[Kesley]

Used a 5% Hcl 5% h2o2 solution to clean iron based grunge off quartz covers for uv bulbs in wastewater plant...worked well for about 20 min. then green color changed to brown and violent bubbling exothermic reaction took off...kinda scary....neutralized with soda ash eventually.  Any thoughts?  Possibly FeOH + Hcl --> FeCL2-4H2O  green..oxidized by peroxide to FeCl3 brown?  BTW we feed ferrous sulfate in sewage system for odor control.

[DevaDevil]

Are you sure you start with Fe (OH)_x, rather than Fe_xO_y? Iron oxide is more common (rust)

in any case, peroxide is a very strong oxidizer, so I would expect your iron to be oxidized first. (in stead of anion exchange as you predict) The bubbling may be nothing more than the peroxide decomposing into oxygen and water

ps. iron (III) oxide is brown, so that may be your product (iron sulfate is green)

[Nobby]

Violet color is strange. brown is iron-III

[DevaDevil]

he was talking about brown color... and violent bubbling :p

[Nobby]

I got it. Sorry.

[eprigge]

I think you are probably right about FeCL2 (green) getting oxidized to FeCL3 (brown).  You should be able to test that it is FeCL3 in a number of ways.  I've done the same reaction with the sulfate salt and peroxide does indeed oxidize green ferrous sulfate to brown ferric sulfate.

Turns out FeCl3 is a catalyst for the decomposition of hydrogen peroxide and I think that's the reaction you saw.  You should be able to test the gas coming off to see if it's oxygen.

BTW, the reaction of peroxide with Fe ions yields a deeper brown color than 'plain' FeCl3, which itself can look amber brown when concentrated, or yellow when not.  A temporary higher oxidation state perhaps.  Color depends on oxidation state as well as how water / hydroxide ions glom on to it which is itself dependent on pH.  But I'm no expert.

[AndersHoveland]

Reaction between hydrogen peroxide and hydrochloric acid

"The catalytic decomposition of hydrogen peroxide in either hydrogen chloride solution or with chlorine has been shown       to be closely related to the two chemical reactions
 
H2O2 + (2)H(+) + (2)Cl(-) ---> Cl2 + (2)H2O
 
H2O2 + Cl2 ---> O2 + (2)H(+) + (2)Cl(-)
 
and is believed to be due to the occurence of these two competing reactions at equal rates."
Livingston and Bray, J. American Chem. Society, Volume 47, p2069  (1925)
 
In this reaction no net generation of chlorine results from the action of dilute solutions of hydrogen peroxide on hydrogen chloride, but there does exist an elemental chlorine intermediate in the reaction, which could potentially chlorinate enols.
 
When 30% concentrated HCl and 30% H2O2 is used then there develops a slight greenish yellow color and a faint but distinctive odor of chlorine, yet the gas from the bubbles is still mostly O2.  I personally have conducted this expiriment and observed only moderate steady bubbling that persisted for several hours.
 
It is mentioned in the literature that chlorine gas is evolved from 30% solutions of HCl and H2O2, although in more dilute solutions, only oxygen is generated.
"Oxidation of Hydrogen chloride with hydrogen peroxide in aqueous solution" V.I. Skudaev, A.B. Solomonov
 

[ajkoer]

First, HCl with equal amounts of H2O2 is a method for preparing HClO (Watt's Dictionary of Chemistry):

                                               H2O2 + HCl  =  HClO +  H2O

 But with excess H2O2, the HClO is reduced:
                                               
                                                HClO + H202 = HCl + O2 + H2O

I have noticed a similar reaction when HClO (created by H2CO3 + NaClO) reacts with Iron. One reference gives the expected reaction of HClO + Fe as Cl2 (a somewhat unique reaction with HClO which usually liberates its oxygen). My guess is:

                                         6 HClO + 2 Fe = 2 Fe(OH)3 + 3 Cl2

Hence, the elemental chlorine viewed as green which then reacts with H2O to form more HClO and HCl, and finally the red-brown FeCl3:
                                          Fe(OH)3 + 3HCl  = FeCl3 + 3 H2O                                           

The red-brown FeCl3 is corrosive.

[ajkoer]

Came across a reference that may be of value to those using an etching solution of HCl and H2O2 (or, HClO) as it may explain some of the observed chemistry, which appears to be a mystery to some. The online available Google book is (thank you Google for making available the pioneering work of scientists):

Chemistry, inorganic & organic, with experiments
 By Charles Loudon Bloxam

To quote a paragraph from page 115:
 "The action of some metals and tneir oxides upon solution of hypochlorous acid is instructive. Iron seizes upon the oxygen, whilst the chlorine is liberated; copper takes both the oxygen and chlorine ; whilst silver combines with the chlorine, and liberates oxygen. Mercury yields, on shaking, the brown mercuric oxychloride. This distinguishes solution of HCIO from chlorine water. Oxide of lead (PbO) removes the oxygen, becoming peroxide of lead (PbO2) and liberating chlorine, but oxide of silver converts the chlorine into chloride of silver, and liberates the oxygen;

Ag2O +  C12O = 2AgCl + O2. "

The link is:
http://books.google.com/books?id=vIQ-AAAAYAAJ&pg=PA455&lpg=PA455&dq=pyrophosphate+%2B+Fecl3&source=bl&ots=PAQkLZIGKr&sig=kw6UofEXvXye1VJC6nOg0G1bJVk&hl=en&ei=Mti9Ta3-DMmbtwf0i725BQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBYQ6AEwAA#v=onepage&q=pyrophosphate%20%2B%20Fecl3&f=false

[ajkoer]

After more research, I now suspect the reaction chain of Fe in HClO is as follows:

Fe + HClO --> FeO + HCl

HCl + HClO <--> Cl2 + H20

Hence, the observed chlorine creation. Continuing:

2 FeO + HClO + 3 H2O --> HCl + 2 Fe(OH)3

This is the oxidation of Ferrous Oxide mentioned in the extract (see below) from Mellor. Continuing with a hypothetical reaction:

Fe(OH)3 + 3 HClO --> Fe(OCl)3 + 3 H20

And, possibly an immediate reaction is that the Fe(OCl)3 oxides more iron and decomposes:

Fe(OCl)3 + 2 Fe --> Fe2O3 + FeCl3

Normally (per Mellor), the reaction of HClO on a metal oxide is a suggested path to creating the metal hypochlorite. Apparently, however, Iron hypochlorite either does not exist or immediately decomposes (as I have proposed above with further oxidation). Interestingly, my speculated behaves parallels the properties of Aluminum Hypochlorite (albeit slightly more stable) which upon contact with a fabric, for example, bleaches and then immediately decomposes with deposits of Al2O3 into the fabric itself.

Note, as the solution clearly becomes reddish brown, it is FeCl3 that forms and not FeCl2 which is yellow in aqueous solutions.

REFERENCE: "A comprehensive treatise on inorganic and theoretical chemistry, Volume 2 By Joseph William Mellor

EXTRACT
"Iron filings immediately decompose hypochlorous acid with a brisk effervescence produced by the evolution of chlorine; the iron is partly oxidized and in part dissolved as chloride without the formation of any chlorate. A. J. Balard commented on this: "The greater number of other metallic substances do not decompose hypochlorous acid, and I am yet entirely ignorant of the cause of the peculiar behaviour of iron." P. Grouvelle passed chlorine through water with iron hydroxidelin suspension and a bleaching liquid along with ferric chloride was produced, and he found the liquid retained its bleaching properties after boiling for a quarter of an hour; but A. J. Balard failed to confirm this statement; he could not make ferric hypochlorite either (i) by the action of hypochlorous acid on iron hydroxide, for hypochlorous acid does not dissolve ferric oxide; or (ii) by the action of calcium hypochlorite on ferric sulphate, for calcium sulphate and ferric oxide are produced. Hence, adds A. J. Balard, "ferric hypochlorite cannot exist;" the results by P. Grouvelle are due to the formation of ferric chloride and hypochlorous acid in dil. soln.; when the mixture was heated, a portion of the acid distilled off, and the reaction which occurred with the cold soln. was reversed, for ferric oxide and chlorine were formed. Ferrous oxide is oxidized to ferric oxide by hypochlorous acid."

A free google book. Link:
http://books.google.com/books?pg=PA275&lpg=PA275&dq=ferric+hypochlorite&sig=zHHdZcLpiC-8m8HCOj-kix87YZ4&ei=11P7TcXXMdHTgQepiZneCw&ct=result&sqi=2&id=7XoGAQAAIAAJ&ots=R0bAIgamIM#v=onepage&q=ferric%20hypochlorite&f=false
____

[ajkoer]

In addition to adding value to this thread (the reaction of Fe in HCl/H2O2, or as I have argued HClO per the Watt's reference), I believe my suggested reaction chain (I apologize for its length) is also important because it implies the creation of Iron hypochlorite (albeit very temporary) as a path to the formation of FeCl3. My limited support for this contention is as follows:

The action of HClO on Fe to create FeCl3 proceeds in even very dilute solutions (my personal observation is an approximately 2% HClO with the solution turning reddish brown occurring in days). Now, in the industrial process for the pickling of steel, using 18% strength HCl, one source has given the reaction as (see http://en.wikipedia.org/wiki/Hydrochloric_acid):

Fe2O3 + Fe + 6 HCl → 3 FeCl2 + 3 H2O

So stronger HCl only produces Ferrous Chloride, FeCl2. The other cited low temperature route is the chlorination of iron ore containing oxides, but as Cl2 in water produces HClO, this is not a low temperature HCl path on iron oxide or iron hydroxide that arrives at a Ferric chloride.

So to suggest a creation route for FeCl3, one has to detail a plausible reaction chain other than via the creation/decomposition of Fe(OCl)3, as the metal hypochlorite is normally being produced by the action of HClO on the metal oxide (see Mellor reference). This I was not able to perform, but others may have better luck. If this cannot be done, however, it lends some support to the creation/decomposition hypothesis that Iron hypochlorite exists and behaves somewhat analogously to Aluminum hypochlorite (which upon bleaching decomposes immediately depositing Al2O3 sediment into the bleached item).

Comments welcomed.