Any substance that reduces Cr2O72- will interfere with COD procedure. One of the most
common interferences is Cl- which is quantitatively oxidized to Cl2 by dichromate. If chloride is
known to be present in the sample, this difficultyis overcome in the method described by adding
HgSO4 to the reaction mixture to tie up Cl- as a_soluble Hg(II) chloride complex.