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COD can increase ????

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NewsMaker:
Hello,
I have a question regarding COD test.As many people know that during chemical treatment of water we add fecl2 to coagulate the suspended particles and then we can filter out easily i just want to know that fecl2 can increase the value of COD as it is chemical and i also want to know that sulphuric acid can increase the COD or not becuause during teatment we add sulphuric acid to reduce the pH of the water or to make it neutral if these two chemicals (fecl2 and H2SO4) are not responsible then what other factors influence in the increment of  COD  ??? plz reply soon

NewsMaker:
Some samples of water contain high levels of oxidizable inorganic materials which may interfere with the determination of COD. Because of its high concentration in most wastewater, chloride is often the most serious source of interference.
Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference.

The following table lists a number of other inorganic substances that may cause interference. The table also lists chemicals that may be used to eliminate such interference, and the compounds formed when the inorganic molecule is eliminated.

Inorganic molecule  Eliminated           Elmination forms 
Chloride                  Mercuric  sulfate    Mercuric chloride complex 
Nitrite                         Sulfamic acid      N2 gas 
Ferrous iron                 Sulfamic acid      - 
Sulfides                       Sulfamic  acid     - 

Hispania:
H2SO4 doesn´t influence COD, because it´s a reagent that is added when the analysis is done (before heating the sample).

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