Talk:Chemical oxygen demand

Bula — Preceding unsigned comment added by 27.123.136.171 (talk) 14:51, 1 September 2022 (UTC)

Untitled

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)

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)

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)

COD of oil and grease

I heard that in actual municipal wastewater, for each mg/L of oil&graease there are about 12 mg/L of COD. I don't see how it comes from the equation, but I see it in actual lab result. Maybe someone knows about it? I also added the wastewater application to article. hope its OK with you.84.229.159.187 (talk) 20:26, 30 October 2011 (UTC)

I suspect something wrong in the lab. A typical paraffin will have a COD value slightly less 4 times the molecular weight. For example decane which has a molecular weight of 142 will require 496 grammes of oxygen for complete oxidation.  Velella  ^{Velella Talk}   23:35, 30 October 2011 (UTC)

mg/L and ppm are not the same

The first paragraph in the article infers that mg/L and ppm are interchangeable as units (meaning they are the same thing), which they are not. For this reason the reference to ppm should be removed. The mg/L expression is a mass per volume expression, while ppm is a dimensionless expression (made possible because the units used on either side of the expression are the same, i.e. litres per megalitre, milligrams per kilogram etc). A common problem occurs when ppm and mg/L are interchanged unwittingly when, for example, working out the dose rates of chemicals in water. Take the example of dosing 42% ferric chloride in water (a common occurrence in the treatment of industrial discharge waters). Since the specific gravity of 42% ferric chloride is 1.45, then 1 litre of 42% ferric chloride is equal to 1.45 kilograms. When 1 litre of 42% ferric chloride solution is dosed into 1,000,000 litres of water, the dose rate is 1 ppm. If I then reported this as 1 mg/L, I would clearly be wrong (since it is 1.45 mg/L). Hence one cannot just call mg/L as ppm (or vice versa) else in this case a 45% error will occur! BluesLewis (talk) 07:40, 24 September 2015 (UTC)

I agree with you. I have never seen COD expressed as ppm and have removed it from the first paragraph. EvM-Susana (talk) 22:13, 26 September 2015 (UTC)

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