Hello. There is standard EN ISO 8467 describing the method with limit of quantification about 0.5 ppm (with certainty), but good laboratories can make validation the method working at LOD between 0.1 and 0.2 ppm, and LOQ about 0.2-0.3 ppm. I doubt you can reach with titration LOD below 0.1 ppm.

The problem is why you need to measure COD at 0.1 ppm? In such circumstance, you'd better to use TOC as the indicator.

If your method is precise enough you could spike your sample with a known amount of COD (50 ppm), compare it to a standard of 50 ppm, and take the difference.

TOC analysis is better instead of COD value for lower concentration. That is more precise than COD for lower concentration.

Analysis of trace level like 0.1 ppm, spike method can refer. However, you can do TOC analysis rather than COD which will give you the more and precise indication of your samples.

Measuring so low COD concentrations is a little weird. I support other comments, its better to measure TOC instead. Anyway, a much more diluted digestion solution from the APHA Standard Method could let you to measure COD values as low as 0.1-1 ppm. You could even try to prepare a solution by yourself, if you prepare the reactives theoretically. I recommend in this way to use ultrapure water for preparing any component, since the minimum impurity could increase the COD values.

I would support Greg Loraine's suggested. Running a blank with a controlled COD level then running a second sample with the same controlled amount mixed into your low COD is going to yield the most accurate measurements.

But also agree that needing to measure anything at such a low concentration is rare, so you must be working on something very special.

The current method doesn't measure below 40ppm because anything lower is hard to measure accurately and can read falsely with 1000 different contaminates in the sample, miss calibrated machines or improperly measured reagents.

Thanks all for your valuable suggestions.. :)

Other possible method could be concentrating the sample by dehydratation, if you have enough sample for doing it.

Any method has a range of application: COD by dichromate method can not be applied to measure 0,1 mg/L. For these low concentration you might try TOC (total organic carbon) or better specific methods by GC or HPLC if you know the nature of the compounds.

As other researcher's said, what is your problem that you need to measure COD at such trace concentration( 0.1 ppm)? In such circumstance, you'd better to use TOC as the indicator test. Also you can use standard addition technique in a precise test. In such manner you should be add a given concentration to all of your samples (such as 100 mg/l or)and then follow the COD test method. Then after analysis completed you can determine the real sample concentration respecting to COD.

The explanation regarding your quest to measure rather low COD concentration had been well expressed by previous. I do suggest you measure Total organic carbon.

Chemical Oxygen demand determination is a measure of the readily oxidization material in water, and furnishes an approximation of the minimum amount of organic and reducing material present . In reality COD is defined by the method used for its determination. If the concentration is very low

Dear Dr. Saxena:

if you have any reference for the same, please kindly let me know. thanks in advance.

Try to search USGS publications.

You'll have to use the old titrimetric method. You can't come anywhere close to this level using colorimetric COD methods. Given the titrimetric method, and using diluted reagents, you can get reproducible results in the 1-3 mg/L range, and with great care you can probably extend this down into the sub-ppm range. You will have severe issues with reagent quality, especially water.

At these low concentrations, COD is not indicated, there is a imprecision non-surmountable measures go to the TOC measurement which is a stronger but more expensive method

For such low concentrations TOC is suitable . But why do you want to measure such low concentrations; Find attached relative article.

I also think that this is rarely used, but if for some purposes (research etc.) have to use the dichromate COD method, there is a way.

Use the titrimetric method. As others suggested, you have to be very precise with the work procedure, by using the most precise pipettes available and very diluted reagent (ferroammoniumsulphate). The final and the most important thing is to use a very fine burrette for titration. Good luck and inform us on how is it going.

Using the colorimetric method, oxidizable compounds reduce

the Cr6+ (dichromate ion, Cr2O7 2–) to green chromic ion (Cr3+). At low concentrations (although I'm not sure if as low as 0.1 ppm) the amount of Cr6+ remaining is determined (440 nm), while at high concentrations, the amount of Cr3+ produced is determined (600 nm). Potassium biphthalate acid is used as standard

Dear Gisela:

can you send me the URL links of such article papers.. !!!

I have the file as a pdf, send it to you by mail

Dear Gisela: Okay..!! my mail id is-

[email protected]

Accuracy of COD for normal samples itself is not confirmed unless otherwise the sample is synthetic carbohydrates.

Hence go for TOC analyzer. Dont forget to analyse for total nitrogen also.

Lower COD is erroneous Hence TOC is best suited for your sample.

There are many more submerged reasons on why COD cant be measured if the sample COD is less than 0.1 ppm. We have carried out COD of distilled water and deionized water and pure drinking water. Surprisingly we found the results in plus or minus 0.5 mg/L. This was possible by taking double extra precautions and it took 3 days to prepare for one set of COD experiment backed with nearly 1 year of research using all ultra grade quality chemicals etc etc.. Otherwise it is very much difficult even to get identical results with the same sample set. Even with standard solution of lactose, which was prescribed by ASTM/ Standard Methods by normal means it is difficult. Ofcourse we got an error of less than 0.1%. We could not get that accurate measurements.

Udaya Bhanu's results are not surprising. Remember, you are not actually measuring organic compounds; you are measuring the oxygen demand of a sample. People forget that there are many, many inorganic species that exert an oxygen demand (can be oxidized). It will most likely not be possible to get a blank that has a COD of less than 0.1 mg/L. I would expect that the lowest you could routinely measure would be somewhere between 1 and 3 mg/L; note that the lower limit on the Hach colorimetric method is listed as 3 mg/L. Udaya's data is consistent with this.

Please Gisella, could you also email me that PDF paper, thanks

What Mark Krause tells is practically acceptable. I do agree with his views. Any inorganic species contributes to COD measurements. As a matter of fact COD and BOD are just indicative and not 100% reliable since they are more likely to get changed periodically in any effluent or waste water especially in industrial scenario.

Any analytical method applied for pure compounds is stable. This is a well known fact. But for applications in environmental monitoring /analysis (air, Water or soil/ sludge) the readings are highly dynamic and cant be argued for 100% accuracy whatsever be hifi technique applied except radio-analytical techniques which again is not accessable easily.

One of our professors /teaching faculty felt that in practical applications upto an error of 2% is acceptable in environmental analysis unless otherwise its really required for proposing a theory/ concept. The analysis may be correct. The instrument may be correct even the samples also are identical. There are many cases we get NOT 100% accuracy results.

LETS RETHINK A WHILE.... WHICH ENVIRONMENT IS STABLE CHEMICALLY OR BIOLOGICALLY?

So lets not worry much about too accuracy readings especially in COD BOD is not 100% reliable in numbers.