You need remove the chloride ion first.

Dear Nguyen,

It depends on the concentration of chlorides. In case of chloride concentrations up to 2,000 mg / L mercury sulphate addition at a ratio of 10:1 (HgSO(4):Cl(-)) can adequately mask the interference of chloride.

I recommend the following article "Determination of Chemical Oxygen Demand for Saline Wastewater"

Abstract : The interference of the chloride ion (Cl-) on the standard method for chemical oxygen demand (COD) determination proposed by American Publican Health Association (APHA) cannot be ignored, especially for samples with high salinity. Increasing the ratio of HgSO4:Cl- and adding some masking agents such as AgNO3, Cr3+, Al3+ are all good modifications of the standard method to eliminate the Cl- interference. Correction method can also be used to calculate the real COD value by analyzing the interfering COD value and the measured COD value. In addition, new techniques such as chemiluminescence (CL) system, chemiluminescence combined with flow-injection method, and new assisted digestion methods can tolerate Cl- at a certain extent and save time simultaneously.

With my best regards

Prof. Bachir ACHOUR

Thanks all! :)

Chloride interference in high salinity wastewater makes it difficult to determine its COD. Two approaches, either dilution of the sample or adding masking agents can be done to measure the COD of high salinity wastewater.

hello

this link could be help

Article Determination of Chemical Oxygen Demand for Saline Wastewater

As mentioned above, you could dilute the samples to a salinity that is within the acceptable range. however do take note of the sensitivity of your water samples. ie if your samples are too diluted, the lower range of the COD test kit, (i am assuming you are using a hach test kit) in my experience is not very sensitive. This is particularly so when comparing different samples in the lower ranges. it will be difficult to draw data if the samples are too diluted. (hence all samples will fall in the lower ranges of the kit)

you could perhaps correlate the data to TOC and BOD to give a better understanding of the wastewater characteristics.