Hello!
I am trying to determinate COD (chemical oxigen Demand ) from sea water , I had analyzed waste water before but as everybody knows High Clorides leves interfer on the analysis, the litarature say that you can anlyze COD on sea water but I couldn't. I need to develop and spectrofotometric method, my COD is arround 65 ppm ( is not open sea water), I really don't thrust on Kits because when I prepare and standar i can not obtain the same concentration.
I need to know if any body heve some expierence with this, and what is the best way to achieve it.

Thanks!

Thanks asqa, but my problem is that my cloride level is arround 16gr of Cl-/Lt  so, if I dilute my sample probally I can not be able to detect COD because my COd level is 64ppm, i followed the Hach method before but if I prepare and KHP std of 80ppm in water with 25gr/L of NaCl I can not ontain a good result I am starting to believe that COD is availabe for seawater. I don't know if I woould have to use more mercuric sulfate, sylver sulfate or a further reagent or treatment.

Thanks Again.

silver sulphate is used here as catalyst for oxidation.Mercuric sulphate  act as suppressing agent for chloride.

Another way, you can find out the Total COD i.e. organic compound + chlorides then find out COD of Chloride. Finally you can substract it.

you will get a very close value.More over mercury sulphate helps to suppress chloride for certain level only.

HELLO AGAIN, AREALLY THANKS FOR YOURHELP,
 I PREPARED 3 STD OF 300PPM OF COD USIN KHP 10000PPM, IN PURE WATER WITH 20.000PPM OF CHLORIDE, I USED 0.2 ML OF STD IN THE HACH HIGH RANGE PLUS FOLLOWING THE HACH PROCEDURE, AND PLACED 0.5 GR OF MERCURIC SULFATE, THE PROCEDURE SAY THAT THIS COULD WORK EVEN FOR 40000PPM OF CLHORIDE BUT AFTER THE DIGESTION THE SOLUTION WAS GREEN.
I WAS LOOKING FOR A REAL PROCEDURE FOR SEA WATER OR SOMEBODY WHO HAS MADE THIS ANALYSIS BEFORE BUT I CAN NOT FIN IT .
I DON'T KNOW WHAT MORE TO DO.
I WOULD LIKE TO KNOW IF I NEED TO DO SOMETIHNG THAHT THE PROCEDURE DONT SAY OR IF EXIST ANOTHER METHOT FOR THIS.

THANKS.

SORRY FOR MY BAD ENGLISH.

I am sorry it wasn't my intention.

Try to add more silver sulfate.

I am sorry about that it should be mercury sulfate not silver sulfate.
We have experience (painful) for the method development of COD determination in 50,000ppm chloride sample. The chloride suppressed by the weight ratio of 10:1 for HgSO4:Cl- (from standard method). The result shows that the detection limit is very high (~40mg COD/L)(which is higher than the clean water sample ~1mgCOD/L).
For the HACH method, I suggest you add more HgSO4 and reduce the volume of sample used.

The Cl- can be removed by solid sodium bismuthate (suggested by Miller) as follow:
"A special chloride removal cartidge containing a glass-fiber filter(upper cartidge, for removal of particulates) and a column packed with a mixture of solid sodium bismuthate and an inert, free-flow agent (lower cartidge) is used. The acidified sample is drawn through the chloride removal cartidge under carefully controlled conditions of contact time, flow rate, and acid strength to allow chloride and bismuthate to ract effectively without destroying smaple organics. Dolid sodium bismuthate is not soluble in the acidified solution and remains in the cartidge. The by-product, trivalent bismuth (Bi 3+), dissolves in the acidified sample solution but has no effect on COD results. The glass-fiber filter containing particules is removed from the top of the cartidge and combined with the treated liquid component in a COD reagent vial for total COD measurement." ---from Encyclopedia of Analytical Chemistry (Elsevier Ltd. 2005)

Reference: Miller DG, Brayton SV, and Boyles WT (2001) Chemical oxygen demand analysis of wastewater using trivalent manganese oxidant with chloride removal by sodium bismuthate pretreament. Water Environment Research 73:63-71.

We use the open reflux-titration method.

Would you please tell me the details of your determination procedure and the standard preparation procedure.

I have same experience as you that the sample contains high chloride and use the close reflux-colorimetric, after digestion the solution turn green. I repeat with excess amount of HgSO4 added to and reduce the sample volume.

At this moment, I think you need to check:
1. distill water. (any color change after reflux)
2. check the calculation of standard (is the right concentration used?)
3. standard check (without chloride added)
4. Test the reagents  (any color change after reflux)
5. sample only (any color change after reflux).

Please try.