Dear Dr.,

I assume that you are using APHA standard for measuring COD. The precipitation upon addition of silver sulphate is due formation of ionic complex between the residual oxygen+ waste ions of water to be tested. 

Thx very much

So how can i do the test without appearance of this precipitate  because it give me a wrong value

The precipate could be AgCl due to too high chloride content in the sample you analyse. If you get a wrong value it is likely due to that all the silver ions are precipated. In some cases you can correct the analysis by adding more Ag2SO4 until all chloride is removed. 

 Dear Sir,

I think, you have to add Ag2SO4 (and not AgSO4) in concentrated H2SO4 for preparing COD.H2SO4

The white precipitate is probably due to high chloride concentration in the sample. For removing chloride interference, you will have to add a pinch of HgSO4

Dear Henrik

if the chloride concentration more than 1000 mg/l it may cause that but in our sample the chloride concentration less than 100 mg/l

I think, that the precipate could be AgCl,  probably due to high chloride concentration in the sample. 

Do you see precipation in the blank and control samples?

Could you describe the origin of the samples.

The sample is synthitic wastewater containing hexan , ethanol, cacl2 and phenols 

To rule out the is an error with reagents - can you say if the control (positive) and blind samples precipate on adding the Ag2SO4+H2SO4?

Dear Dr.,

We use a little modified APHA procedure in our lab to estimate COD. I am enclosing the complete procedure for you reference. Give it a try  hope you will find no precipitation

Reagents and method of COD with open reflux calorimetric following standard methods as per American Public Health Association (APHA), method of analysis.

Preparation of reagents                                          

a)    Standard potassium dichromate solution (0.0417 M or 0.25 N, dissolved 12.259 g primary standard grade K2Cr2O7, previously dried for 2h at 103ºC in 1l distilled water)

b)    Sulphuric acid reagent (5.5 g Ag2SO4 /kg H2SO4 was dissolved. 1-2 days was required for complete dissolution of Ag2SO4 )

c)    0.25 N Ferrous ammonium sulphate standard reagent (dissolved 98 g Fe(NH4)2(SO4)2 in water and added 20 ml H2SO4. Final volume was made up to 1L. This solution is standardized daily against 0.25 N K2Cr2O7 solution)

d)    Ferrion indicator (dissolved 1.48 g 1,10-orthophenanthroline and 0.7 g FeSO4.7H2O in 100 ml-distilled water).

e)    Mercuric sulphate (HgSO4, crystal or powder is used).

f)     Potassium hydrogen phthalate (KHP) standard (Dissolve 425 mg KHP in distilled water and dilute to 1000 ml. KHP has theoretical COD of 1.176 mg O2/mg).

Apparatus

Round bottom flask (500-ml capacity) connected to 30 cm (12”) Allihn condenser.

Procedure

50 ml of appropriately diluted sample was added to 500-ml refluxing flask. Added 1 g HgSO4 along with several glass beads followed by a slow addition of 5.0-ml sulfuric acid reagent along with regular mixing to dissolve HgSO4.  The flask was kept cool while mixing to avoid possible loss of volatile materials. Added 25 ml 0.0417 M K2Cr2O7 solution and mixed the contents properly by gentle swirling. Attached the flask to condenser followed by addition of the remaining sulfuric acid reagent (70-ml) through open end of condenser with continuous swirling and mixing. The open end of condenser was covered with a small beaker to prevent foreign material from entering refluxing mixture and the contents were refluxed for 2 h. After cooling the system, wash down the condenser with distilled water. Disconnected reflux condenser and diluted mixture to about twice its volume with distilled water. The unreacted K2Cr2O7 was titrated against FAS, using 0.10 to 0.15-ml (2 to 3 drops) ferroin indicator. Although the quantity of ferroin indicator is not critical, use the same volume for all titrations. The end point of the titration was noted as color change from blue-green to reddish brown. The blank was refluxed and titrated using the same procedure. The quality of the reagents was evaluated by standard potassium hydrogen phthalate solution.

The COD was calculated in mgml-1 (ppm) as:

                                      COD = [(B-S) x N x 8000]

                                                                 V

Where,

B = Volume of ferrous ammonium sulphate used with blank (ml)

S = Volume of ferrous ammonium sulphate used with sample (ml)

N = Normality of ferrous ammonium sulphate

V = Volume of sample used (ml)

Regards

I presume that the white precipitate is indicating the Silver (Ag+) present

Dear Henrik

yes there are the same participate in the blank sample

Dear Oilva

Thank you very much

Do you mean Ag2SO4 or AgNO3

Silver nitrate is quite soluble; silver sulfate is not very soluble.  Adding sulfate (such as sulfuric acid) to a silver nitrate solution will precipitate silver sulfate.

As mentioned colleague silver sulfate dissolves slowly in sulfuric acid, but if a white solid colloidal persists may be that the acid is impure with chloride and AgCl is formed.

In that situation I had to change service provider's acid.

Dear Hernan

I think that the problem in the sulfric asid so i will change it 

One of my colleagues today has faced similar problem while performing COD testing. A gelatinous white colloidal substance is formed which remains in the COD tube even after adding acid and sample. What should be the reason? problem is with acid?? or AgSO4??

Dear Kshitiz:

I try to change reagent, sulfuric acid is cheap, if the fault disappears then it was acid, but we must keep trying.

Does it only with the sample or with the blank?

Did you try with a standard?

Dear Kshitiz:

I try to change reagent, sulfuric acid is cheap, if the colloidal disappears then it was acid, but we must keep trying.

Does it only with the sample or with the blank? Did you try with a standard?

I aggree with Henrik.

why we are adding Ag2SO4 along with sulphuric acid? Can you give me an answer please?

The COD worldwide is estimated using APHA method. You are confusing the reagents used at differen steps. The most general COD determination uses potassium dichromate (K2 Cr2 O7 ) in excess in an acidic medium (maintained by adding 50% H2SO4) while the silver sulfate (Ag2 SO4 ) as a catalyst, and mercuric sulfate (HgSO4 ) added to remove interference of chlorides. Hope the problem is answered

Regards

Mukesh

Yes Sir Thankyou so much. So this catalyst will help in oxidation of organic contaminants to carbon dioxide and water, isn't it?