Hi Kamal

Sodium tiosulphate is quite often used to stop Fenton reaction. It reacts very fast with H2O2. You should check the stoichiometry of the reaction of H2O2 with thiosulphate in order to calculate which is the amount of thiosulphate that you need.

You can probe wtih a peroxydase too (of course, it is more expensive but very selective)

the reaction can be quenched by adding

1 ml of sodium sulfi te (1 M).

Thank you all for your answers. Does the test mentioned interfere with other ions (especially phosphates) present in the waste-water or increase or decrease the pH of the wastewater. These factors are very important for my research. I have used NaOH before but it will drastically increase the pH.

I have red in some papers one of the best way is to keep your samples in ice. when the temperature is below 0 celsius. in this way no interaction would happen and the reaction stops.

Although I agree with most of the methods proposed, some of these reagents are expensive or can contaminate your samples. To keep it simple and following your first intuition you can simply add sodium carbonate or diluted sodium hydroxide up to pH 7...

Right, but you can get rid of carbonate by CO2 evolution at tipical pHs of Fenton reaction...

Fenton reaction is the decomposition of H2O2 catalyzed by iron complexes. You have to add a chelating agent to suppress the catalytic activity of iron. EDTA may work.

We have been using a solution contaning KI, Na2SO3, and NaOH, all at 0.1 mol/L, to quench the Fenton reaction in samples before analysis. Samples are then filtered using a membrane of 0.22 micra and neutralized with a drop of H2SO4 solution.

Edta doesnt work! The best way is to use sulfite or bisulfite since you dont generate any sludge, I2 or other free radicals like by adding carbonate, for example!

Thank you everyone for your kind suggestions. I have tried using sodium thiosulfate recently, but I encountered some problem. If i am using all these quenching reagents, how do I know if all the hydrogen peroxide is gone? Do i have to calculate the exact mole using stoichiometry ? I just used couple of drops of concentrated sodium thiosulfate solution. Any help will be much appreciated.

I'd recommend using Ferrozine - a very strong iron complexant. You can make a fairly concentrated solution (e.g 50mM) so you don't have to add much to your sample to stop the reaction. Just make sure it is in at least a 3:1 molar excess of your iron concentration. I wouldn't recommend using EDTA. Under some conditions it can make the Fenton reaction go faster!

Good morning! Normally I used 10% of Methanol

Dear Kamal, Yes, you should add some excess of thiosulphate considering the stoichiometry of the quenching reaction...

You can use DTPA which is not ideal but much better than EDTA. If you use iron ions to induce Fenton reaction you can use deferoxamine which is strong iron specific chelator.

You can use Catalase, check this paper http://www.ncbi.nlm.nih.gov/pubmed/12867337

I am agree whit Guo-Dong Fang; but if you need to follow TOC concentration you can't use methanol or ethanol

I think the answers given are mostly pretty good, except that which reducing agent or chelating agent or scavenger is the best choice clearly depends on the objectives of the experiment. Chelators such as those mentioned are effective, but leave unreacted H2O2. The reducing agents produce new products and may glom up the reaction mixture as far as product characterization is concerned. Scavengers such as ethanol form oxidation products such as acetaldehyde or formaldehyde, which are reactive intheir own rights with many classes of compounds

I'm using Fenton reaction for protein oxidation. To stop Fenton reaction I used 10% TCA, and then centrifuged samples three time at 1000 RPM. then, added phosphate buffer on protein pellets at the bottom of tube and dissolved them into buffer. Finally, I put samples into freezer (-20) to use for different assays like carbonylation assay, DTNB assay and so on. I wonder if TCA has an effect on my different by products which has produce on the protein during Fenton reaction? 

Dear kamal,

I have read all the possible answers (i have also tried so many) , however, each of the methods have their own pros and cons depending on the type of wastewater, strength of WW, composition, experimental conditions, etc. etc. Therefore, based on my personal research experience, I would like to suggest you to use NaOH (conc soln) or NaOH pellets directly to stop the Fenton rxn. The pH first increases slowly and then after pH=8 it increases sharply. This is due to formation of Fe(OH)3 complex. Do not add further NaOH , and also do not try to neutralise at this point. All the Fe-catalyst gets chelated by this time, but H2O2 may remain present which must have to be removed. Immediately heat the rxn mix. to boiling for 2 - 5 min @ 100 degrees in order to remove the remaining H2O2. Now cool the rxn mix and proceed for SVI. Filter the samples and remove fenton sludge. Now neutralize the fenton filtrate at this point and proceed for further analysis. Enzymes are much costlier and ethanol/methanol may also increase the solubility of organic compound giving erroneous results. Further, i would like to say that this is widely acceptable and basic method and other methods may be case specific for WW used.

Thank you

I recommend you not use methanol or ethanol if COD analysis is conducted in the following step.

However, some literature used MnO2 for quenching excess H2O2.

2 MnO2 +4H2O2 ------->  2 Mn(OH)2 +3O2 +2H2O

 Good luck