Photo 2011-01-23 00:05:02
As a photographer hobbyist, with some chemistry knowledge, I came
across the following ‘riddle’. Besides being a photo hobbyist, I’m
also interested in chemistry. I’m just curious about what happens
here. I cannot explain it with my knowledge, one of you may be able to
I did the following experiments:
Add a granule of iodine to a solution of sodium sulfite. This little
piece of iodine quickly dissolves and the liquid becomes colorless,
just as expected. The reaction, which occurs, I think, is the
following (highly simplified, highlighting main redox effect):
I2 + [SO3]2- + H2O –> 2I- + [SO4]2- + 2H+,
with the H+ quickly being neutralized by the slightly alkaline excess
I also did this experiment in a fairly acidic environment. I took a
small spatula of sodium sulfite and dissolved this in a few ml of
dilute H2SO4 (appr. 1 mol/l). I added a small piece of iodine and
again this dissolves. To my surprise, the liquid does not become
colorless, it becomes yellow. I first thought that I did not add
sufficient sulfite, but adding more sulfite did not change the color.
Besides that, the liquid has a very pungent odour of SO2, indicating a
large excess amount of sulfite.
In the strongly acidic environment, I expected the following reaction
to occur (again strongly simplified):
I2 + SO2 + 2H2O –> 2I- + [SO4]2- + 4H+.
However, the products of this reaction are all colorless, so something
else is occurring as well. Maybe there is some equilibrium reaction?
I2 + SO2 + 2H2O <--> 2I- + [SO4]2- + 4H+?
Because of lower pH, more H+ is present and this drives the reaction
to the left, such that a clearly visible amount of iodine is present
I did two counter-experiments to test the equilibrium hypothesis:
1) Dissolve some potassium iodide (KI) in a solution of sodium
sulfite. This liquid is colorless, as expected. Then add some
dilute sulphuric acid. The liquid becomes yellow at once and
there is a pungent odour of SO2. This supports the hypothesis
of the equilibrium.
2) Dissolve some KI in dilute sulphuric acid. This liquid is
colorless (after several minutes, it turns very pale brown,
but this color is MUCH lighter than the yellow described
above). When some solid sodium sulfite is added, then the
liquid at once turns yellow. Besides this, the smell of SO2
can be observed again. This does not support the hypothesis.
The addition of KI to sulphuric acid should already result
in the yellow color, but the sulfite really is needed to get
the yellow color.
All observations above do not depend on the acid. I also tried all
experiments with dilute hydrochloric acid (appr. 10% commercial acid
from GAMMA, a dutch hardware store, which carries a nice colorless
grade of dilute hydrochloric acid, other stores often have acid with
yellow/green impurities, which are bad for this experiment).
The experiments were also carried out with potassium bisulfite from
another supplier, and the results were consistent with the results,
described above, except, of course that the bisulfite is fairly acidic
on its own already.
In order to test my hypothesis about the equilibrium further, I did
the following two additional experiments:
Make two equal parts of the yellow liquid with excess of sulfite
(strong smell of SO2), each appr. 2 ml large.
Take 1 ml of ligroin (bp. range 40 – 60 C) and add to 2 ml of the
yellow liquid and shake. The ligroin does not become purple, it does
not extract iodine from the aqueous layer.
Take another 1 ml of ligroin and dissolve a small piece of iodine in
this. The ligroin becomes dark purple. Add this to the other half of
the yellow liquid and shake. The ligroin quickly looses its iodine. It
becomes colorless and the aqueous layer remains yellow.
The ligroin experiments do not support the equilibrium hypothesis. I
expected to see at least some purple/pink color in the ligroin, but
not even the faintest pink could be observed in it after shaking a
long time and letting the layers settle again.
So I wonder, what can the yellow stuff be? I’m really surprised by the
results of these experiments. Just simple iodine and sulfite… Does
iodide or iodine form some colored compund with SO2? I never heard or
read about such a thing and a search on Internet did not give any
answer to me. I’m posting these questions, just driven by curiousity
and in order to learn a little more about chemistry. Remember, I’m not
a professional in the field.
The chemicals, used in the experiments are all ‘photo grade’
chemicals, obtained at internet shops for raw photography chemicals. I
think, that these are sufficiently pure and that the yellow color is
not due to some impurity. If you have doubts, just check up in your
labs with real reagent grade chemicals!
Please, no discussion about safety on acids, SO2 or the like. I am
aware of the risks of performing chemical experiments and I’m pretty
confident about me knowing what I’m doing. I have worked with nastier
things, like sulfide baths, giving off ‘rotten egg’ H2S, or dichromate
If someone has any idea, I would be pleased.
PS: The word ‘photo’ must be replaced by ‘foto’ if one wants to send a
personal mail to me. The address firstname.lastname@example.org may be removed soon,
if it is spammed too much.
Farooq_w 2011-01-23 00:05:04
You will find much details of the reaction on the internet when you a
search for *iodometric determination of sulfite* eg
en/motion/support/processing/h243/ecr1303.pdf this website mentions
that photographic sulfite does contain some other sulfur containing
(I am not a professional too) I tried to repeat your experiment
qualitatively with relatively pure (General Puropse Reagents-GPR )
using sodium bisulfite NaHSO3 , potassium iodide, and extra pure
sulfuric acid since I thought the acid might be the source of
contaminants. As acidified potassium iodide was mixed with sodium
bisulfite solution the mixture immediately acquired a yellow color.
Further addition of iodine solution (brown in color: made by mixing KI
with hydrogen peroxide) to that solution had no effect on the yellow
color and iodine seemed to dissolve in it. In another similiar mixture
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.
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.
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
2. Colloidal sulfur is not formed in that reaction?
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
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
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?
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.
Photo 2011-01-23 00:05:11
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.
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).
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.
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.
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.
That kind of nice things are completely out of reach for me 🙁
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.
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
Farooq_w 2011-01-23 00:05:13
This is making more interested in solving the mystery behind the
yellow color. This is such a common reaction but unfortunately I am
unable to find specfic mention of yellow coloration during the
reaction. I will see if I get permission to use the a UV-Vis
spectrophotometer to compare the spectrum of tri-iodide with the
yellow color so obtained from this reaction. This will not take more
than 15 minutes if I get permission.
Try diluting the solution of KI and iodine till it matches the color
of the iodine-sulfite-acid mixture. Indeed a concentrated tri-iodide
is dark yellow brown but my dilute tri-iodide solution really matched
the yellow color of this system.
As you say that tri-iodide can be detected by starch, that is true but
we would not be sure which “species” either free iodine I2 (aq) (no
matter how little may be present) or tri-iodide ion which is in
equilibrium with iodine is turning the starch blue.
Can somene else help here?
Farooq_w 2011-01-23 00:05:15
Just thinking of very well known *clock reaction* called Landolt
reaction,( which uses iodine-sulfite system), the solution changes
color after certain time intervals in a periodic fashion, google it if
you are more interested in this system, in the meantime I will try to
obtain the spectra.
Farooq_w 2011-01-25 06:46:42
I repeated this thing a bit more carefully, added 33% extra pure drop
of H2O2 to the yellow solution , and the color disappeared.
Similary the reaction of *solid* sodium bisulfite with 33% H2O2 was
too violent , lots of steam and slight explosive like sound.
Infact I added sodium thiosulfate to the yellow solution to confirm if
it were tri-iodide ion, the solution should have immeditely
decolorized but nothing happened! What is causing the yellow color now?
Finally took the spectrum of both solutions using distilled water as
*blank*, this subtracts any absorption of light due to water.
Here is the data using Beckmann quartz cuvettes:
The spectrum of tri-iodide has three peaks, a peak near 200 nm is too
intense so it is out of paper, this is not our concern, another far
less than the previous one at 287 nm and another at *347 nm* of
nearly equal intensity.
The spectrum of yellow solution has the same out-of-scale peak at near
200 nm, and surprisingly another peak at *346 nm* !!! Too close to one
of the peak in tri-iodide, indicating that the yellow color is due a
species which is either identical to or very closely related to the
I do not think now that a complex between sulfur dioxide and elemental
iodine is formed because the absorption maxima of both spectrums are
very very close.
Do you have Holleman and Wiberg’s “Inorganic Chemistry” (originally in
German but English translation is available now), My library doesn’t,
but I have heard that it is a very comprehensive +2000 pages book,
just check it in a public library, I am sure that book might have
discussed the reaction. I have checked Cotton’s “Advanced Inorganic
Chemistry” but could find specific mention of yellow coloration.
Beavith 2011-01-25 06:46:45
exactly! the H+ is neutralized and the soln remains slightly
alkaline. as long as the soln is alkaline, and any H+ is sucked up by
the alkalinity, the reaction will go to the right, under normal conditions.
this would be expected. you are essentially starting off with the
right side of your redox reaction and expecting it to go backwards.
this would be more like
I2 + SO2 + X H+ –> ?
at some point, you’d get preferential release of the SO2 as gas, which you do experimentally.
no. it supports the evolution of SO2 gas in an excess acid environment
this should be telling you that the acid environment is the factor,
not the particular acid. time for a new hypothesis.
you get points for thoroughness
yeah. not a professional. i think i’m doing your homework for you!
tell me this isn’t a school lab. for petes sake this is July!
Beavith 2011-01-25 06:46:47
On 5 Jul 2004 23:40:58 -0700, email@example.com (Mohammed Farooq)
slow down there! interesting reference, tho.
ever hear the expression “when you hear hoof beats think horses, not
Farooq_w 2011-01-25 06:46:51
Modify zebra to hinny, Landolt reaction is based on the
iodine-sulfite system and is not totally unrelated!
You have still haven’t tried to solve the main question: What is
causing the bright lemon-yellow color?
For basic spectral data, read one of my follow-up…if interested.
Photo 2011-01-25 06:47:00
It looks the signal on this channel is contaminated with some biased
random noise. The source of this random noise may be either arrogance
or ignorance. More analysis of this phenomenon may be necessary…..
OK, let’s get serious again. I do not say that this is rocket science,
but it sure is not the plain schoolbook chemistry which is involved
here. Also have a look at the very nice investigations of Mohammed
Farooq. Sometimes simple things are really surprising!
Photo 2011-01-25 06:47:02
This surprises me. You did not first get a brown color, due to
elemental iodine? I’ll try this at home again with more H2O2. Maybe
there is excess H2O2, which oxidizes the iodine further to colorless
iodate? I’ll send an update if I have some time to experiment again.
Yes, concentrated H2O2 can give funny things 🙂
A nice suggestion – a little off-topic. Carefully add some HTH calcium
hypochlorite (65% – 70% active chlorine) to some 30% H2O2 in the dark.
A violent reaction and a beautiful red glow! However, please be
careful and don’t use much H2O2!
A high concentration of SO2 stabilizes thiosulfate considerably. This
is used in photography in fixers. Having a high bisulfite content in
the fixer allows a fairly acidic solution to be combined with high
concentrations of thiosulfate, without the thiosulfate decomposing to
sulphur. This may explain, why it does not almost immediately become
Of course, this is no answer to your question, why the liquid does not
become colorless with thiosulfate. Apparently there really is
something, which strongly stabilises tri-iodide in a solution with
acid and SO2, or even oxidizing iodide to iodine, see remark further
This is a very nice result (BTW, thanks for all your efforts and
taking this ‘riddle’ so serious, I really appreciate that). Your
result convinces me that this really is I3(-), it hardly can be
anything else. This makes the situation very interesting. The only
conclusion I can make with this now is that SO2 or some acidic
species, derived from sulfite is an _oxidizing_ agent, oxidizing I(-)
to I2, although the reaction is not driven to completion.
I think this conclusion is right, because with only iodide, sulfite
and hydrochloric acid, the yellow color is observed. Hydrochloric acid
for sure is not oxidizing, so the only thing left is SO2, or an acidic
derivative of sulfite. This is interesting, because normally SO2 is a
fairly strong reductor.
In inexact equation form.
2I(-) + x SO2 + y H(+) <---> I2 +
Secondary reaction then is:
I2 + I(-) <---> I3(-)
point of research. I have the book “Chemistry of the elements”,
written by Greenwood. This desribes many oxo-anions of sulphur. I’ll
look at that. If I have an idea, I’ll feedback to you.
With this result I can explain the lemon yellow color. A much higher
concentration of iodine is taken up by excess SO2, according to the
following counter reaction, which removes iodine.
I2 + SO2 + 2H2O —> 2I(-) + SO4(2-) + 4H(+)
This was observed already by means of dissolving I2 in acidic sulfite
solution and iodine being extracted from the ligroin.
I’ll see if I can find this in a second hand book shop. In the recent
past I have obtained some very nice old german books on chemistry for
just a few euros. Here in the Netherlands we have a lot of german
literature on the subject. I like these books, because they do not
only give explanations about the principles, but also give detailed
descriptions of a lot of real-life compounds.
Beavith 2011-01-25 06:47:09
i appreciate that. its just that during the school year, we see many
kids obviously doing their homework on the usenet.
if you really are doing this out of curiosity, my hat is off to you.
my days of tinkering at the chemistry set are pretty much over, even
though the set is pretty extensive.
the question that i was focusing on was the original redox reaction,
which was correct. then you change a condition (an important one!)
and get an unexpected result. my contention is that it shouldn’t be
i’m curious about the yellow color too. if MF has the opportunity to
post the UV VIS spectra, i’d guess that the yellow color will either
be I2 or a sulfur dioxide soln. does the mixture smell? does it go
clear when you boil it?
Farooq_w 2011-01-25 06:47:11
Thats why I was surprised.
I don’t know how to upload a image, neither I do have a scanner but I
have described it in one the follow-ups. It will not take more than 15
minutes to obtain a qualitative spectrum if you (beavith: if you are
seriously interested) have a spectrum recorder.
As I originally assumed that the yellow color is due to iodine as
tri-iodide ion, still the yellow color is not decolorized by sodium
thiosulfate indicating this is not tri-iodide, secondly its spectrum
shows a single sharp peak very close to one of the tri-iodide peaks.
Secondly the solution is very stable.
Secondly sulfur dioxide solution in water is colorless, and if SO2
were causing this color, only acidification of sulfite would have
resulted in a yellow color (the acidified solution does smell of SO2)
but this color is not developed in the absence of iodine or iodide. As
Wilco has written all the three are necessary: acid-sulfite-iodide.
Farooq_w 2011-01-25 06:47:14
Yes I do, when iodide is in excess.
You are lucky to observe a red glow somehow I was always fascinated by
chemiluminiscent reactions of hydrogen peroxide. This experiment was
tried long time ago without success with sodium hypochlorite using 6%
medicinal H2O2. What does HTH stand for? Did you use solid calcium
hypochlorite or used a kind of its slurry.
But why reaction is taking place id HCl and why thiosulfate does not
react with it?
The formation of slight yellow color after adding H2SO4 to KI solution
is that sulfuric acid does oxidize some iodide to iodine. Sulfur
dioxide can not oxidize iodide to iodine under any condition, because
it is a very poweful reducing agent as you have already said.
Sulfur dioxide is usually reduced to sulfur, but the solution is
pefectly clear. I don’t understand this is such a common reaction with
wide application still no one has described the cause of that yellow color.
I also like german books ( I can read mainly translations of them)
because of they are really comprehensive and start from basic history
of the concept. In the meanwhile I try to self-learn a working
knowledge of German. Unfortunately neither English nor German books
are available easily nor there are any good chemistry books worth
mentioning in the local langauge.
If you have time take a look at
and the references therein.
Photo 2011-01-25 06:47:23
I promised to send an update about my investigation of the sulphur oxo
anions. The book I have, mentions that the acidic sulfite / SO2 system
is very complex and several species are in solution at the same time,
and possibly others
The metabisulfite (O2S2O3(2-))is the interesting species here, because
it has two sulphur atoms in it, one having oxidation state +5 and the
other having oxidation state +3 (I always thought that both S-atoms
have oxidation state +4 in a metabisulfite ion and I always though the
structure to be like O2SOSO2, but this is false, the two S-atoms are
connected directly, without bridging oxygen). This gives it very
interesting redox properties. The book states that the following net
half-reaction can occur at pH equal to 0:
4SO2 + 4H(+) + 6e <--> S4O6(2-) + 2H2O
The redox potential for the half-reaction, going from left to right
equals +0.509 V, hence SO2 surely can act as oxidizer. The real
reaction is much more complex, than the one given above, it is going
through the O2S2O3(2-) species. The reaction above is just a net
equation, without taking into account all intermediates and
The lower the pH, the easier the reaction proceeds to the right (the
oxidizer needs H(+)). The redox potential depends on pH. For
decreasing pH, the redox potential becomes even larger.
In the same book, the redox potential for the half-reaction
I2 + 2e <--> 2I(-)
is mentioned as +0.535 V. This does not depend on pH, no H(+) is
This is close to the redox potential of the reaction above. If pH
becomes lower than 0, then the reaction indeed can proceed somewhat
(at least thermodynamically, and according to our observations, there
apparently is a kinetic route for the reaction as well, otherwise we
would not see the yellow).
The book does not relate iodine with the complex SO2 system. The link
between the two is made by me.
This _may_ be an explanation of what we observe. If this theory is
correct, then it should be possible to observe S4O6(2-) in the
solution, but this will be very difficult to establish (at least for
me, because I do all this at home, without lab facilities). The
S4O6(2-) ion is colorless. This ion is the same as the one, which is
formed when thiosulfate is oxidized by iodine, the so-called
Tetrathionate can be created quantitatively from thiosulfate by
oxidizing this with iodine, but (if the above theory is correct) it
can also be created by reducing acidic sulfite/SO2 with iodide, but
certainly not at quantitative yields.
PS: The book I used is “Inorganic Chemistry”, by Barett Barnet. It is
old book of 1953, and I was lucky enough to find it second hand.
Beavith 2011-01-25 06:47:25
On 7 Jul 2004 08:59:39 -0700, firstname.lastname@example.org (Mohammed Farooq)
i don’t have one. sorry
that implies one species or multiple species that are extremely similar.
but not odorless.
true, but so should a neutral soln. a small amount of alkalinity is
necessary to eliminate the odor.
knife edge equilibrium between the 3 components?
Farooq_w 2011-01-25 06:47:32
For the first time a seemingly simple reaction has perplexed me to
this extent. We will find the answer to this problem soon, God
AN IMPORTANT DEVELOPMENT:
I have found a reference that studies the kinetics (and hence
mechanism) of this very reaction ie iodide-sulfite system in a very
famous journal named “Inorganic Chemistry” published by American
Chemical Society, you will be happy to read its title, a surprising
thing is that it is a very recent article, it means this reaction is
by no menas simple and is still the subject of current studies.
“Non-Metal Redox Kinetics: Reactions of Iodine and Triiodide with
Sulfite and Hydrogen Sulfite and the Hydrolysis of Iodosulfate”, Yiin,
B. S.; Margerum, D. W. Inorg. Chem. 1990, 29, 1559-1564.
Sadly our library does not have recent issues of the journal
“Inorganic Chemistry” but any good library would have them. Can you
devote some more time to this problem and just have a look at this
paper. I would like to hear more about it, I assume you have access to
good chemical libraries.
Can anybody help by having a *glance* over this article. I am sure
this would have discussed the possible species responsible for the
yellow color formed in this not-so-simple reaction
Photo 2011-01-25 06:47:34
The calcium hypochlorite, used is solid, just add a spatula of this to
a small amount of 30% H2O2. The letters HTH do not have a special
meaning. This is just a brand of chlorine bleach powder, fairly
commonly available in the Netherlands (I think it originally comes
from the USA). The calcium hypochlorite must be real hypochlorite, not
hypochlorite/chloride. The stuff I use contains Ca(OCl)2, with just a
few percents of other stuff (mostly Ca(OH)2, CaCO3 and also
a little CaCl2). It is technical grade stuff, used for cleaning
toilets, floors etc. and it is also used in swimming pools.
See posting, just posted before this one: tetrathionate is proposed as
a _possible_ related compound. Just a theory, no proven fact!
The reason that thiosulfate does not react still is not clear to me. I
also would expect that to react.
Yes, I agree, at least if the concentration is sufficiently high.
Another source of the slight color may be slow oxidation by oxygen from air.
No, I’m not sure anymore of this, see previous posting, as mentioned already.
This also surprises me. I really think it is strange that nowhere a
treatise on this reaction can be found.
This is interesting. It also is about tetrathionate. What surprises
me, however, that nowhere on this site mention is made of the specific
reaction, we are studying now. I cannot imagine that these people
never have seen what we have observed, the more so, because they are
working both with iodine and with oxo-sulphur compounds.
Taking everything into account, I still believe that the yellow color
is due to I3(-), primarily, because the spectrum is so convincingly
close to the one, derived from a reference solution.
What still puzzles me, however, is the extreme stability of the yellow
color (as long as the medium remains acid). Even thiosulfate does not
destroy it. I’ll try lateron with a large excess of thiosulfate. If a
small amount of thiosulfate is used, then I can imagine that all
thiosulfate is oxidized by the SO2 and its derivatives, by means of
the following pathway:
1) SO2 et al. are converted to tetrathionate and
iodine is formed.
2) iodine converts thiosulfate to tetrathionate, itself being
reduced to iodide.
If this is true, then iodide would be a catalyst for reaction between
thiosulfate and sulfite, both of them being converted to
tetrathionate. This is just some thinking further and of course all
this must be establised (or disproved) by means of experiments. I’ll
put some time in this, within the next days and give an update.
Photo 2011-01-25 06:47:38
I tried adding a large excess of thiosulfate to the yellow solution.
Even with the large excess the color does not disappear. Now, this
makes me less confident about that the yellow color is caused by
I3(-). With a large excess of thiosulfate, the liquid becomes milky
after a while, but not immediately. This is just the well-known
decomposition of thiosulfate by acid.
I also tried the following:
Prepare a solution of tri-iodide, by dissolving some I2 in a
concentrated KI-solution and very carefully add thiosulfate-solution
to this, until the liquid JUST becomes colorless. Then it contains
only iodide and tetrathionate (besides of course Na(+) and K(+) ions).
Adding hydrochloric acid (10% by weight) to this colorless liquid does
not give any visible change, the liquid remains colorless. Even after
many hours, the liquid remains clear and colorless. Apparently
tetrathionate is quite stable in a strongly acidic medium. The yellow
color definitely is not due to some iodine-tetrathionate complex.
As soon, as I add a spatula of Na2SO3 to the acidified
tetrathionate/iodide solution, it becomes lemon yellow and it remains
Now I’m totally lost!
Unfortunately I do not have that kind of access. I’m just a hobbyist,
with self-builtup knowledge of chemistry. I use this knowledge for
enhancing my possibilities in my photo-hobby (see e.g. posting on
vanadium toner in this group and in rec.photo.darkroom as well). My
profession is software engineering and I do not have any professional
connection with chemistry. Obtaining chemicals through photography
shops is not that difficult for me, o.t.o.h., obtaining good journals
on the field is almost impossible for me, they simply are way too
expensive for a private person. A friend of mine works in a chemical
manufacturing company, he _may_ have access to that kind of
literature. I’ll ask him, but it will take some time and I cannot promise anything.
If there is someone out there with access to the above mentioned
article and with some spare time, I would be really appreciated if one
could give a clue.
My experience with chemistry is that even with very simple things
there are really remarkable things. During my hobby time I have come
accross several riddles, of which this sulfite/iodine riddle is just
one example. Each time, when I find such a riddle, I’m really
surprised to see that no one else apparently ever encountered it (or
the results are simply hidden for me, because they are written down in
journals, not accessible for me).
Beavith 2011-01-25 06:47:43
On 8 Jul 2004 10:51:56 -0700, email@example.com (Mohammed Farooq)
a 1992 follow on article by the same group is titled
“Non-Metal Redox Kinetics: Reactions of Iodine and Triiodide with
Thiosulfate via I2S2O32- and IS2O3- Intermediates”, Scheper, W. M.;
Margerum, D. W. Inorg. Chem. 1992, 31, 5466- 5473.
iodine and iodide stabilized thiosulfate anion? hmmm! that’d
correspond to your single vis peak (or an unresolved double peak).
i can’t get into the article either. amazingly, Inorganic Chemistry
archives don’t seem to be posted to the web. in this day and age, i
find that to be surprising.