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Topic: COD method for sea water  (Read 24819 times)

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BAL

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COD method for sea water
« on: August 19, 2005, 04:47:15 PM »
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!

asqa

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Re:COD method for sea water
« Reply #1 on: August 22, 2005, 01:38:08 AM »
First determine the chloride content and dilute sample to get the lower conc. chloride then add mercury compound to suppress the chloride.
Finally go for dichromate oxidation method

herewith i attached link for further information
http://www.hach.com/fmmimghach?/CODE:L70531494%7C1

Link was corrupted, hopefully it is OK now. If not, check if adding //true at the end won't help and please let me know - Borek, moderator.
« Last Edit: August 23, 2005, 08:08:55 AM by Borek »

BAL

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Re:COD method for sea water
« Reply #2 on: August 22, 2005, 12:24:38 PM »
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.

Offline kevins

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Re:COD method for sea water
« Reply #3 on: August 22, 2005, 01:37:00 PM »
Try to add more silver sulfate.

asqa

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Re:COD method for sea water
« Reply #4 on: August 22, 2005, 11:42:06 PM »
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.


asqa

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Re:COD method for sea water
« Reply #5 on: August 23, 2005, 11:13:03 PM »
instead of link attached above, now i am attaching the file.
« Last Edit: August 23, 2005, 11:16:16 PM by asqa »

BAL

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Re:COD method for sea water
« Reply #6 on: August 24, 2005, 04:57:12 PM »
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.
« Last Edit: August 24, 2005, 04:58:11 PM by BAL »

Offline Borek

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Re:COD method for sea water
« Reply #7 on: August 24, 2005, 06:18:25 PM »
SORRY FOR MY BAD ENGLISH.

Please don't abuse capital letters.

ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

BAL

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Re:COD method for sea water
« Reply #8 on: August 24, 2005, 07:36:31 PM »
I am sorry it wasn't my intention.

asqa

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Re:COD method for sea water
« Reply #9 on: August 24, 2005, 11:37:07 PM »
BUT AFTER THE DIGESTION THE SOLUTION WAS GREEN.

Mr. BAL,

what you have followed is correct except the reagent chromate solution added.

if you are not added enough chromate solution, after digestion the solution will turn green.

So add more chromate solution then digest, now will get it

all the best

Offline kevins

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Re:COD method for sea water
« Reply #10 on: August 28, 2005, 04:37:16 AM »
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.

BAL

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Re:COD method for sea water
« Reply #11 on: August 29, 2005, 02:02:40 PM »
Thanks Kevins for your advice, my problem is that i am in a very very far away place, and for me is a little dificult to get sodium bismutate, I dont think that the deteccion limits for me be a problem, because my level shoul be arround 70ppm, and I could improve it by Standard adition, I am already waiting for my own reagants, but when a use The kit following the prossedure, (taking 0.2ml of sample, adding extra 0.5 of HgSO4 or even 1.0gr, this dont work!!) I used high range for my test, There are people that say "you have to buy this, or that" I dont want to buy anything with out to be sure that this will work, because I had tested with this kits usin different batches and neither work! at least taht all the lot that I used be spoiled.
I would like to know your procedure for COD with 50.000ppm of Cl-

Thanks A lot.

Offline kevins

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Re:COD method for sea water
« Reply #12 on: August 30, 2005, 09:48:55 AM »
We use the open reflux-titration method.

Would you please tell me the details of your determination procedure and the standard preparation procedure.
« Last Edit: August 30, 2005, 09:49:46 AM by kevins »

BAL

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Re:COD method for sea water
« Reply #13 on: August 30, 2005, 10:17:13 AM »
I use colorimetric closed reflux-method.
I am ussing at this moment kit vials, I use Ultra high range reagents, the detection range is form 200ppm-15.000ppm, I blend my sample, I know that my COD is arround 70ppm so I add an aliquote of KHP STD solution to reach ~300ppm, I take 0.2ml of spike sample, put it in the cap vial (sorry for my inglish)add 0.5g HgSO4 and digest it at 150C 1.5hr, also I run a blank with detilled water and NaCl (27gr/L).
At the end my blank and my sample are green!
The salinity of my sample is ~26gr/L.

I decided to prepare my own reagents Dichromate (0.25N), AgSO4, HgSO4, H2SO4, but they are in costums, I hope to get it this week but my fear is to have the same problem. I do not want to say the name of the company, but his procedure says that the reagent work for seawater, I can not obtain good ressult.

Should I develop the titration with Mohr Salt?
I can not work with open reflux I don't have the facility.
But i Will try to run my samples at same way with reagent prepared by my selft, but before that any advice is wellcome.

Thanks!!
« Last Edit: August 30, 2005, 10:30:32 AM by BAL »

Offline kevins

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Re:COD method for sea water
« Reply #14 on: August 30, 2005, 12:06:49 PM »
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.


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