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happyfooddance
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Do you know, offhand, how light or uv radiation affects this equilibrium? I would imagine it to be pretty significant.
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unionised
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Quote: Originally posted by AJKOER  |
A technical point, my understanding is that a ferrous salt REDOX commencing with oxygen (and no H2O2) is not referred to as a fenton reaction.
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OK, so why did you say
"he latter hypochlorous acid can even more vigorously engage in a fenton-type redox reaction than H2O2, also producing hydroxyl radicals (see, for
example, "
if it's not a Fenton reaction?
I'm quite happy to accept that there could be a radical reaction- chlorine is good at those.
And I also agree that HOCl is likely to be involved.
It's hard to see how you could rule out a Hoffman degradation like reaction with proteins.
But there really was no need to drag Fenton into it, was there?
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aga
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Very thorough unionised.
As a side-question, doesn't Cl2 mainly stay as-is without some external input, such as UV light ?
If so, the conc of HCl & HOCl would diminish further if it was cold and dark.
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unionised
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It's hard to define the idea of "equilibrium" in the presence of light.
Enough hard UV would turn water Cl2 etc to HCl and O2
Not many factories are that sunny- so it hardly matters.
However Cl2 is very reactive (through a number of mechanisms) with a lot of biologically important molecules.
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LearnedAmateur
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I wouldn’t have thought that the disproportionation of chlorine into water proceeds via a radical substitution, which is where the UV matters: Cl2 +
hv -> 2 Cl•
Rather, this is what I’m picturing:
2 H2O <-> OH- + H3O+ (through Kw)
Cl2 + 2 OH- <-> HOCl + Cl- (Cl2 = 0 -> Cl = +1,-1; an induced dipole attacked and caused by the lone pair on the oxygen atom)
Cl- + H3O+ <-> HCl + H2O
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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AJKOER
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Perhaps we all can agree on the fact that dry chlorine is greenish-yellow, in concurrence with the very origins of its name, and in other conditions,
possibly grass green.
The cause of the color change with respect to chlorine does not appear to be greatly discussed in the literature, so I am hesitant to be dogmatic on
the mechanics, but do suspect interaction with either water, or especially water containing transition metal impurities.
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I will let Unionised explain the issue with his equilibrium comments following this observation, a solution of chlorine water in the presence of say
ferrous or cuprous will engage in a fenton-type redox consuming first HOCl, as hypochlorous acid has the larger oxidizing electrochemical potential
over chlorine in such acidic conditions (and more than HOBr,..., see for example, comments and Table 32 of electrochemical potentials on page 205 to
206 at https://books.google.com/books?id=Mtth5g59dEIC&pg=PA205&... ). When all the HOCl is consumed, then, and only then, could dissolved chlorine be
the driving part of an electrochemical reaction in such a system.
However, I would argue that the consumption of HOCl could force the equilibrium of chlorine in water, with time (and likely rapidly so), to the right,
so I don't expect a leading role for elemental chlorine in a system having electrochemical components that favor reagents with higher oxidizing
electrochemical potential under acidic conditions than chlorine.
In essence, a mist of chlorine, water and transition metal impurities likely in air has different dynamics (and possible coloration) than absolutely
pure dry (or moist) Cl2!
Here is a simple example to think about. Hypochlorous acid is a very weak acid, but it does a good job in bleaching in short order (referring to an
acidic heterogeneous system with organics/metal impurities).
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Interestingly, following my logic above on how a possible Cl2/H2O/Fe(ll)/Cu(l) system can be moved to consume HOCl adding further chloride, where high
chloride (or chlorine) concentration may promote the formation of the polyatomic species (see https://books.google.com/books?id=pu3HBQAAQBAJ&pg=PA276&... ):
Cl2 + Cl- = Cl3-
which may have further coloration effects in addition to the strongly colored ferric/cupric salts visible in low concentrations. In particular, with
respect to the HCl3 speciation, to quote a source (see https://www.researchgate.net/profile/Laszlo_Kotai/publicatio... and a confirming source, see https://www.researchgate.net/publication/7981068_Solubility_... ):
“Although the addition of excess hydrochloric acid or chloride ion is favourable; yet too much HCl inhibited the hydrate formation [referring to
Cl2.nH2O] by decreasing the chlorine content in the hydrate by forming trichloride ion (Cl3−).”
and also:
"An addition of 1-2 M HCl, however, induces a stronger absorption of the chlorine on account of the formation of the HCl3 [26, 27]."
Also, as the fenton-type redox produces radicals:
Fe(ll)/Cu(l) + HOCl --> Fe(lll)/Cu(ll) + Cl- + .OH
With some recycling:
Fe(lll)/Cu(ll) + .OH --> Fe(ll)/Cu(l) + OH-
Fe(lll) + Cu(l) = Fe(ll) + Cu(ll) (redox couple)
And also to a lesser extent, some possible radical species as well:
Cl- + .OH = .Cl + OH-
.Cl + Cl- = .Cl2-
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Some related report of chlorine gas absorption into chloride:
| Quote: Originally posted by Melgar | You can make copper (ii) chloride from HCl, H2O2, and copper with little if any chlorine gas formation, as long as you only add a very small amount of
H2O2 initially. Cl2 can combine with Cl- to form Cl3-, much like iodine and bromine can. As the metal chloride level increases, the solution is able
to dissolve much more chlorine this way, and you can add higher levels of peroxide accordingly.
Interestingly, the solution gradually takes on a mild, slightly pleasant smell, which are not words that are normally associated with compounds
composed of chlorine, oxygen, and hydrogen. My only theory is that copper and nickel (which does the same thing) can catalyze the formation of
chloric or perchloric acid, which imparts the smell. In that case, those species would have to be removed or decomposed in order to not have them
oxidize your copper (I) salt. |
Link: http://www.sciencemadness.org/talk/viewthread.php?tid=74546
[Edited on 6-4-2018 by AJKOER]
[Edited on 6-4-2018 by AJKOER]
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