Sciencemadness Discussion Board

I2 crystals from tinctures - The chemistry behind it

cnidocyte - 18-12-2010 at 11:22

I need some I2 crystals for staining TLC plates so I looked up how to get them out of iodine tinctures. Found plenty of methods but none of them explain the theory behind it. From what I've gathered, the acid is just to convert the KI into I2 (tinctures usually contain 5% I2 and 5% KI dissolved in IPA). The rest I don't understand. The methods I've seen instruct you to use bleach (is there a standard NaOCl concentration for bleach) or 3% H2O2 solution.

From what I read, when enough oxidising agent has been added, I2 crystals start to precipitate out. I have no idea what is going on here though, can anyone inform me about the theory behind all this?

DJF90 - 18-12-2010 at 11:40

The reaction you're looking at will be something like this:

6I- + 6H+ + 3H2O2 => 3I2 + 6H2O

The acid is in there to allow the peroxide to work. Other oxidants work similarly. For the preparation of bromine, the following reaction can be used:

5Br- + BrO3(-) + 6H+ => 3Br2 + 3H2O.

Note how this is the reverse reaction of halogen with hydroxide (balance the H+ with OH- on both sides):

3Br2 + 6OH- => 5Br- + BrO3(-) + 3H2O (this reaction occurs with hot hydroxide solutions)

For completeness, with cold hydroxide we get:

2OH- + Br2 => Br- + BrO- + H2O

Note that the halogen involved may be substituted appropriately - they should (and do, to the best of my knowledge) all behave the same in this reaction.


[Edited on 18-12-2010 by DJF90]

ScienceSquirrel - 18-12-2010 at 12:38

Your best way to do this is to react the tincture of iodine with fine steel wool.
This causes the reaction;

Fe + I2 ----> FeI2

to occur,

Treatment with potassium hydroxide, boiling and filtration will remove the iron hydroxide, leaving you with a solution of potassium iodide in aqueous alcohol.
Evaporate or distill off the alcohol to leave a strong aqueous solution of potassium iodide.
Acidify and add hydrogen peroxide in slight excess, the iodine will precipitate out as violet black plates, filter off, press dry and then dry in a sealed bottle over silica gel.
Removing the alcohol improves the yield as the iodine has a significant solubilty in alcohol.
Alternatively you can use potassium hydroxide to convert the iodine to a mixture of the iodide and iodate, evaporate off the alcohol, acidify and add peroxide.

cnidocyte - 20-12-2010 at 05:12

Nice one ScienceSquirrel, first time I heard of that route, I'm gonna try that. Don't have any KOH right now though so I'm gonna try extracting with H2SO4 and NaOCl.

cnidocyte - 20-12-2010 at 08:19

I tried this but I don't think it worked. I added 98% H2SO4 to some diluted tincture and the very first drop I added caused HI to form of the surface but after that no gas. I then added "thick bleach" slowly while stirring and I saw a colour change but no visible precipitate. I was following a method I found on the internet but this method said to look out for precipitate. I'm leaving it sit for 20 minutes just in case I2 precipitates out but I'm doubtful that it'll happen.

This bleach was viscous like a syrup. I always remembered bleach having similar viscocity to water. I'm guessing my mistake was using "thick" bleach.

[Edited on 20-12-2010 by cnidocyte]

ScienceSquirrel - 20-12-2010 at 09:31

It is best to convert the iodine in the tincture to iodide if there is alcohol present so you can remove the alcohol.
If you are using an aqueous solution like Lugol's iodine solution then there is no need.
Thick bleach is full of shit, I would not use bleach, full stop.
You want to use just over the calculated amount of sulphuric acid and hydrogen peroxide, then filter and wash with deionised water, dry over calcium chloride at room temperature.

cnidocyte - 20-12-2010 at 11:28

Quote: Originally posted by ScienceSquirrel  
It is best to convert the iodine in the tincture to iodide if there is alcohol present so you can remove the alcohol.
If you are using an aqueous solution like Lugol's iodine solution then there is no need.
Thick bleach is full of shit, I would not use bleach, full stop.
You want to use just over the calculated amount of sulphuric acid and hydrogen peroxide, then filter and wash with deionised water, dry over calcium chloride at room temperature.


Yeah there'd be no reason for an aqueous salt solution to be viscous like that so I shoulda put more thought into it before deciding to use that crap.

I'll go with peroxide next time. Don't need many crystals so a bottle of 3% H2O2 should be more than enough. I'll react with Fe first this time. I'll have to use NaOH instead of KOH though.

[Edited on 20-12-2010 by cnidocyte]

ScienceSquirrel - 20-12-2010 at 12:50

Sodium hydroxide added to the tincture and then warmed to make a mixture of iodide and iodate will be fine. Stick it on top of a radiator to get rid of the alcohol and then acidify with sulphuric acid and add the hydrogen peroxide.
That should make iodine of around good reagent grade eg 99%+ when dried, that should be more than fit for purpose.

User - 21-12-2010 at 01:33

I would like to hop in for a sec.
There is this bottle in my lab that contains etOH and pure iodine.

What would be a clean way to separate this.
If possible without wasting other chemicals.

ScienceSquirrel - 21-12-2010 at 04:49

Quote: Originally posted by User  
I would like to hop in for a sec.
There is this bottle in my lab that contains etOH and pure iodine.

What would be a clean way to separate this.
If possible without wasting other chemicals.



Follow the procedure with steel wool that I give above, easy ;)

cnidocyte - 21-12-2010 at 08:27

I left the solution out in the cold overnight and black crystals precipitated out. I'll collect them but I've no idea how pure they are so I'm going to try again, this time using steel wool first.

The steel wool they had in my supermarket literally was steel. Do I need pure iron for this method or will carbon steel do? Heres my calculations for a 25ml tincture (2.5% I2 w/v, 2.5% KI w/v, 89% ethanol v/v).

25 mL (2.5% I2 w/v, 2.5% KI w/v, 89% EtOH v/v

I2 and KI
(2.5g / 100ml) = 0.025g/ml * 25ml = 0.625g per tincture

RMM of I2 = 253 g/mol
0.625g / 253g/mol = 0.00247 mol

RMM of KI = 166 g/mol
0.625g / 166g/mol = 0.00377 mol

I'll add an excess of steel wool and end up with 2.47 mmol of FeI2. Then
2NaOH + FeI2 -> 2NaI + Fe(OH)2
so I'll add about 5 mmol of NaOH and end up with 4.94 mmol of NaI

Now I need 7.41 mmol of H2O2 to convert the NaI and KI into 3.7 mmol of I2.

0.0037 mol * 253.8 g/mol = 0.94 g

I still don't know where the acid comes in. May have made a mistake somewhere in there but 0.94 of I2 per tincture looks about right.

[Edited on 21-12-2010 by cnidocyte]

ScienceSquirrel - 21-12-2010 at 08:33

I have used fine steel wool for making iron II sulphate, ferric ammonium sulphate, etc.
It is can be used anywhere iron filings is required and you can chop it up into tufts so it does not form a clump in the flask.

An easy test for purity with solid iodine is to warm it gently in a test tube, it it all sublimes as a violet vapour condensing as crystals in the top of the tube then it is pure.
It is a good way to purify a fair amount as well.

[Edited on 21-12-2010 by ScienceSquirrel]

cnidocyte - 21-12-2010 at 09:19

Quote: Originally posted by ScienceSquirrel  

An easy test for purity with solid iodine is to warm it gently in a test tube, it it all sublimes as a violet vapour condensing as crystals in the top of the tube then it is pure.
It is a good way to purify a fair amount as well.

The surfactants made it a nightmare to filter the crystals out. Its not worth the effort trying to separate the I2 from that crap.

cnidocyte - 22-12-2010 at 15:27

The only info on the I2 + Fe --> FeI2 reaction I could find involve reacting hot iron wool with I2 gas. Would the reactivity of an I2 solution be similar to a gas? I tore up some steel wool with my hands (didn't have a decent scissors) and added a bit to a flask. I added 2 x 25mL tinctures to the flask. So far no signs of a reaction occuring. I'm going to warm the flask but should I be adding water or anything to get this reaction going?

[Edited on 22-12-2010 by cnidocyte]

ScienceSquirrel - 22-12-2010 at 15:40

I have done this reaction and heating the solution is needed to get the reaction going.
It was a standard way to make pure potassium iodide.

cnidocyte - 22-12-2010 at 16:37

I have it mildly refluxing. The solution hasn't turned green but it has lost its dark brown colour, now its clear yellow. I also noticed some bright red/orange solids (or possibly spots of immiscible liquid) floating on the surface of the solution at one point. No idea what they were. I'm going to do a spot test to see if I can precipitate out any Fe(OH)2 after basifying and boiling.

EDIT: There is a lot of this bright orange stuff floating on the surface now and the solution has turned orange. There was some water in these tinctures, I think this is iron oxide.

2nd EDIT: I'm outta NaOH so I'm gonna have to leave this until tomorrow. I'm contemplating adding H2SO4 now. Don't think I'll even need to add H2O2 since H2SO4 is a decent oxidiser but I don't know how it'll react with FeI2.
H2SO4 + FeI2 --> FeSO4 + I2?

[Edited on 23-12-2010 by cnidocyte]

Sedit - 22-12-2010 at 18:01

Its a bit off topic and not exactly the equations you seem to desire but when dealing with tinctures they are normally NaI/I2 in alcohol.

I always added some NaOH and evaporated to dryness and beyond with a little extra heat. This gives me mostly NaI and perhaps some iodate but it seems to be a non issue. At that point I setup for a cold finger type system and start to drip in concentrated H2SO4 on to the NaI at which point the heat generated combined with the acidic nature of sulfuric acid as well as its oxidative properties always caused my I2 to condence dry as a bone at the top of the cold finger.

All the other methods of oxidation seem to precise and if you mess up you end up with nothing more then a yellow solution from over oxidation or something and the only possible way I have found to recover is to add Aluminum foil and place a non polar solvent above the solution. Overnight the Nonpolar will become deep purple from the iodine released and the Al will be mostly consumed.

not_important - 22-12-2010 at 19:34

I must point out that there in iodine tincture there is about equal amounts of iodine as I2 and as iodide (Na or K). Rather than just going for the I2, it might be worth recovering all the iodine. You can do as Sedit does, about 1/6 of the I2 iodine ends up as iodate; acidic conditions cause the iodate and iodide to react to produce I2.

Alternatively add 1 mole of glucose per mole of elemental I2 and 1.5 moles of Na2CO3. The I2 oxidises the glucose (sucrose does not work), forming gluconic acid and HI, the Na2CO3 neutralises the acids. After that the alcohol of the tincture can be distilled off, and the remaining solution evaporated. Extration with warm acetone gives a solution of pretty pure NaI in acetone, fom which it is recovered by evaporation. If I need elemental I2 I heat NaI with MnO2 and H2SO4, subliming off the I2.



MagicJigPipe - 23-12-2010 at 18:17

I remember back in the day, before "they" took tincture off the shelves, I would simply add some H2O2 (30%; amount should be easy to figure out) and then add a couple of drops of H2SO4. I think more dilute H2O2 would work but the reaction would be really slow. After adding these to the tincture just place it in the freezer until it gets very cold.

Why is that method not viable?

cnidocyte - 24-12-2010 at 10:39

I basified with NaOH and the solution went from clear yellow to murky black. There was brown condensation on the walls of the beaker so there was definitely I2 in there. What the hell happened? How did I convert FeI and/or KI to I2 with NaOH? I filtered out some of this black precipitate with coffee filters but a lot of it got through the filter. I'll extract the rest with proper filter paper.

[Edited on 24-12-2010 by cnidocyte]

cnidocyte - 27-12-2010 at 11:13

I'm completely lost now, I have no idea whats going on with this iodine solution. From start to finish, heres everything I observed:

1.) Reacted the tincture with steel wool and it went from dark brown to clear yellow.
2.) Basified the solution and it went from clear yellow to murky brown.
3.) Filtered out a significant amount of brown goo and was left with a clear, slightly yellow solution.
4.) Added around 15mL of 3% H2O2 solution, no visible changes occured.
5.) Added about 3mL of 98% H2SO4 and the solution instantly turned dark red.
6.) Added about 50mL of dH2O and ended up with what looks like a red solution.

Quote from wikipedia
Quote:

If the solution was not deoxygenated and the iron reduced, the precipitate can vary in color starting from green to reddish brown depending on the iron(III) content.

maybe that explains what happened when I basified. Whats responsible for turning the solution red after I added the H2SO4 though?

MagicJigPipe - 27-12-2010 at 16:35

No wonder you are lost! Look at the convoluted way you are going about this should-be-two-step process. If you are trying to get I2 from tincture simply add enough 30% H2O2 and H2SO4 to destroy the solvent, create HI and oxidize I- to I2. Then, bring temp. below 0*C and collect crystals. If you need them very dry I don't know what to do but I'm sure there are simple methods out there.

It has been many moons but I recall yields in excess of 90% or so from this method.

Also, to purify without sublimation just run a bit of cold water over them. I mean, what are you using them for? Analytical work??

I have done this before several times and I can think of no reason to do anything more complicated (less steps = less losses generally). Maybe someone could inform me if I am missing a piece of the puzzle.

cnidocyte - 27-12-2010 at 22:01

Quote: Originally posted by MagicJigPipe  
No wonder you are lost! Look at the convoluted way you are going about this should-be-two-step process. If you are trying to get I2 from tincture simply add enough 30% H2O2 and H2SO4 to destroy the solvent, create HI and oxidize I- to I2. Then, bring temp. below 0*C and collect crystals. If you need them very dry I don't know what to do but I'm sure there are simple methods out there.

Yep, it seems to be as simple as that. I checked the beaker after the solution had cooled to about 5C and there was a small pile of I2 crystals at the bottom of it. I got way more the first time I did this and I used 2 tinctures this time though so I added some more H2O2 and a few drops of acid and I'll see later if more crystals form. Next time I do this, I'm just gonna use H2O2 and H2SO4 like you described. I don't see why the order matters at all, I speculate that if I just added some piranha solution I'd have the same results as if I added the acid first, then the oxidiser like most people recommend.

Quote: Originally posted by MagicJigPipe  

Also, to purify without sublimation just run a bit of cold water over them. I mean, what are you using them for? Analytical work??

Staining TLC plates. I'm not sure how pure they need to be for that. Yeah I don't see any reason that a cold dH2O wash wouldn't do an adequate job of cleaning the crystals.

Quote: Originally posted by MagicJigPipe  

I have done this before several times and I can think of no reason to do anything more complicated (less steps = less losses generally). Maybe someone could inform me if I am missing a piece of the puzzle.

The first time I tried this, I pretty much used the method you described, only different being I used NaClO instead of H2O2 and added the acid first and by the looks of it, I got a very high yield. Only used 1 tincture but got a fairly large pile of crystals. If I hadn't used thick bleach, I would have been able to recover those crystals easily. In summary, I don't think theres much point in converting the I2 to a salt and boiling off the alcohol because the reason they add KI is to make the I2 more soluble in the alcohol so once the KI is gone, the I2 solubility in ethanol/isopropanol wont be an issue anymore.

[Edited on 28-12-2010 by cnidocyte]

cnidocyte - 28-12-2010 at 12:05

I'm guessing the red colour was just dissolved I2 because adding more H2O2 and H2SO4 took the colour out of the solution. I'm guessing the extra acid and oxidizer converted more NaI and KI reducing the I2 solubility even further. I recovered a small amount of crystals by the looks of it. I'm pretty sure I lost some when I filtered out the iron hydroxide after basifying.

bigad - 29-12-2010 at 20:19

why does kings chemistry survival guide preperation 1 for I2 crystals not use an acid

entropy51 - 30-12-2010 at 06:33

Quote: Originally posted by bigad  
why does kings chemistry survival guide preperation 1 for I2 crystals not use an acid
Because it was "written" by Jared Ledgard and any resemblance to chemistry is purely accidental.

nitric - 30-12-2010 at 20:40

Another procedure, much more unpleasant but has worked in the past for me is bubbling chlorine gas through the tincture, and that oxidises the KI into I and KCl and also helps precipitate the I2 already dissolved.

tnphysics - 30-12-2010 at 22:52

What about boiling off the liquid and reacting the ppt. with conc. H2SO4 (or dilute acid + oxidizer)?

nitric - 31-12-2010 at 12:21

Quote: Originally posted by tnphysics  
What about boiling off the liquid and reacting the ppt. with conc. H2SO4 (or dilute acid + oxidizer)?


I think that would work, but would be harder to work with, it would be preferable just to remove the ethanol then add sulfuric acid to the conc KI + I2 solution with H2O2 or some other oxidizing agent