Editing Chemical oxygen demand (section)

==Inorganic interference==
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. Its reaction with potassium dichromate follows the equation:

:<math>\mathrm{6Cl^- + Cr_2O_7^{2-} + 14H^+ \rightarrow 3Cl_2 + 2Cr^{3+} + 7H_2O}</math>

To eliminate chloride interference, [[mercuric sulfate]] can be added to the sample prior to the addition of other reagents.

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

{| class="wikitable"
! Inorganic molecule
! Eliminated by
! Elimination forms
|-
| [[Chloride]]
| [[Mercuric sulfate]]
| [[Mercuric chloride]] complex
|-
| [[Nitrite]]
| [[Sulfamic acid]]
| N<sub>2</sub> gas
|-
| [[Ferrous|Ferrous iron]]
| -
| - 
|-
| [[Sulfide]]s
| -
| -
|}