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Author: Subject: Report on attempt to make Cromium Iodide
Lion850
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[*] posted on 29-11-2019 at 18:52
Report on attempt to make Cromium Iodide


Reading online it was not immediately clear to me which was the best solvent to use for reacting Chrome powder and Iodine crystals; however some sites claimed CrI3 was soluble in water so I decided to give water a go.
4g Chromium powder and 10g Iodine was added to 120ml water in a beaker and boiled. There was little sign of a reaction, and Iodine fumes kept coming out. After 60 minutes the solution became brown, it was boiled for another 30 minutes. The solution was then filtered, the filtrate was pale brown but there was a lot of unreacted Chrome powder and Iodine left in the beaker. This filtrate was later boiled off and leftover was a minuscule amount of light brown powder - just enough to be visible in the empty beaker and not enough to do any tests with. It was discarded.
150 ml Xylene was then added to the beaker with the leftover powder and Iodine, also the Iodine crystals that formed when the Iodine vapored off in the earlier part was added again. The xylene solution was then boiled under reflux (not sealed to the atmosphere), after 90 minutes the solution seemed more clear and it was filtered. This time the filtrate was a wine red color, but when boiled off a similar looking very small amount of light brown powder was left - basically just enough the stain the beaker. And there was again a lot of black sludge left over in the beaker. At this stage it was the end of the day.
I returned the next day planning to get rid of the black sludge and clean the beaker. However I saw the sludge was now wet and a beautiful dark green color. And heating a tiny amount in a flame showed no Iodine vapors. I then dissolved the sludge in 100ml water; it dissolved easily. A green filtrate was obtained and there was now only a small bit of Chrome powder remaining.
I boiled down the green solution in the beaker directly on the hot plate. But as the last water disappeared the leftover solid because yellow-brown and a sticky mess like chewing gum. I added a small bit of water and all dissolved into the dark green solution again. This time I placed a watch glass on a boiling beaker with water to ensure it is dried at no more then 100 deg; again a dark green / brown sticky mess resulted. I then added some 5ml of water; all dissolved back to the nice dark green solution. Total weight of the solution was 9g; so the solid must be around 4g.
I tested a few drops of the solution with lead nitrate to confirm it contains Chrome Iodide and the familiar bright yellow lead iodide immediately appeared.
I did not try to extract crystals from the solution again; that will have to wait until I can attempt to it at room or at least reduced temperature.
I was a bit confused as to whether I made CrI3 or CrI as both exist. "Treatise on inorganic chemistry V11 chapter 60 Cr" which is in the Sciencemadness library mentioned dark green very hygroscopic crystals in chapter 27 "Chromium Iodides and Oxyiodides" but I am not sure to which Iodide of Chromium this applies.
I am also not sure when this compound formed: was it during the xylene boiling or when the sludge in the beaker was left unattended for more than 12 hours, open to atmosphere?
I will re-run this experiment under xylene when I have a proper reflux setup.
Attached a photo of the initial water and xylene filtrates and the final green solution - although the green does not show that well on the photo.


Water filtrate & Xylene filtrate.jpg - 467kB Green CrI3 (I think) solution.jpg - 441kB
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DraconicAcid
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[*] posted on 29-11-2019 at 19:47


CrI exists? I doubt it. CrI2 would be very hard to make, as Cr(II) is extremely air-sensitive and easily oxidized.

I'm fairly certain you've made hydrated chromium(III) iodide. However, this will form a number of different compounds, because chromium(III) forms complex ions which are very slow to react to change ligands. You may have [CrI3(H2O)3] (two possible isomers), [CrI2(H2O)4]I (also two possible isomers), [CrI(H2O)5]I2 and [Cr(H2O)6]I3. Reaction with lead nitrate shows that you have at least one compound where iodide is not coordinated to the chromium ([CrI3(H2O)3] will not react with either lead nitrate or silver nitrate, at least not over short time scales).

Coordination chemistry is great fun, but with chromium(III), it can often just give a mess.




Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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j_sum1
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[*] posted on 29-11-2019 at 20:18


This is so true that it is worth highlighting.

Quote: Originally posted by DraconicAcid  

Coordination chemistry is great fun, but with chromium(III), it can often just give a mess.


This is probably even more true of iron than chromium.
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[*] posted on 29-11-2019 at 22:00


Quote: Originally posted by j_sum1  
This is so true that it is worth highlighting.

Quote: Originally posted by DraconicAcid  

Coordination chemistry is great fun, but with chromium(III), it can often just give a mess.


This is probably even more true of iron than chromium.


At least iron will exchange ligands quickly enough that you can actually get one main product.




Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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