[Pippo89]

Good day,
I work for a chemical company and I have a "simple" question concerning the removal of Iron precipitate in swimming pool.
Minerals can become a problem in the water when they are oxidized by chlorine (or bromine).
The following reaction is what happens:
HOCl + 2Fe²⁺+5H₂O 2Fe(OH)₃ +Cl^-+5H⁺

One empirical solution is use Hydrogen Peroxide (35% v/v). The brown precipitate disappear...
What happens chemically?

I'm wait for your reply (and sorry for my bad english).

[Pippo89]

The discussion continue here.

[Arkcon]

The redditors are at least half correct.  Iron (III) might be reduced by peroxide to iron (II), but that doesn't make it soluble.  Maybe in some obscure way, the color fades, or its grip on concrete is loosened, or it reacts again into something soluble, or more likely, something insoluble that washes away.

You can try writing balanced equations until something makes sense to someone.

See if Chemistry by Paulings has some hints.  He wrote a lot about iron chemistry.

[Arkcon]

Did I say reduce?  Whoops, sorry.  Peroxide is an oxidizing agent.  But Fe (III) is already as oxidized as its going to get.  Something else must be happening.  Maybe oxidizing some organic that lets the stain stick?

[Intanjir]

Actually Iron(III) is reduced to Iron(II) by hydrogen peroxide. At the same time Iron(II) is oxidized to Iron(III) by it as well. This ping-ponging is the catalytic machinery that makes for the Fenton reaction. The free radicals produced are extremely potent.

Also Iron(II) is appreciably soluble at neutral or acidic pH. Here is a nice solubility chart for iron's two oxidation states.

Perhaps if there isn't too much iron then it will cycle between oxidation states faster than iron(III) can precipitate? At least until the hydrogen peroxide runs out, and then the O₂ will eventually bring back the iron(III) hydroxide if your hypochlorite doesn't do it first. So I would expect the brown to come back somewhere unless something else sequesters the iron.

Maybe it just ends up reprecipitating as larger particles of hydroxide and settles. Or maybe it never totally dissolves and the H₂O₂ just sort of accelerates Ostwald ripening?

[Arkcon]

Or the iron dissolves in the phosphoric acid used as a stabilizer.  Heh.  That one is a little silly.  Maybe the peroxide oxidizes something else, that removes the grip the precip has.  But your idea, make and break and reform the precip as large wash away flakes ...  now that has a lot of merit.  Sound like a way to solve lots of problems, if you can swing it.

[Pippo89]

Or the iron dissolves in the phosphoric acid used as a stabilizer.

The quantity of phosphate isn't to low?

I quote US 4534945 A Patent:"An aqueous 25% to 35% solution of hydrogen peroxide with a residue on evaporation of 20 mg/l or less are stabilized against decomposition by a maximum of 1.4 mg/l tin. The tin is maintained in the solution in the form of a very fine colloidal sol by the addition of a maximum of 2-5 mg/kg of phosphate added as a phosphonic acid and a maximum of 5.5 mg/kg of a hydroxycarboxylic acid."

Perhaps if there isn't too much iron then it will cycle between oxidation states faster than iron(III) can precipitate? At least until the hydrogen peroxide runs out, and then the O₂ will eventually bring back the iron(III) hydroxide if your hypochlorite doesn't do it first. So I would expect the brown to come back somewhere unless something else sequesters the iron.

I can confirm that, in practice, then when you restart the chlorination there aren't brown precipitation. Not in a short time, at least.

[jasongnome]

Did I say reduce?  Whoops, sorry.  Peroxide is an oxidizing agent.  But Fe (III) is already as oxidized as its going to get.  Something else must be happening.  Maybe oxidizing some organic that lets the stain stick?

Hydrogen peroxide undergoes a disproportionation reaction when it decomposes (oxygen in the peroxide is both oxidised and reduced) so it can be used as an oxidising or reducing agent depending on the conditions. It will, indeed, reduce Fe³⁺ to Fe²⁺.

Iron (II) hydroxide is a pale green colour so in the tiny amount present in a whole swimming pool it probably wouldn't be noticed as easilythe bright rusty colour of the Iron (III). This makes me wonder if this is masking the problem rather than solving it.