Water and Wastewater.com Help Forums - Difference in COD with Vials of different ranges

Muhammad Afzal Khan

Ex Member

Difference in COD with Vials of different ranges
Dec 21^st, 2010, 12:20am

HI All

we have a treated effluent from textile processing plant. We have MERCK COD TEST vials of 10 - 150 ppm range and other one is 0 - 1500 ppm range.

The problem is
'When I check COD using High range i-e 0-1500 ppm vial the result come out as 130 ppm.

But if same sample is checked using 10-150 ppm vials after two time dilution the result exceeds 300 ppm and if with out dilution the result photometer shows reading > 150 ppm???? I am really confused
Please advise.
Regards

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kukkkatz Full Member Offline WaterandWastewa ter.Com is the best! Posts: 185 Gender: Company or Organization: KUKK Ltd.	Re: Difference in COD with Vials of different ranges Reply #1 - Dec 21^st, 2010, 6:28am 1. use proper scale for vials, they might read at different wavelength (check standard methods and you'll learn about this) 2. use vials of same supplier for the given unit
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David S. Senior Member Offline I have nothing clever to say here. Posts: 373 Gender: Company or Organization: Environmental Express	Re: Difference in COD with Vials of different ranges Reply #2 - Dec 21^st, 2010, 11:24am According to Standard Methods, the low range vials are only to be used for samples with less than 100 mg/L COD. I don't think they will properly read anything higher than that, even with a dilution. You may also have some chloride interference, which is more pronounced at the lower level.
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DrHenry

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Re: Difference in COD with Vials of different ranges
Reply #3 - Dec 21^st, 2010, 12:39pm

This can happen for a couple of reasons and without seeing and testing your sample, it is hard to tell. I have seen it before.

1. There is turbidity or collodial material left in the 150 that is scattering light, but in the 1500 it is removed.

2. There is color in your sample that messes up the 150 or there is color created in the 150. This happens with colored samples.

Try this. Dilute out the 150 until it gets on scale and then compare. At half dilution, it may not work, but lets say at 1/4 strenth, you get 30. That would match the high range. For samples above 100, I always check them both to make sure there is not something odd in the sample. In fact I usually just use the 1500, because if you are getting 100-150, one day the samples are going to be 160 and out of range. 150s are best for samples consistently less than 100.

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Victor YaBB Moderator Offline Escherichia coli Happens! Posts: 1269 Company or Organization: www.santacruzproductions.com	Re: Difference in COD with Vials of different ranges Reply #4 - Dec 21^st, 2010, 10:39pm Muhammad: Check your saline levels. You may want to take a look at an old post in regards to performing COD on wastewaters high in Cl: http://www.waterandwastewater.com/cgi-bin/yabb/YaBB.pl?num=1145429827
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Muhammad Afzal Khan Ex Member	Re: Difference in COD with Vials of different ranges Reply #5 - Dec 22^nd, 2010, 12:16am Dear Al Many thanks to comment. It is very useful for me.
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gadfly YaBB Newbies Offline I love Water and Wastewater.com Posts: 1 Company or Organization: U&D Enviromenal Tech	Re: Difference in COD with Vials of different ranges Reply #6 - Dec 22^nd, 2010, 4:34am I‘m a newer in the field,Could you tell me what "ppm" means? there are many things I can't feagure out thank you my email is gadfly@gzud.com Could you sent it to my email?
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sludge judge Hennessy Ex Member	Re: Difference in COD with Vials of different ranges Reply #7 - Dec 23^rd, 2010, 7:57pm ppm= parts per million mg/L is also parts per million
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Jim Royer

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Re: Difference in COD with Vials of different ranges
Reply #8 - Dec 30^th, 2010, 9:49am

It would appear that you are using the Hach methods for COD. The first thing you need to do is analyze a standard to determine that you can get proper results. Analyze a 1000 mg/L COD standard on high range and make a 1 to 10 dilution, 100 mg/L COD, and analyze it on both ranges. If you do not get close to 1000 and 100 mg/L then you need to find out what the source of error is.

If your QC samples are OK then the problem is with the sample if the two ranges do not agree. Make sure the water used for blanks and dilution is organic free. If it is a color apsorbing problem you might have to titrate the COD's.

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sandeep Dey YaBB Newbies Offline I love Water and Wastewater.com Posts: 16 Varanasi Gender: Company or Organization: Delhi Jal Board	Re: Difference in COD with Vials of different ranges Reply #9 - Jun 29^th, 2011, 3:55am Check your chloride level at high chloride conc.it happen.use pre added Mg in COD Vails of low and high range.
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