Re: iodine and sulfite in acidic medium

From: Wilco Oelen (photo_at_woelen.nl)
Date: 07/05/04 

Date: 5 Jul 2004 15:09:42 -0700

Mohammed Farooq wrote:
> addition of hydrogen peroxide had no effect on it indicating that the
> yellow color is not due to a reducing species which I was first
> thinking of as some sort of (perhaps yellow) oxo-anions of sulfur
> mainly pyrosulfite ( synonym: metabisulfite) formed by the reaction of
> SO2 and HSO3(-) in equilibrium. So the reaction with hydrogen
> peroxide helped to rule out such reducing species in solution.
I have some sodium metabisulfite and potassium metabisulfite at home,
and these are white solids. The metabisulfite S2O5(2-) ion is
colorless. In water it goes into equilibrium with bisulfite (H2O +
[S2O5]2- <--> 2[HSO3]-), so this cannot explain the yellow color.
Hydrogen peroxide does react with these species, but the reaction is
not visible. The fact that a reaction occurs, however, can be clearly
deduced, because mixing a solution of metabisulfite with hydrogen
peroxide of sufficient high concentration results in clearly
noticeable warming of the liquid.

> Secondly I thought due to some impurity , mainly thiosulfate,
> colloidal sulfur would have formed when your post was first read but
> when your experiment was repeated the solution was perfectly clear.
Indeed, the solution remains perfectly clear. I kept the yellow
solutions for several hours and they remained perfectly clear and
there was absolutely no precipitate at the bottom.
Another reason for excluding the presence of thiosulfate is that
photo-grade sulfite must be ABSOLUTELY free of thiosulfate. The
sulfite is used in developers and even very small amounts of
thiosulfate would dissolve all silver in the undeveloped
paper/negative, resulting in ruining the undeveloped print. (Ag[+] is
complexed very well by thiosulfate. This is the principle behind
fixer, which removes unexposed Ag[+] from the developed print).

>
> Do you agree that:
> 1. The yellow color is not due to some complex oxo-anions of sulfur
> (or a hypothetical iodine-SO2 compound) because it has no reducing
> property. Also acidificatin of sodium bisulfite solution did not
> produce coloration, indicating that yellow color is due to a iodine
> containing molecule?
I agree that the yellow color is not due to some oxo-anions of sulfur,
otherwise the yellow color could be observed by careful oxidation of
sulfite with other compounds than iodine. The yellow color is specific
to the iodide/sulfite/acid system. I did some acid/sulfite experiments
with other oxidizing agents (H2O2, KMnO4, Br2), but the resulting
liquid always was colorless, when excess sulfite was used. Especially
the experiment with Br2 is interesting. With Br2 no yellow color is
formed. The Br2 was created by adding a pinch of KBrO3 to an acidified
solution of KBr, such that the liquid becomes orange/brown and a pale
brown vapour of Br2 was above the liquid. At this orange/brown liquid,
an excess of solid sulfite was added, resulting in immediate
disappearance of the orange/brown color and appearance of the smell of
SO2. At this, just a few small crystals of KI were added and
immediately, the colorless liquid turned yellow again!
I'm not sure, whether I can agree with the statement that the yellow
color is not due to some iodine-SO2 or iodide-SO2 complex. I did an
experiment with a drop of H2O2 (10%) added to excess amount of yellow
liquid and I observed formation of a brown cloud, which however,
disappeared on shaking. This can be explained by assuming that iodine
is formed, which however is reduced again, when the liquid is shaken.
The excess of reducing agent (the excess of SO2 in the acidic sulfite
solution) reduces the free iodine again.

>
> 2. Colloidal sulfur is not formed in that reaction?
I completely agree with this, the liquid remains clear, no milky
appearance at all and after many hours, no white/pale yellow
precipitate at all.

>
> It is strongly suspected now that the yellow color is due to the
> triiodide ion I3(-), since very similiar color is obtained when you
> drop very small quantity of solid iodine in potassium iodide solution
> and shake for a while ( triodide is known to form by free iodine and
> iodide ions).
This is an interesting thing. I will think of ways to check whether
this is true or not. I think I3(-) can be detected with starch, also
in acidic environments, I will try this.
Indeed, I3(-) is formed easily. I added a very small piece of iodine
to a solution of potassium iodide. The little piece of iodine did not
dissolve completely, but the liquid was coloured brown. However, I
found that the color is not exactly the same, it is more brown/yellow.
The sulfite/iodide/acid system produces a more bright yellow color. Of
course, I must admit, that this kind of observations is always very
subjective and personal, so I would not add too much value on this
observation.

>
> A UV-Vis spectrum would have immediatly helped to decide if I had time
> and quartz cuvettes (for a complete spectrum in the UV region) to
> compare the spectrum of a known tri-iodide solution with the yellow
> solution so obtained. If they were same then the suspect would indeed
> be tri-iodide.
That kind of nice things are completely out of reach for me :-(

>
> Another thing which is perplexing that even in hydrochloric acid the
> same reaction occurs, because HCl is unable to oxidize iodide to free
> iodine (which is causing slight yellow coloration in H2SO4)!. I
> couldn't repeat your experiment with extra pure HCl . The formation of
> sulfur dioxide is explainable here but not the yellow color ie from
> where did free iodine come to form yellow triiodide ion?
I spent a ml or so of my expensive analytical grade dilute HCl (2
mol/l) on this 'riddle', but again, yellow stuff, when combining this
with iodide and sulfite. But the yellow stuff only appears when all of
HCl+iodide+sulfite is used. Leave out one of these ingredients, and
you are left with a colorless liquid. I also tried with bromide
instead of iodide, but then the result is a colorless liquid.
Especially the result with HCl really makes me believe that a species
of sulfite or SO2 and iodide is formed, which has the yellow color.
Your hypothesis about I3(-) being formed may also be valid, but then
the only oxidizer which I can imagine is SO2. Normally SO2 acts as a
reductor for iodine, but it is known, that it can act as oxidizer as
well, albeit only in rare cases. May be this is such a rare case? What
the reduced species of SO2 then will be, I have no clue about that.

>
> A suggestion: Try carrying out the experiment quantitatively ie use
> every solution of known concentration and use known masses if you have
> a nice balance at home, this would help in further analysis of the
> mystery ( I never had a practical experience of analysing sulfite with
> iodine) and post a update.
Unfortunately I do not have an accurate balance (normally,
photographic recipes only need resolutions at the level of a few
grams), but I'll try whether I can derive results from solutions of
known concentrations. If I have any results in this, I'll post an
update.

Wilco

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