The only way that the value could be negative is if oxygen increased in your incubation. This could result from several errors, but most likely light was allowed to penetrate your bottle allowing photosynthesis to occur, otherwise you have to check the calibration of the instrument.
Among other less likely factors, it could be your blank being misread or badly prepared.
Dear Iresha,
You have got minus values of COD due to some mistakes ( you should find out yourselves which) in the process of chemical analyses. COD cannot be minus even theoretically, it is always more or equal to zero. Theoretically even distilled water contains some organic matter. In my practice I observed the cases of minus BOD (biological oxygen demand) which is quite possible, as it was stated by Mr. Panagiotis.
With best wishes
Bakhtiyor
As Dr. Karimov answered above, COD values should be somewhere between zero and a positive value. Negative value means that an error occurred in your test settings or apparatus. Therefore, you must check it carefully before repeating your measurements.
Best regards
SM Najim
COD values can not be negative. My suggestion is that you have to recheck your observations and calculations and also calibrate the measuring instrument.
Thi Lagan correctly pointed out that CHemical Oxygen Demand value can not be in negative. It is better to recheck your analysis precisely.
Minus values could result of the only presence of oxygen in the best way or lack of accuracy in blank preparation in the wrong way. Recheck your blanks and calculations.
In addition to the comments above regarding blanks and calibration, my experience with pre-prepared closed digestion vials has shown additional interference from suspended solids that have not fully settled before reading and from excess chloride in the sample or blank.
The COD determine through chemical oxidation by dichromate , the majority of the orgainc matter present in water or wastewater it needed for mass balance.
reduced Cr+3 is equivelent to COD in sample, actually we measure residual Cr6 by reducing it to Cr3 using FAS, difference between is actually COD, if negative which is not true, charecterize sample, it may contain some interfering constituents, ( titration)
colrimetric, spectrophotometre, will measure Cr3, wave length 600nm, complementry of near blue,
This is tricky; I have never experienced such. If you use a blank digested in parallell this could be contaminated. Do you titrate with an indicator or use some other end-point detection?
I think the samples were highly polluted thus you should dilute them by DW.
Well,
Supposing, the procedure of test is going ahead without any errors.
-What is the amount of DO concentration in water sample?
- Eh (Redox Potential) value is it negative or positive?
you can find the solution
Sincerely
Can anyone please suggest a method which is to be used to measure C.O.D, because I am also facing the same problem. How generally blank is prepared is there any references?
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