Talk:Chemical oxygen demand

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Here at CAI, we make premade reagents for the COD test using potassium dichromate and mercury (II) sulfate as the chloride precipitator. This article refers to the "permanganate" mehtod as an alternative to the dichromate method.

Can anyone provide us with more information? We want to develop new products for the COD test that are accurate and precise, but free of the hazardous mercury waste. mgilbert@caisc.com

Alghough I wrote this article, I'm not an expert in the field (at the time I just happened to be having a lengthy conversation about COD/BOD with my girlfriend, who is a civil/environmental engineer; later I filled in the article from a few of her textbooks). Not being an expert won't prevent me from giving casual advice, however :-)

From what I've heard about potassium permanganate, while it's a strong oxidizing agent, it doesn't appear to completely oxidize all organic compounds (hence BOD values are often higher than COD when using KMnO₄). So it's unlikely that the permanganate method is valid for all samples. Plus, it seems to me that you'd still end up with chloride interference, which would again probably require the use of mercuric sulfate. Back to square one.

I've heard that there are ways to eliminate mercury from samples by precipitating it as mercuric iodide, which I believe is harmless. If I recall correctly, potassium iodide may be used for the precipitation, and may recover 70% to 80% of the mercury. (A quick Google search shows that these folks also suggest potassium iodide and say you can get up to 95% recovery.) It seems that this would be worth checking into. Best wishes, Diberri | Talk 19:17, Aug 30, 2004 (UTC)

[edit] Ammonia Contribution

The article states that oxidation of ammonia is part of the COD reaction, but I don't believe that this is generally considered to be the case. Free chloride ions can lead to the oxidation of ammonia, but under typical conditions these are controlled and ammonia oxidation should be minimal.

If you can provide a reference for that, please feel free to update the article. --David Iberri (talk) 02:34, 27 May 2006 (UTC)

[edit] References

I haven't edited a Wikipedia page before and I'm not sure I know the accepted way to go about it, so I thought I would post a touch more discussion.

My reference is Standard Methods for the Examination of Water and Wastewater. I am using an old edition (17th Edition, 1989) and would have to check a new one to see if the wording remains the same, but this one states: "Ammonia, present either in the waste or liberated from nitrogen-containing organic matter, is not oxidized in the absence of significant concentration of free chloride ions."

I also have a copy of a paper from 1989 titled "Effect of ammonia on COD analysis" by Byung R. Kim from the Journal WPCF, Volume 61, Number 5, page 614 in which the author attempted to quantify the effect of free chloride on ammonia oxidation in COD analyses.

My only edit to the original post would be to delete the line detailing the ammonia oxidation in the introductory section. The information below that does not imply that ammonia oxidation occurs.

Ammonia is generally oxidized to nitrate and thus has oxygen demand. The Overview sections seems to be more theoretical than practical so some discussion of ammonia would seem reasonable. However I do agree that the COD test does not usually oxidize the ammonia to nitrate. High concentrations of chloride cause other problems for the test (it is a direct interferent) so it is probably not even fair to say that chloride will cause the ammonia to interfere. Initially I must have been convinced otherwise, but I am now convinced that reference to the COD test needs to be removed. But I do think that since this section is theoretical some discussion of nitrification should be included.--Chris 22:30, 11 July 2006 (UTC)