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Topic: (Cu(NH3)4)SO4 + KI = ?  (Read 544 times)

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Offline nino_

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(Cu(NH3)4)SO4 + KI = ?
« on: October 24, 2020, 09:23:47 PM »
Hi, I recently made tetraamminecopper(II) sulfate and I thought I'd experiment with it a little bit. I know copper(II) salts react with KI to give CuI and iodine but I didn't know what the results with the ammonia complex would be. I mixed a solution of the complex with a KI solution and saw a precipitate, I then filtered it (picture attached) and I was left with that green powder. My guess is that the copper(II) got reduced to copper(I) since there was a brownish colour on the walls of the beaker, indicating the presence of iodine. Any help on what that compound could be ?

Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #1 on: October 25, 2020, 03:23:40 AM »
This is probably not very pure CuI [copper(I) iodide].
AWK

Offline nino_

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #2 on: October 25, 2020, 05:24:27 PM »
This is probably not very pure CuI [copper(I) iodide].

I washed the precipitate with water which should have dissolved the remaining copper-ammonia complex. I then washed it with acetone and let it air dry with the vacuum still on.
I also tried dissolving it in acetonitrile which should have worked if that was CuI, it didn't move at all.

Sorry, I should have mentioned those steps in the first post too.


Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #3 on: October 26, 2020, 03:27:18 AM »
The reaction you used is not suitable for obtaining pure CuI. Many side reactions take place during the reaction and lead to colored impurities. In addition, CuI itself oxidizes in the air (which you yourself noticed) and slowly turns green-blue, and such a preparation already loses its catalytic properties, and the colorless CuI catalyst is most often used.
AWK

Offline nino_

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #4 on: October 26, 2020, 08:13:21 AM »
The reaction you used is not suitable for obtaining pure CuI. Many side reactions take place during the reaction and lead to colored impurities. In addition, CuI itself oxidizes in the air (which you yourself noticed) and slowly turns green-blue, and such a preparation already loses its catalytic properties, and the colorless CuI catalyst is most often used.

I think there's a misunderstanding here... I didn't notice any air oxydation.
Plus, the color is perfectly homogeneous, I dont think i left it long enough in the air to have it oxidized all the way, so even if it did, shouldn't I notice that the color is uneven ? Does it oxidize that fast ?


Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #5 on: October 26, 2020, 08:57:53 AM »
Did you conduct the reaction and work it up under an inert gas atmosphere? Cu (I) compounds are extremely sensitive to oxidation. If you want to obtain colorless CuI, you must first reduce Cu (II) to Cu (I), then do the exchange reaction with an exactly stoichiometric amount of KI, and all operations, including drying, must be performed under an inert atmosphere, best under hydrogen. Commercial CuI becomes colored on the surface after a few days after opening the jar once.
AWK

Offline billnotgatez

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #6 on: October 26, 2020, 10:44:08 AM »
... best under hydrogen ...

is this a typo?

Offline nino_

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #7 on: October 26, 2020, 12:57:53 PM »
Did you conduct the reaction and work it up under an inert gas atmosphere? Cu (I) compounds are extremely sensitive to oxidation. If you want to obtain colorless CuI, you must first reduce Cu (II) to Cu (I), then do the exchange reaction with an exactly stoichiometric amount of KI, and all operations, including drying, must be performed under an inert atmosphere, best under hydrogen. Commercial CuI becomes colored on the surface after a few days after opening the jar once.

No, all I did was mixing two solutions together and then filtering, no inert atmosphere, the filtered product was green when wet, stayed green when dry. And if it doesn't oxidize that fast, what you think is CuI should dissolve in acetonitrile, at least most of it, but it doesn't.
Could it not dissolve because it is impure ?

Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #8 on: October 26, 2020, 01:42:56 PM »
Do you know the exact proportions in which you mixed the reagents: CuSO4, NH3, and KI?
My guess is that the basic copper sulfate is responsible for the color, and also the product of CuI oxidation by oxygen from the air in the presence of water.
AWK

Offline nino_

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #9 on: October 26, 2020, 02:38:25 PM »
Do you know the exact proportions in which you mixed the reagents: CuSO4, NH3, and KI?
My guess is that the basic copper sulfate is responsible for the color, and also the product of CuI oxidation by oxygen from the air in the presence of water.

Well, the (Cu(NH3)4)SO4 was from a previous experiment, (CuSO4 + NH4OH, then precipitated with methanol), I then filtered it and dried it. I dont know how pure it is, but honestly it must be good enough since the powder was crystalline. I dont think there was much copper sulfate remaining, i washed it with lots of methanol/ammonia to make sure it's as pure as it can be (might need a recryst. though).

And for the KI... I didn't really write down how much I used, since it was just to see if there was a reaction. I think it was like 20g in 100ml of water ? Mixed with maybe 5g of the copper-ammonia complex in 100ml of water. I don't really remember and I don't have enough KI to redo the experiment, might have to order some.

Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #10 on: October 26, 2020, 02:42:20 PM »
Try to write a balanced reaction equation and think about what side reactions can occur - one I have already mentioned - the formation of basic copper sulfate.
AWK

Offline nino_

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #11 on: October 27, 2020, 09:40:23 PM »
Ok, so I've conducted a new test.
My reasoning was if the following equation: Cu2+ + 2I-  :rarrow: CuI2  :rarrow: CuI + 1/2I2 works for my complex, I should then see the blue coloration disappear and the white-ish CuI precipitate. I weighed 5g of copper complex and added 3 (2 for the equivalence and 1 more for excess) molar equivalents of KI, the solution stayed blue.
I then added what I had left of KI, I didn't weigh it but there may have been like 10g of it left. Still blue, though darker with some dark precipitate (probably that same green powder). I then noticed the solution lightly smelled of ammonia, so I thought I should probably neutralize it with some acid ? I took a small sample of it, added like 5 drops of 96% H2SO4, and there it was, white precipitate in a brown solution, the CuI and I2. There's little to no doubt that the ammonia in solution plays a role in the formation of the green precipitate, and neutralizing it with an acid allows the iodide ions to react with the Cu2+.
But if the basic solution is still blue after adding an excess of KI, would it mean that maybe another complex is made, thus needing more iodide equivalents for the complex to fall out of solution ?
I think I should have added the copper solution to the KI solution, as opposed to what I did, adding the KI to the copper solution, this way I could have seen better (the copper solution is really dark).
The more I experiment on this, the more questions I have  ???.
I dont have the time for a filtration tonight, I'll try one of these days.

@AWK, I really don't see how this reaction could lead to basic copper sulfate, I can't seem to understand the side reactions you're talking about.

Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #12 on: October 27, 2020, 10:19:56 PM »
Every chemist begins a synthesis by writing a balanced main chemical reaction and considering potential side reactions.
Did you know that CuI with ammonia gives a water-soluble complex, did you know that CuI dissolves in excess of KI, did you know that I2 forms I3- with iodide ions and may be also CuI3 is formed? The Cu(I) ammonium complex contains less ammonia molecules than the Cu(II) complex and therefore basic copper sulphate with a variable composition will precipitate. All this has to be written down with balanced reactions and consider whether this way of synthesis makes sense.

You're just wasting chemicals this way.
AWK

Offline nino_

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #13 on: October 27, 2020, 11:20:25 PM »
Every chemist begins a synthesis by writing a balanced main chemical reaction and considering potential side reactions.
Did you know that CuI with ammonia gives a water-soluble complex, did you know that CuI dissolves in excess of KI, did you know that I2 forms I3- with iodide ions and may be also CuI3 is formed? The Cu(I) ammonium complex contains less ammonia molecules than the Cu(II) complex and therefore basic copper sulphate with a variable composition will precipitate. All this has to be written down with balanced reactions and consider whether this way of synthesis makes sense.

You're just wasting chemicals this way.

Well, as I was doing this experiment just for fun, I didn't think of writing down all the possible side reactions, sorry.
I do know about most of these reactions but I didn't take all of them in consideration, since I'm not familiar with all the conditions for these to happen.
I don't like to see this as a waste of chemicals, chemicals are never wasted if you can learn something from the experiment. What I was doing certainly looked like messing around to you, I'm ok with that, the point of this experiment wasn't about synthesizing something in particular, I just wanted to see if had maybe created some poorly documented complex, that's it.

Thank you for your answers, I'm moving to another experiment :)

« Last Edit: October 27, 2020, 11:51:15 PM by nino_ »

Offline AWK

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Re: (Cu(NH3)4)SO4 + KI = ?
« Reply #14 on: October 28, 2020, 12:06:09 AM »
Want to see what CuI looks like? Got some sodium thiosulfate? In one test tube you can get a few to several dozen mg of the compound in 5 minutes from any soluble Cu (II) salt, but not from the ammonia complex. Mix concentrated copper salt solution with potassium iodide, then add sodium thiosulfate until the solution becomes discolored. Rinse the sediment thoroughly with water several times (no filtration and prolonged contact with air, no drying). Leave precipitate under the water. It will be almost white and it will be CuI.
AWK

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