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edgeofacliff
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If you add the acid after dissolving the KI in water you can then add the bleach a little at a time and watch for the crystals to form, its really not
that difficult. On filtering if the solution is still dark or if the filter is stained dark you can continue and get a little more. When the
solution resembles weak tea and the filter shows no stain its pretty much over.
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woelen
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Still this is not optimal. Of course you can see formation of solid iodine and stop adding bleach if solid iodine is separated, filter the iodine and
see whether adding additional bleach to the remaining solution gives more solid iodine. But this is quite cumbersome and still you most likely loose
quite a lot of your iodine.
If you can get H2O2, use that. It gives nearly 100% yield. If you can't get H2O2, try to determine the amount of hypochlorite in your bleach, e.g. by
titrating with Na2S2O3 or even Na2SO3.
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ChemSwede
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My experiment with KI + H2SO4 + H2O2 --> I2
Hello.
My first post on ScienceMadness, and it's about isolating iodine. I borrowed this thread instead of starting a new one.
I've made two experiments so far. The reagents were of analytical quality.
Experiment #1:
10g of KI was dissolved in some water.
5ml of conc. H2SO4 was dripped into the KI solution. This resulted in brown/yellow gasses and the solution turned dark brown.
30ml of 10% H2O2 was added to oxidize all the I- to I2.
Water was added and the iodine collected at the bottom.
The product was then gravity filtered and rinsed with water.
The moist iodine was stored in a pyrex-bottle.
I tried to clean and dry it by heating and letting it sublime against a cold surface. Not effective at all. The iodine just formed very thin crystals
and the water condensed as well, of course.
Quite a crappy experiment. Using conc. H2SO4 was a bad idea. It leads to a mixture of gasses and it doesn't oxidize I- to I2 very well.
Gravity filtration is not recommended either.
Experiment #2:
20g of KI was used.
Dilute (50%) H2SO4 was slowly added. Not much rise in temp. The solution turned very yellow.
20ml of 10% H2O2 was added slowly, and the iodine precipitated.
The product was filtered with a sintered glass filter and rinsed with water. Very effective, and the filter was easily cleaned in a solution of
Na2S2O3.
The moist iodine was transferred to the same pyrex bottleas before.
I have not been able to determine yield since I haven't dried the product. Theoretical yield from 20g KI should be 15,7g I2.
---
Now I'm looking for ideas for how to isolate iodine with a method that gives the highest purity and yield.
Is it better to use HCl instead of H2SO4?
Is it better to oxidize KI by bubbling Cl2 through a solution of it?
H2SO4 + KI --> HI Should I perform this step in a closed system to prevent loss of HI, or is the gas mostly dissolved in water?
Is the best way to purify iodine by melting it under conc. H2SO4?
Can I dry the product by dissolving it in methanol (or some other solvent) and then let the solvent evaporate?
---
I also have a few questions about the reactions:
The remaining water after filtration is brown in colour. Is that because of dissolved I2 or I3-? Adding some more H2O2 did not precipitate any I2 from
the solution.
When 50% H2SO4 was added to KI the solution turned very yellow. Why is that? I guess HI is colourless?
I used NaHCO3 to neutralize the acidic brown water. Upon addition of NaHCO3 the solution turned from brown to colourless. How come? I2 or I3- +
NaHCO3--> I-?
Sorry about the wall of text, I had a lot of questions 
Tried to upload some images from the experiments, but it didn't work.
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blogfast25
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Quote: Originally posted by ChemSwede  | Now I'm looking for ideas for how to isolate iodine with a method that gives the highest purity and yield.
Is it better to use HCl instead of H2SO4?
Is it better to oxidize KI by bubbling Cl2 through a solution of it?
H2SO4 + KI --> HI Should I perform this step in a closed system to prevent loss of HI, or is the gas mostly dissolved in water?
Is the best way to purify iodine by melting it under conc. H2SO4?
Can I dry the product by dissolving it in methanol (or some other solvent) and then let the solvent evaporate?
---
I also have a few questions about the reactions:
The remaining water after filtration is brown in colour. Is that because of dissolved I2 or I3-? Adding some more H2O2 did not precipitate any I2 from
the solution.
When 50% H2SO4 was added to KI the solution turned very yellow. Why is that? I guess HI is colourless?
I used NaHCO3 to neutralize the acidic brown water. Upon addition of NaHCO3 the solution turned from brown to colourless. How come? I2 or I3- +
NaHCO3--> I-?
Sorry about the wall of text, I had a lot of questions
Tried to upload some images from the experiments, but it didn't work.
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H2SO4 is capable of oxidising iodide to iodine. HI prepared from KI or NaI is best done using H3PO4, not H2SO4, for that reason.
While Cl2 will easily displace iodide to iodine, it also has a tendency to form interhalogens like ICl. Oxidation of iodide to iodine with chlorine is
therefore to be avoided.
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woelen
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Your second experiment with the 50% H2SO4 is quite optimal. It gives a nearly 100% yield, the losses are very small.
You can isolate the iodine very well with conc. H2SO4. Filter the iodine with a sintered glass filter. Rinse it with distilled water (you can do that
again with the iodine in the little bottle) to get rid of adhering KI and other chemicals. The iodine can be dried quite well already by putting a
piece of tissue paper on the other side of the sintered glass filter. It sucks in the water and makes the iodine quite dry.
Scrape the fairly dry iodine from the filter and put this in a tiny beaker and pour concentrated H2SO4 on it, such that it is covered by half a cm of
acid. Stir the iodine/acid slurry with a glass rod (no plastic, no metal) and then carefully heat the beaker until the iodine melts. You get a layer
of iodine under the layer of H2SO4. Let the iodine and acid separate from each other while still molten and once you have two nicely separated layers,
take away the heating and allow the material to cool down. The iodine solidifies. Now you can pour the acid from the iodine and try to get the iodine
as a single smooth piece (or as a few large smooth pieces). Rinse these pieces with distilled water to get rid of all acid and then dry them
with paper tissue. The pieces of iodine then can be crunched to smaller pieces and in this way you get nice dry iodine of high purity. You can get
even better purity by subliming it and condensing it again on a cold piece of glass.
[Edited on 23-8-13 by woelen]
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ChemSwede
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| Quote: Originally posted by woelen | Your second experiment with the 50% H2SO4 is quite optimal. It gives a nearly 100% yield, the losses are very small.
You can isolate the iodine very well with conc. H2SO4. Filter the iodine with a sintered glass filter. Rinse it with distilled water (you can do that
again with the iodine in the little bottle) to get rid of adhering KI and other chemicals. The iodine can be dried quite well already by putting a
piece of tissue paper on the other side of the sintered glass filter. It sucks in the water and makes the iodine quite dry.
Scrape the fairly dry iodine from the filter and put this in a tiny beaker and pour concentrated H2SO4 on it, such that it is covered by half a cm of
acid. Stir the iodine/acid slurry with a glass rod (no plastic, no metal) and then carefully heat the beaker until the iodine melts. You get a layer
of iodine under the layer of H2SO4. Let the iodine and acid separate from each other while still molten and once you have two nicely separated layers,
take away the heating and allow the material to cool down. The iodine solidifies. Now you can pour the acid from the iodine and try to get the iodine
as a single smooth piece (or as a few large smooth pieces). Rinse these pieces with distilled water to get rid of all acid and then dry them
with paper tissue. The pieces of iodine then can be crunched to smaller pieces and in this way you get nice dry iodine of high purity. You can get
even better purity by subliming it and condensing it again on a cold piece of glass.
[Edited on 23-8-13 by woelen] |
Thanks! I will definitely attempt to dry my iodine that way. Would you say that dissolving I2 in methanol and letting the alcohol evaporate would also
work? H2SO4 is hard to get here, so I don't want to use it if I don't have to. Maybe I should start using H3PO4 instead then when isolating iodine.
The yellow colour in my experiment where I used 50% H2SO4, is that iodine then? I thought only conc. H2SO4 was capable of oxidizing I- to I2. What
else could it be?
Do you have any idea as to why the remaining brown water (the colour is from I2 and/or I3- I suppose) turns clear when I add NaHCO3?
By the way, I really like your webpage. Lots of info and nice experiments for the hobby chemist.
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woelen
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The yellow color indeed is iodine, but only a very small amount of it. Oxygen is capable of (slowly) oxidizing iodide ion at low pH and as most water
has some oxygen dissolved in it, you'll see the yellow color when acidifying iodide solutions.
Dissolving I2 in methanol and evaporating that does not work well. On evaporation any water will remain behind and the iodine most likely will hold
back quite some methanol. I'm quite sure you'll make a nasty mess of it, hard to solidify and impossible to crystallize.
Using concentrated H3PO4, free of water, may work, but I have no experience with that. I do not expect iodine to dissolve well in H3PO4, but never
tried that personally. So, I first would try on a small scale before using up a lot of (expensive) chemicals.
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ChemSwede
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| Quote: Originally posted by woelen | The yellow color indeed is iodine, but only a very small amount of it. Oxygen is capable of (slowly) oxidizing iodide ion at low pH and as most water
has some oxygen dissolved in it, you'll see the yellow color when acidifying iodide solutions.
Dissolving I2 in methanol and evaporating that does not work well. On evaporation any water will remain behind and the iodine most likely will hold
back quite some methanol. I'm quite sure you'll make a nasty mess of it, hard to solidify and impossible to crystallize.
Using concentrated H3PO4, free of water, may work, but I have no experience with that. I do not expect iodine to dissolve well in H3PO4, but never
tried that personally. So, I first would try on a small scale before using up a lot of (expensive) chemicals. |
Ah, I thought the water would evaporate with the methanol, but I give up that idea then.
So the yellow is iodine. Nice to know.
What about the brown colour of the wastewater. I'm guessing small amounts of I2 and I3-. And adding NaHCO3 to that somehow turns that into I-
(colourless)? Is I2 and I3- unstable under alkaline conditions?
Blogfast suggested H3PO4 to be used.
The H2SO4 is used for H2SO4 + KI --> HI, but is it possible with H3PO4(85%) + KI --> HI?
I guess you could use H2O2 immediately without first adding H2SO4 but that's a bit wasteful since KI catalyzes the breakdown of H2O2. Better to
convert KI to HI first, right?
I tried to post an image of the nice yellow colour, but I failed to upload it. The size was under 2MB.
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ChemSwede
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I once again isolated some iodine from KI.
This time I used 40g of KI, 20ml of 50% H2SO4 and 40ml of 15% H2O2.
I slowly dripped the H2SO4 into a solution of KI. A yellow solution formed.
I then slowly dripped H2O2 into the yellow solution to precipitate iodine.
The solutions never heated up.
I then filtered the iodine with a sintered glass filter. To my surprise I discovered that more iodine had formed in the filtrate. Perhaps I didn't let
the reaction proceed long enough before filtering. I could salvage the extra iodine formed.
I placed the wet iodine crystals in a small beaker and poured conc. H2SO4 over it, so that the acid covered the iodine with one cm or so. Some purple
iodine vapor formed when the acid reacted with water in the wet iodine.
I then placed the beaker in a sand bath and started heating with my hotplate. That was too slow, so I decided to use a gas burner.
I heated the bottom of the beaker until all iodine had melted. Some product was lost as vapor since the temperature was a bit hard to control with a
gas burner.
I then let the iodine cool, and it formed a round "lump". The acid was removed and the lump of iodine was rinsed with a lot of distilled water until
pH was almost neutral.
The iodine was dried with a piece of paper and put in an exsickator with some NaOH at the bottom.
After a day the lump was weighed.
Theoretical yield from 40g of KI: 30,5g I2
My yield: 29,3g = 96%
Any waste containing iodine was treated with a solution of sodium thiosulfate.
Advise, comments, questions?
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