bottle.
COD detector microwave method: The microwave method is a COD test method developed in recent years. This method uses the sulfuric acid-potassium dichromate digestion system as the reflux method. After the water sample is digested by microwave heating, the excess dichromic acid Potassium was tested with ferriline as the indicator, titrated with ammonium ferrous sulfate, and the COD value was calculated. One of the biggest features of this method is that the heating of the reaction liquid is carried out using electromagnetic wave energy with a frequency of 2450MHz. Under the action of high-frequency microwaves, the reaction liquid molecules will produce frictional movements. In addition, the sealed digestion method is used to make the digestion tank pressure. Rapidly increased to 203kPa, so the advantage of this method is that the reaction time is short, and the determination of high chlorine water can be achieved.