[curiouscat]

What's a good way to assay H2O2 content of an aq. solution where HCl is also present? Would standard redox titration against KMnO4 work with the HCl staying a spectator?

The H2O2 conc. we expect are between 10 - 25% and HCl also in similar ranges.

Typically, the assay protocol does have co-added H2SO4 so not sure if HCl there will cause a big difference to it. Pre-neutralising HCl would be one option but the NaOH needed may cause some H2O2 decomposition I fear.

My application isn't very exacting i.e. I'd be happy with a method that's 2-3% accurate for H2O2 conc.

[Borek]

Standard permanganate titration may not work in the presence of chlorides, as they can get oxidized to chlorine.

[curiouscat]

Standard permanganate titration may not work in the presence of chlorides, as they can get oxidized to chlorine.

Other options may be if I can

(a) find some solvent to extract away HCl very selectively

(b) find a HCl neutralizing agent that won't degrade H2O2

(c) Do some sort of total H2O2 decomposition (perhaps catalytically) and quantify weight loss of evolved O2. That seems iffy.

[Borek]

(b) find a HCl neutralizing agent that won't degrade H2O2

Define neutralizing.

[curiouscat]

(b) find a HCl neutralizing agent that won't degrade H2O2

Define neutralizing.

Yeah, won't help. Unless I can precipitate away the  chloride ions?

[Archer]

You could add KI and titrate against sodium thiosulphate, adding a little starch solution at the end to determine the exact endpoint.

This should work, I used to determine peroxide values of oils using this method in the presence of acetic acid. I can't see any reason why HCl would interferre with the results:

H₂O₂ + 2H⁺ + 2KI -> 2K⁺ + I₂ +2H₂O
 
I₂ + 2Na₂S₂O₃ -> 2I^- + Na₂S₄O₆

[Borek]

Yeah, won't help. Unless I can precipitate away the  chloride ions?

Yes. Alternatively, to make pemanganate titration in the presence of Cl^- possible, you can add a lot of Mn²⁺, that lowers the potential. But I have no idea if Mn²⁺ will not interfere in some other way, it is used in Fe²⁺ titration, not in H₂O₂ titration.

[Borek]

You could add KI and titrate against sodium thiosulphate, adding a little starch solution at the end to determine the exact endpoint.

This should work, I used to determine peroxide values of oils using this method in the presence of acetic acid. I can't see any reason why HCl would interferre with the results

Thiosulfate decomposes in the low pH, iodides in low pH can get oxidized to iodine by atmospheric oxygen, thus it is better to do the titration close to neutral pH.

As OP states ±2% is OK this can work.

[Archer]

Thiosulfate decomposes in the low pH, iodides in low pH can get oxidized to iodine by atmospheric oxygen, thus it is better to do the titration close to neutral pH.

As OP states ±2% is OK this can work.

Thiosulphate reaction is very quick, not enough time to degrade at high pH.

This reaction is also carried out to determine the residual chlorine level in waters and effluents which uses 50% H₂SO₄ as the source of acid.

See reference below for details.

http://www.environment-agency.gov.uk/static/documents/Research/chlorine218_1985298.pdf

[Borek]

Thiosulfate decomposes in the low pH, iodides in low pH can get oxidized to iodine by atmospheric oxygen, thus it is better to do the titration close to neutral pH.

Thiosulphate reaction is very quick, not enough time to degrade at high pH.

I meant low pH, but I suppose that's what you meant too

Warning about low pH decomposition as a possible interference is from a popular Polish textbook (Minczewski, Marczenko), for many years that was a basic text here used by almost everyone. Doesn't make it automatically right, but I trust this book.

This reaction is also carried out to determine the residual chlorine level in waters and effluents which uses 50% H₂SO₄ as the source of acid.

See reference below for details.

http://www.environment-agency.gov.uk/static/documents/Research/chlorine218_1985298.pdf

Interesting. It doesn't mention low pH interference, at the same time procedure described is for "Raw, drinking and swimming pool waters", so perhaps there is an indirect assumption that the sample is neither strongly acidic nor alkaline.

[Archer]

I meant low pH, but I suppose that's what you meant too

Low pH, yes 

I would agree with you, if it is not specified in the reference it is probably because low pH is not encountered in the matices described.

I guess the best approach is to try it with H₂O₂ without HCl and then add a known volume of HCl and see if the data correlates acceptably.

[curiouscat]

I guess the best approach is to try it with H₂O₂ without HCl and then add a known volume of HCl and see if the data correlates acceptably.

Agreed! That's what I'm going to try!

I'll report what I find. Will be interesting to find how much the naive KMnO4 approach gets thrown off by Chlorides too.