Dear Maharaja,

The COD test is an empirical test and is applicable to almost any aqueous sample as an index of pollution. The test is not subject to inhibition by toxic components that would be affected by tests that are dependent upon biochemical oxidation. Samples are oxidised in a defined manner by refluxing with sulphuric acid and potassium dichromate. Silver is added to catalyse the oxidation of certain compounds, such as alcohols and low molecular weight acids. Mercury is added to suppress the effects of chloride and ammonia. In the mercury-free methods, chromium(III) is added, as chromium(III) potassium sulphate, which together with excess silver suppresses chloride interference and the effects due to the presence of ammonia. The resulting mixture is refluxed for two hours and residual dichromate is determined by titration with standardised iron(II) ammonium sulphate solution or spectrophotometrically. The amount of dichromate reduced is expressed as COD in the form of milligrams of oxygen consumed per litre of sample. Blank and analytical quality control determinations should always be undertaken with every batch of samples.

In COD test process, the reactions taking place during the oxidation can be expressed by the following series of equations.

Oxidation reaction: Cr2O72- + 8H+→ 2Cr3+ + 4H2O + 3O

Iron(II)/dichromate titration: 6Fe2+ + Cr2O72- + 14H+→ 6Fe3+ + 2Cr3+ + 7H2O

Chloride interference reactions: Cr2O72- +6Cl-+ 14H+→ 2Cr3+ + 3Cl2 + 7H2O 

Hg2+ + 4Cl-→ [HgCl42- 

Cr3+ + 6Cl-→ [CrCl6]3- 

Ag++ Cl-→ AgCl

For more details, you can find out in the following reference

Cheers,

Dang

Dear Dang,

Thank you for  your response to my query. You mentioned the conventional COD estimation technique and its significance. This procedure based on the oxidation-reduction reactions between the organics of wastewater and the oxidant (K2Cr2O7). We estimate the residual dichromate after the complete digestion followed by the titration against FAS in the presence of ferroin indicator(oxidation-reduction indicator).

I have the doubt that if already wastewater contains chromium ion (III) or Cr(VI) means what we are followed the equation above mentioned would be disturbed.

Because the excess Cr ions (present in wastewater) also involved in the Redox reactions which reflect in the titartion with FAS .Rather in blank experiment there is no interference of these ions.

Such cases how can we justify the COD estimation?

Regards,

Maharaja.P

Dear Maharaja. P,

I understand your thinking. Actually the composition of waste water is very complex. When we would like to determine COD, that means the total of Chemical oxygen demand for oxidation all of metal cations as well as another reduction ions (Fe2+, Cr2+, Cr+3,...). However, ion Cr+6 in your sample is an oxidizing ion. Therefore, you do not need to determine COD for this ion. However, if you are sure that in your sample, Cr6+ only exists in the form salts with chlorine or sulfate (CrCl6, Cr(SO4)3) , you are ready to do test COD without any worrying about significance.  For me, if ion Cr+6 in your sample exists in the form (Cr2O7)2-, maybe your result is a little bit error.

But  for me, I think that in any waste water sample, there is not ion (Cr2O7)2-. So this method can apply for your experiments without any errors

Good luck,

Dang

Dear Dang,

Thanks for your reply. In most tanning industry , the wastewater contains some amount of Cr (in III or IV state) used in chrome tanning process. For this reason only i want to clarify my doubt on COD estimation.

Thanks and regards,

Maharaja.P

You can try with a double essay: Make a "blank titration" of the sample before the digestion and, then, use the standard method. Any Cr III in the sample should be detected and corrected in this way.

Dear Prof.Iñigo Legórburu,

I agree what you mentioned. I have the same idea like we have to assay twice.

 But, i think that one titration in the absence of dichromate (wastewater+ COD acid) after digestion, have to titarte with FAS.(if Cr of wastewater interfere)

Another titration in the presence of dichromate. From the two titrations we have conclude the estimation.

Is it good?

Thanks and regards,

Maharaja.P

Dear Maraha,

I would make several essays in order to evaluate the interference:

-Try titrating, after adding dichromate, at several pH values without adding silver.

-Look for information on the reaction between CrIII and CrVI.

-Use synthetic and spiked samples.

Good luck,

Dear Prof. Inigo Legorburu,

Thank you for your suggestions.Definitely it will help in my studies.

Regards,

Maharaja.P 

Dear Maharajaji,

I am having the same problem. I think if we measure the TOC of the sample instead of COD we may have some accurate picture of the wastewater sample.

Regards.

Dear Sameer Sayeed,

Yeah, its better to analyse TOC instead of COD to such kind of samples. Thank you for your suggestion.

Regards,

Maharaja Pounsamy

What will be the corrected or modified method for estimation of COD of Chromium complex azo-dye conaining waste waters? Dang Nguyen Thoai , Maharaja Pounsamy , Iñigo Legórburu