E-mail: michael@shboqu.com   |   Expert in Water Quality Measurement and Water Treatment Project

Comparison of COD detection and rapid digestion spectrophotometry

by:BOQU     2021-12-09

Sewage COD detection refers to CODCr, that is, the COD chromium method, which mainly measures the total amount of organic matter in the water body; it is different from the oxygen consumption CODMn of surface water. Today I will tell you about the advantages and disadvantages of traditional manual reflux titration and rapid digestion spectrophotometry.

1. Dichromate reflux method:

Classic standard method, with good reproducibility, accuracy and reliability. Disadvantages: the reflux device occupies a large experimental space, the water and electricity consumption is large, the reagent consumption is large, the operation is inconvenient, and it is difficult to quickly measure in large quantities. Measurement principle: In a sulfuric acid medium, potassium dichromate is used as an oxidant, silver sulfate is used as a catalyst, and mercury sulfate is used as a masking agent for chloride ions. The acidity of the digestion reaction solution is 9mol/L. Heating makes the digestion reaction solution boil, 148℃ The boiling point temperature of ±2°C is the digestion temperature. Heat the reaction with refluxing water for 2 hours. After the digestion solution is naturally cooled, use ferrous ammonium sulfate as an indicator to titrate the remaining potassium dichromate with ferrous ammonium sulfate solution. Calculate the water sample based on the consumption of ferrous ammonium sulfate solution. The COD value.

2. Fast digestion spectrophotometry:

The advantage is that the acidity of the digestion system is increased from 9.0mg/l to 10.2mg/l, the reaction temperature is increased from 150℃ to 165℃, and the digestion time is reduced from 2h to 10min~15min. The disadvantage is that the types of microwave ovens are different, and the power and time of the test are different. The classic standard method is the 2h reflux method. In order to improve the analysis speed, various fast analysis methods have been proposed.

It is mainly to increase the concentration of oxidant in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, increase the co-catalyst, and other conditions to increase the reaction speed. Including the advantages of small space occupation, low energy consumption, a small number of reagents, reduced waste liquid, low energy consumption, simple operation, safety and stability, accuracy and reliability, and suitable for mass determination. Rapid digestion spectrophotometry refers to the use of a sealed tube as a digestion tube, a small number of water samples and reagents are taken in the sealed tube, placed in a small constant temperature heating dish, heated at a constant temperature for digestion, and the COD value is measured by spectrophotometry.

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