I am using HACH kit to measure COD but i am facing some problems which i have listed below .
1) I did measure COD of wastewater by high range COD vial. It showed 2300 mg/l and after digestion the color turn in to greenish shade. But high vials have the range 0-1500 mg/l. Is this value is correct without doing dilution ?
2) I did use Ferrate (VI) for the removal of COD. After the treatment I did measure the COD using High range and low range vials for same sample but both vials showed different result for same sample , high range vials were showing 2324 mg/l, 2475 mg/l, 1920 mg/l and low range vials were showing 70 mg/l , 240 mg/l, -130 mg /l, Which one result i have to chose ?
3 ) After the treatment some of low range vials show negative result can i considered these result as zero ?
Hi!
1) No, the value is not correct without dilution. Measuring COD by this process relies on the use of Cr6+ as oxidizing. If the value of COD is too high, the amount of CR6+ available in your kit might not be enough to oxidize all the oxidizable content of your sample. This means that the oxidizing agent could be the limitant reactant. Furthermore, you are relating absorbance to the concentration and you may also be working outside the linear range assumed by the kit to give a concentration value. Perhaps you should read a bit more about Beer-Lambert law?
2) What do you mean by "use Ferrate (VI)"? What was the treatment process? Fenton's reaction? About the values "high range vials", see the previous point. Unlike the high range method, in the low range your setup will measure the absorbance of dichromate (Cr6+) and not the resulting Cr3+ (in this case the calibration negative absorbance vs concentration). Iron 3+ also absorbs in this wavelength, which justifies the values you obtain.
3) No. Those are nonsense results resulting of matrix effects.
Regardless of how you are using iron to treat your samples, you should make sure that it is properly removed before evaluating its effects. Furthermore, if you are using Fenton's reaction, then be also extremely careful with the removal of excess peroxide.
Finally, keep within the working ranges of your kit and go a bit more into all possible reactions that can occur during the COD measurement involving all components you know are present.
Cheers!
You need to dilute the sample ro bw in calibration range.
If you have high conc. of ions they can interfere, thus affecting your result.
Regards
GB
The initial concentration of COD is so high, you need to do the dilution.
Mário Silva Thanks for your cooperation.
I am using Potassium Ferrate, its Fenton's like reaction.
After adding the calculated amount of Potassium Ferrate in glass beaker and then stirred it for calculated time for homogeneous mixture. Then stirring was stopped after calculated time. Then 20 minutes were given for Sedimentation. After that the sample were used for COD measurement. I didn't add anything to stop the reaction.
2) Should i used any thing like Na2SO3 to stop the reaction ?
Of course you need to dilute your sample. But, why using Hack vials for which you need to pay a considerable amount of money, and not constructing your own method using the raw materials and either determining COD photometrically or titrimetrically by using FAS (Ferrous Ammonium Sulphate)...
I’ve also experienced this. Dilute your samples. For wastewater maybe 150 times. You can do a standard calibration curve for 50 - 200 times dilution to come up with the best fit for your sample. Run triplicates of each.
All the best!
The concentration of the initial measured is high you need to dilute it. It should be diluted and calibrated the instrument.
Dilutions of 10 fold would be okay for the first question.
https://www.hach.com/cod-digestion-vials-high-range-pk-150/product-details?id=7640193207
Useful links:
https://mail.yahoo.com/d/folders/1/messages/43785
https://www.tandfonline.com/doi/abs/10.1080/19443994.2016.1172030?journalCode=tdwt20