Cloudiness occurs during COD colorimetry
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Recently, we encountered such a problem while analyzing water samples from retention tanks. We used the colorimetric method to determine the COD level in the wastewater; no changes occurred when the sample was taken and reagents were added to it. However, after digestion in the instrument, the solution became extremely turbid, making it impossible to conduct accurate measurements using a colorimeter. The reading was initially over 1000 and kept rising, eventually reaching nearly 3000. Once the suspension settled, the reading dropped back to around 1000. Have any of you experienced this issue? What causes this, and how can it be solved? Please share your opinion :)
It might be that the chloride ion level is too high; try adding a chloride ion shield
It is recommended to test the salt content and chloride concentration in the wastewater; if the chloride level is high, the wastewater sample should be diluted or a masking agent should be added to mask the chloride ions.
It is recommended to use the national standard distillation and reflux method for greater accuracy.
Could it be that chloride ions affect the determination of COD? I would appreciate some guidance from experts – are there any other ions or substances that influence COD as well?
There are no chloride ions; calcium ions are suspected. When silver sulfate reagent is added, calcium sulfate that is slightly soluble in water is formed
Chloride ions can interfere with COD monitoring by forming silver chloride precipitates, which leads to insufficient amounts of catalyst; therefore, mercury sulfate is commonly used to suppress chloride ions. The precipitate you mentioned could be silver chloride.
Silver chloride precipitates rapidly, but in our case a turbid suspension is formed, and it takes some time before it settles
Hello, this situation is most likely caused by an excessive level of chloride ions in the water, which interferes with the COD measurement. It is recommended to add some mercuric sulfate after adding the potassium dichromate reagent, stir well, and then proceed with the test!