cnidocyte
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Converting iodides and iodates to I2
I managed to obtain some clean I2 crystals by following instructions but I still don't know the chemistry behind it all so I'm doing some
experimenting in order to learn. I just a spatula tip of solid NaOH to some iodine tincture and as expected, I observed the solution turn from murky
brown/black to clear yellow. I assume this is a result of all suspended (or dissolved) I2 being turned into NaI and NaIO3. Pretty cool how quickly the
reaction occured.
Now I'm gonna boil off the IPA but everything about converting these salts back into I2 is a mystery to me. For the iodides someone gave me this
equation
6I- + 6H+ + 3H2O2 => 3I2 + 6H2O
but where does the acid come in? In my case I'm using H2SO4. Also what happens to the NaIO3? I found this equation in google
5 NaI + NaIO3 + 3 H2SO4 = 3 Na2SO4 + 3 I2 + 3 H2O
but I'm not sure how accurate that is. Is that what will happen if I add H2SO4 or do I still need to add H2O2 or another strong oxidiser?
[Edited on 3-1-2011 by cnidocyte]
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DJF90
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The NaIO3 is your oxidiser and the second equation is correct. Analogous chemistry is used also for making elemental Bromine (using bromide and
bromate in place of the Iodine compounds). The first equation is a different reaction utilising hydrogen peroxide as the oxidant in place of
iodate(V). Recall that the spectator ions have been omitted from the equation, so it actually looks something like this:
6NaI + 3H2SO4 + 3H2O2 => 3I2 + 3Na2SO4 + 6H2O
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cnidocyte
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Ah right now I get it. Thanks a lot. I like this route way better than the H2O2 one. Theres still KI in there though so I'll have to add a bit of H2O2
if I'm gonna convert that into I2. My plans to boil off the IPA and recover the solid salts then add some dilute H2O2 followed by the acid.
[Edited on 3-1-2011 by cnidocyte]
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cnidocyte
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Heres what happened. I boiled off all alcohol and was left with solid salts. I added about 20mL of 6% H2O2 solution and it started sizzling and
bubbling. I added about 20mL of dH2O. I added a few mLs of conc. H2SO4 and a rapid reaction took place, turning the whole solution dark red and this
time, dark red gas even evolved. Was that gas I2? I was doing this outdoors so I didn't stick around to see what it was but it definitely looked red,
not purple. Scared the crap outta me whatever it was. 
The back garden reeks of H2S at the moment. I'm guessing I should have let the solution cool down before adding the H2SO4.
EDIT: I got a decent amount of crystals but this is the 2nd time the H2SO4 did a lot of the oxidising. Is this an indication that theres not enough
H2O2?
[Edited on 4-1-2011 by cnidocyte]
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ScienceSquirrel
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That is not chemistry.
If you want to make a product by mixing reagents together then you have to try and understand what you are doing and how the reaction works.
You would have been better off dissolving the salts in water, adding excess acid and then adding hydrogen peroxide to complete the oxidation.
Stoichiometry, dear heart, stoichiometry rules the chemical kingdom 
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cnidocyte
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Quote: Originally posted by ScienceSquirrel  | That is not chemistry.
If you want to make a product by mixing reagents together then you have to try and understand what you are doing and how the reaction works.
You would have been better off dissolving the salts in water, adding excess acid and then adding hydrogen peroxide to complete the oxidation.
Stoichiometry, dear heart, stoichiometry rules the chemical kingdom
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Thats basically the reason behind me doing this experiment and starting this thread. Like I said I can extract I2 crystals with ease but I'm
experimenting so I can understand exactly whats going on. Don't have a scale yet so I can't do any stoichiometry with the solids. Can you elaborate on
why I would have been better off dissolving the salts in water, adding excess acid then adding H2O2? Why not add the H2O2 first?
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Sedit
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The H2O2 is to oxidise the HI formed to I2. Adding oxidiser first causes to much to happen at once.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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DJF90
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Acidify with 2M H2SO4, not concentrated acid. Then add sufficient peroxide solution (3-9%).
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Neil
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KI to I2
Hi all,
I had (am having) a strange experience with KI. About a month ago I placed a small amounts of KI into a vial with some gold scrapings and then
covered the mixture in HCl. (all in all about .8 cc)
I observed that after 24 hours of being kept in the dark the solution tuned a light golden color, after a further 24 hours of being in the dark it
became the dirty red color of iodine in water.
It ceased to change after that and I took it out the dark and have had it in light since then. For two weeks nothing changed. The solution stayed a
dark dirty red, some of the gold flakes as well as the base metal remained (they were scrapped of circuit board fingers so).
Recently I noticed tiny black crystals forming. They continue to grow and the remaining solid KI steadily vanishes into solution. the solution has
turned to a dark amber, which shows green if splashed against the sides of the vial, just like a solution of ferric chloride.
The vial is well sealed and no H2O2 was added. The vial is only a 1 dram vial - so the tapped air volume is very small.
This seems... odd.
No measurements on the ingredients. I had read that using an excess of KI with HCl would strip gold and I was skeptical but could not find anything
which directly denied the possibility, this was only supposed to confirm that gold would not be dissolved without adding an oxidizer...
I do not have any pure Tin and so am unable to test for gold in the solution, however my observation is that it appears some of the gold has been
dissolved.
Thoughts? I do not see how KI/HCl would result in elemental I2..
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bbartlog
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It's certainly hard to say what would be doing the oxidizing here... pretty much everything in your vial is a reducing agent. Nonetheless, gold (I)
iodide is greenish-yellow and is solubilized by KI. Further I would think that the AuI may also increase the solubility of the KI (paralleling the
situation with CuCl and NaCl for example).
As for the black crystals, my CRC handbook says that gold iodoaurate (KAuI4) forms black lustrous crystals. I think that is likely what you have,
though again I have no idea what would oxidize the gold to +3 or even +1 in your experiment.
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Neil
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It's not in direct sunlight but there is dim light on the vial all the time. Perhaps it was a photolysis reaction?
http://www.jstor.org/pss/2415173
The descriptions I found for KAuI4 do fit the description of these crystals but they also have nearly the same description as Iodine crystals.
I guess I'll have to test them to find out.
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bbartlog
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I suppose it's possible that there is some equilibrium reaction involving the HCl, which is then slowly driven to completion by the fact that the
hydrogen gas escapes. Of course that supposes that hydrogen could escape from your well-sealed vial, but it is notoriously difficult to keep it from
diffusing out of and through many materials. I recall doing an experiment on a similar timescale involving a copper penny and HCl, and over time the
HCl did attack the penny even in a sealed test tube.
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Neil
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I'm sorry I did not refer to my notes when I wrote that post.
*There was a small amount of gas produced during the first day, the lid was only screwed onto the vial lightly allowing the gas to escape.
*There was never any visual effervescence.
Between now and then the vial was wrapped in plastic and the cover 'cracked' barely open enough to verify there was no pressure build up. There was
never any sign of pressure, nor any sign of escaping iodine vapor.
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Neil
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I've opened the vial. Sadly it seems the seal was damaged, which means it has not been sealed. Strangely, the solution tests positive
for iodine, but I can not find any escaped Iodine. Who ever heard of well behaved Iodine?
The liquid was tested with pH paper and showed a pH of 2.
Some of the liquid was pulled out of the first vial with a capillary tube and added to a dilute solution of NaOH. The liquid lightened to a pale
yellow and formed a pale blue precipitate. The precipitate then vanished and the solution became a clear yellow, the pH is 12. There was no other
precipitate.
Several crystals and a small amount of fluid was extracted from the first vial and these where added to a small amount of cold water. The solids
completely dissolved and formed a pale yellow/green liquid.
Looks like it was a salt not I2 as I had hoped, which means this is completely off topic, opps.
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