We precipate iron from the solution by raising pH to about 11, decant the sample from the precipate and then neutralize pH before COD analysis.

Do not forget H2O2 from Fenton reaction also interferes with the COD analysis (catalase enzyme treatment).

Thank you Henrik for your reply.

Is the addition of a base is not likely to precipitate or react the organic matter present in the medium to be analyzed?

Indeed, H2O2 is oxidized by dichromate, could you give me more information about treatment with catalase.

These pretreatments could they alter the sample before analysis and distort the result?

Dear Henrik,

I should note that the iron hydroxide particles are very small and are very hydrophilic, which this results in a very low rate of sedimentation. Furthermore, filtration is not adequate since it also would eliminate the organic matter in suspension.

In my experience ironhydroxides and carbonates settle in a few hours. We pipet or decant the sample for COD from the beaker we used for the experiment and leave at least 1/3 of the volume with the iron precipate.

If your experience is different I can see two reasons. You have mainly iron(III) in the solution (Fe(OH)3 is fluffy) or too little carbonate in the water. I have heard of a variation of the procedure there the pH is increased by adding Na2CO3 rather than NaOH. This could solve the problem as both iron(II) and iron(III) carbonate settle reasonable well.

Dear Henrik

Thank you for these informations that I will use.

Manganese dioxide (MnO2) can catalyze the decomposition of H2O2 to H2O and O2. You can use MnO2 to quench the residual (unreacted) H2O2 in the sample in order to prevent the interference of H2O2 on COD analysis.

If you found that iron hydroxide particles had very low rate of sedimentation in your work, I suggest centrifuging instead of filtering.

Iron(III) ions can be rendered inactive during the determination by the addition of 4 cm3 phosphoric acid or 2 cm3 potassium fluoride solution.