If chloride concentration is less than 2000 mg/L, it is possible to measure COD correctly with the supplementation of extra mercuric sulfate. It is explained in standard Methods. It says that the presence of chloride can be overcome largely by complexing with mercuric sulfate. Although 1 g HgSO4 is specified for 50 mL sample, a lesser amount may be used where sample chloride is known to be less than 2000 mg/L, as long as, 10:1 weight ratio of HgSO4:Cı- is maintained.

I suggest you to check the procedure with artificial samples containing known amount of COD and chloride. If it gives acceptable results, then use the procedure to measure the COD of your sample.

HI Erkan. I thank you very much from your guidance.

Add the mercury sulphate in A Sample.(Hg So4 )

A simpler method is to put carefully the water to be analyzed in the concentrated sulfuric acid, add 1-2 ml more of acid in the apparatus used for analysis then heat up to the boiling point, then keep for about up to the nearly complete evaporation of HCl, then carefully add first the dicromate and then the catalyst, heat and do the 2 hours required for analysis. It is well known (and is one of the methods to produce HC) that suluric acid shift HCl from NaCl...

@Luca. Have you evaluated if this procedure with boiling off the chloride as HCl interfers with the COD measured? I suspect that sulfuric acid could oxidize some organic material and therefore cause an erroneous lower COD result.

Mercury is not very popular as a laboratory chemicals due to the environmental problems related to this element and the general ban on its use.

In some samples it is possible to add silver nitrate to precipitate AgCl. Then no more AgCl precipates with addition of more AgNO3 the chloride free sample is pipetted into the test tube for the standard COD analysis.

Hi Henric. I tanks for Participate in academic and research questions and answers. I'm now using HgSO4 method and I've earned the right results. In this experimental,

I use AgNO3 for measurement of chloride in my wastewater and silver nitrate well enough to use it. To me, silver is a toxic metal too that is dangerous enough for Eco. But this could be one of the negative results of any scientific investigation.

I agree that both mercury and silver are toxic, but silver is not very toxic to humans.

We have used silver knives, forks and drinking cups for centuries without toxic effects, whereas mercury is one of the most toxic metals for humans with the possibility to bio-accumulate and form organometals which are volatile and also very toxic.

A worldwide ban on the use of mercury is eminent and this is the reason for the interest in removing mercury from common laboratory procedures such as the COD analysis which will be covered by the ban.

We who work with environmental issues can set an example for the other experimental sciences by making sure that environmental laboratories consider their environmental impact and how it can be minimized.

http://www.un.org/apps/news/story.asp?NewsID=43963&Cr=mercury&Cr1=#.URV0mqX918F

Tank you very much Henrik. You're right.I agree with you. but a question. if this method is correct,then Why are not included in the standard method for wastewater?

Firstly the COD procedures in the standard books are very old and it is not easy to make a prove that a new procedure is equivalent to and old one. Secondly, mercury has some catalytic effects in oxidation reactions. E.g. known from Kjeldahl analysis. Therefore I suggest that the silver titration procedure is used to bring the chloride level down to the limit that is specified in the COD procedure and then the procedure can be performed according to the procedure.

Then the mercury ban becomes effective it is unclear if we are going to make a new COD procedure or - as I have heard the Swedish are planning to do - that the COD parameter will be replaced with the parameter TOC/DOC for regulation.

thanks again Henrik for your important guidance. I am so happy that discuss with you and took advantage from it. I hope that you be successful always. Maybe next time you can get help me. I definitely say your recommend to my supervisor.

I too agree that AgNO3 can be used to precipitate Cl, instead of using HgSO4 due to its environmental impacts. however, if Cl concentration is less than 2000mg/L, there would be no problem with COD, but still you want to remove Cl then use silver...:)

It's never too late to give a suggestion, I agree with you when you say if you have less than 2000 mg/L you don't have to worry about interferences of Cl-, I have treated water with more than 29 000 mg/l of Cl-, I used hach COD vials but I got interferences of 1400 mg/L only for Cl-. I have prepared samples with same concentration of Cl- using NaCl and I got the interferences of these samples.

you can precipitate chloride ion by silver nitrate as silver chloride then separate it but AgNO3 has another interference (NO3) for COD Test. Another suggestion is using resin for chloride removal. it is called (styrene base cation exchange ion in silver) .

In COD measurement with spectroscopy, there is a standard method that it has some materials such as HgSO4 that help for having right answer for COD, it can cover up to 2000 ppm of Chloride interference, if we refer to articles and standards if use HgSO4 10times more than Chloride we can be confident for removing Chloride, but this procedure is tedious and boring,