l0k1
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electrolytic separation of iodide salts and ion exchange for reactions
Firstly, I was just reading and watched a few youtube videos showing electrolysis of salts. With potassium or sodium iodide you get the formation of
elemental iodine on one electrode and hydrogen gas on the other.
Obviously since the goal is isolation of the iodine, there would have to be a membrane or salt bridge to stop the iodine reacting with the alkali that
builds up in the solution around the electrode emitting hydrogen. What is a good and cheap way to make a salt bridge? Vinyl or silicone tube stuffed
with cotton wool and soaked in saturated sodium chloride?
Am I right in saying that as the reaction proceeds the sodium or potassium hydroxide will tend to migrate into the salt bridge and back out into the
chamber emitting hydrogen? Is there some other way to make sure the elemental halide does not recombine? Such as, for example, since the solution is
brine, running it at the temperature of iodine sublimation, around 120C, since the reduction to iodide requires the iodine to be in solid or liquid
phase to happen efficiently. Another thought that occurs is filling the half cell with the iodide salt with powdered carbon which will adsorb it, and
then recover it by heating it dry and collecting on a cool surface.
I am just wondering about this because I would like to find ways to eliminate the presence of corrosive and irritating hydrochloric acid vapor during
chemical processes. The adsorption idea is particularly neat because it allows you to get 100% dry iodine without the hassle of dealing with iodine
which also makes irritating vapours. It also allows one to dispense with the need to acquire peroxide or other oxidiser.
Secondly, if one were to get zeolite powder and treat it with a solution of hydriodic acid, it would adsorb onto the surface of the zeolith until some
other salt ion comes along. If this treated zeolite were then exposed to a solution containing a proper amount of a chemical which reacts with
hydriodic acid in a 2:1 molar ratio and heated (perhaps in a microwave) then the ions of the other chemical will displace the HI (my guess is that the
hydrochloride would replace it) then putting the HI in solution with our desired reagent. So long as the volume of water is correct for the released
HI to be equivalent to saturated ~56% percent HI and the only base in there is the substrate the amount of HI required above the molar ratio would not
be great, only to account for the small amount of oxidation of HI to I2 by oxygen. The iodine could be recovered from this reaction by exposing it to
a coldfinger type thing.
I have read about other reaction systems using ion exchangers and microwaves and the reactions are extremely efficient, reaction times as short as
10-20 minutes and almost complete conversion and reduced production of unwanted side products. Since in the whole system the only two microwave
heatable materials are the acid ions and the water, the precursor and the zeoliths are transparent, there would be some degree of ionisation occurring
(microwaves induce currents in conductors of the wavelength of the microwave), which reduces the kinetic energy threshold for a reaction.
Something else that I am wondering about is ion exchange materials like zeolite, is it possible to somehow make the acid ion go into one portion of
the ion exchanger, and the alkali into the other? For the microwave/zeolite reaction proposed above, one needs hydriodic acid adsorbed onto the ion
exchange matrix. However if you put potassium iodide into contact with it, you would get a bit of both. The alkali metal would poison the reaction by
reducing the pH of the solution. Could one use some kind of method akin to electrophoresis combined with the ion exchange to cause the ions to migrate
to their respective electrodes?
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elementcollector1
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"Obviously since the goal is isolation of the iodine"
Why on earth would you go to this length to get iodine when there are so many other methods that can give you more product, faster, and can be
purified so easily?
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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l0k1
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the goal is HI adsorbed to ion exchange beads. mincing about with hydrochloric acid and oxidisers is a lot dirtier and untidy.
Continuing on with searching for information I have learned about the way that water deionisers work - they include two different ion exchanger types,
one which prefers negative the other positive. I can't seem to find any information about the different types however, but the technique would be
somewhat similar to capillary electrophoresis where the current induces the ions to tend towards the appropriate electrode. I'm not sure about the
details.
However, since zeolite or any other ion exchanger except for one special type do not adsorb nonpolar materials simply by filling a bath of potassium
iodide brine with zeolite, when the potassium hydroxide forms it is adsorbed, giving the iodine nothing to react with. a non-ionic mechanical
separation between the two sides of the cell such as some fine nylon mesh would allow one side to be filled with the ion exchanger and the other with
a nonpolar adsorbent like carbon. Elemental iodine adsorbed onto carbon would be very easy to get a pure dry end product by washing it with distilled
water and letting it sit in room temperature conditions. Having said that, instead by just using say zeolite as the alkali ion adsorbent one can then
after the reaction has been completed simply filter out the zeolite and add water (er, switch those around) and the iodine falls out of the water/salt
solution, and from there the standard drying it out as with acid/oxidiser methods of preparation.
But as I am saying, the goal is to get HI onto an ion exchange matrix to test a theory that using ion exchange and microwave heating that the problem
of regenerating the I2 is eliminated, because the ion exchanger limits the rate at which HI is released into the solution such that half the precursor
salts are adsorbed, half not, and in the process releasing sufficient HI to react the other half, and once the reaction has completed on a given
molecule it becomes insoluble iodine in the solution thus not participating in the reaction. For this reason there would need to be a mild excess of
some kind of acid, probably HCl, maybe acetic acid, which ensures the reaction conditions remain acidic, essential for halide reactions. Acetic acid
could be used here in the form of standard 5% distilled white vinegar as the solvent for the precursor, mixed so as to be just shy of full saturation.
So, if one can find a way to go direct to the HI adsorbed onto an ion exchanger, that would be better. The scanty information I can find so far seems
to suggest that an electrolysis bath containing a salt and filled with certain kinds of ion exchangers allows continuous flow deionisation of water. I
am going to continue investigating this and see what I can find because if this can be made to work it is a far shorter process and eliminates
entirely any need for a reducing agent for the I2 that forms after reaction. Some sort of coldfinger type thing, perhaps stack a container full of dry
ice which doesn't heat from microwaves, and you get all your reagent back.
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l0k1
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I just realised that the simplest way to do this directly is to mix KI with concentrated phosphoric acid, then boil and vent through a flask full of
zeolite, then suckback trap and finally water bubbler. I think that since the silica of zeolite won't participate in the reaction it could be used
directly as is mixed with the other reagents afterwards and will act as a solid phase buffer for all ions in the reaction (and those simply spectating
as well of course). I don't know if there is any specific benefit to this except perhaps that for example HI adsorbed onto zeolite in excess of
required to perform a HI reaction (like 10+ times as much as required, since making the doped zeolite is cheap and easy.
the iodine that is formed could be captured by adding carbon to the reaction as well, which will have the benefit of ensuring any oil soluble gak can
just be filtered straight out, and the iodine then recovered by sublimation under heat or maybe cooked in sodium hydroxide solution which will make it
cease to have affinity with the carbon and yields something that can be turned back around in the phosphoric acid gas phase adsorption apparatus. The
zeolite is then washed with sodium chloride to recover the adsorbed ions for recovery of the iodine and reaction product.
[Edited on 15-11-2012 by l0k1]
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