Sciencemadness Discussion Board - silver iodide or copper iodide?

Try dissolving it in conc. ammonia. Silver iodide will not dissolve; copper(I) iodide will, and the solution will turn blue.

Wow, cool! I'll make some copper iodide just to try this.

jamit - 8-11-2014 at 02:43

Try dissolving it in conc. ammonia. Silver iodide will not dissolve; copper(I) iodide will, and the solution will turn blue.

I'll try it and report back. Thanks.

gdflp - 9-11-2014 at 07:44

I believe that silver iodide will dissolve. Silver(I) forms an amine complex as well as copper, and the CRC lists silver iodide as being soluble in NH4OH.

DraconicAcid - 9-11-2014 at 12:04

I believe that silver iodide will dissolve. Silver(I) forms an amine complex as well as copper, and the CRC lists silver iodide as being soluble in NH4OH.

Silver does form an amine complex, but the iodide is really insoluble. I was under the impression that silver chloride dissolves easily in dilute ammonia, silver bromide requires concentrated ammonia, and silver iodide stays put. If it is soluble in conc. ammonia, it's not very.

Anyway, the silver complex is colourless and stays that way. The copper(I) complex is colourless, but rapidly turns dark blue on exposure to air.

[Edited on 9-11-2014 by DraconicAcid]

Metacelsus - 9-11-2014 at 12:48

http://alevelchem.com/aqa_a_level_chemistry/unit3.2/sub3205/...

Quote:

Silver iodide does not dissolve in even concentrated ammonia.

Edit: Didn't notice above post (facepalm)

[Edited on 10-11-2014 by Cheddite Cheese]

gdflp - 9-11-2014 at 13:01

I apologize then. Maybe I need to find a newer CRC.

The Volatile Chemist - 9-11-2014 at 13:16

Quote:

Wow, cool! I'll make some copper iodide just to try this.

It works with any Copper(II) ion salt.
Edit: most salts, eg. CuCl₂

[Edited on 11-9-2014 by The Volatile Chemist]

DraconicAcid - 9-11-2014 at 13:21

The blue is from the ammonia complex with copper(II)- as the volatile one says, it will work with any copper(II) salt. Copper(I) iodide isn't worth making specifically for it.

DrMario - 9-11-2014 at 13:24

Quote:

Wow, cool! I'll make some copper iodide just to try this.

It works with any Copper(II) ion salt.
Edit: most salts, eg. CuCl₂

[Edited on 11-9-2014 by The Volatile Chemist]

I've tried this with most Cu(II) salts as a youngster, but never with copper iodide.

The Volatile Chemist - 9-11-2014 at 14:23

Quote:

Wow, cool! I'll make some copper iodide just to try this.

It works with any Copper(II) ion salt.
Edit: most salts, eg. CuCl₂

[Edited on 11-9-2014 by The Volatile Chemist]

I've tried this with most Cu(II) salts as a youngster, but never with copper iodide.

I see. I suppose it would be a bit more dramatic with the CuI₂.

DrMario - 9-11-2014 at 16:59

Quote:

Wow, cool! I'll make some copper iodide just to try this.

It works with any Copper(II) ion salt.
Edit: most salts, eg. CuCl₂

[Edited on 11-9-2014 by The Volatile Chemist]

I've tried this with most Cu(II) salts as a youngster, but never with copper iodide.

I see. I suppose it would be a bit more dramatic with the CuI₂.

Totally worth it!

A few comments: I synthesized the CuI2 via a simple substitution reaction between CuSO4 and KI - but to my surprise, elemental iodine was also forming. The iodine formation was slow but after a quarter of an hour it made the liquid very clearly brown-yellow and the iodine smell (one of my favorites) was unmistakable. What up with that?
The other thing is the colour of the precipitate: it was light grey, not "light brownish yellow" as jamit describes. I assume his sample is still contaminated with some iodine. I have, on my part, carefully decanted my product multiple times.

The change from light gray precipitate to splendid royal blue with the addition of ammonia, was stark and, as I said, totally worth the effort.

gdflp - 9-11-2014 at 17:01

The elemental iodine is due to the fact that you didn't synthesize CuI2, you synthesized CuI. CuI2 is unstable and rapidly decomposes 2CuI2 --> 2CuI + I2

DrMario - 9-11-2014 at 23:13

The elemental iodine is due to the fact that you didn't synthesize CuI2, you synthesized CuI. CuI2 is unstable and rapidly decomposes 2CuI2 --> 2CuI + I2

jamit, you hear that? So now we at least know which compound you do NOT have.

DraconicAcid - 9-11-2014 at 23:26

jamit, you hear that? So now we at least know which compound you do NOT have.

I think it was a given that it wasn't CuI₂; it was CuI that was considered a possibility.

unionised - 10-11-2014 at 00:30

Is it just my imagination, or is this a rather long thread to see if someone has silver iodide (which is yellow) or copper iodide (which is white)?

DrMario - 10-11-2014 at 00:50

jamit, you hear that? So now we at least know which compound you do NOT have.

I think it was a given that it wasn't CuI₂; it was CuI that was considered a possibility.

It wasn't necessarily a given, since CuI2 is indeed yellow.

DrMario - 10-11-2014 at 00:51

Is it just my imagination, or is this a rather long thread to see if someone has silver iodide (which is yellow) or copper iodide (which is white)?

It's not your imagination, but you have to consider that some of us (me, for instance) didn't know that CuI2 is so unstable. But I suspect there may be other thread participants that were blissfully unaware of this.

DJF90 - 10-11-2014 at 01:11

The elemental iodine is due to the fact that you didn't synthesize CuI2, you synthesized CuI. CuI2 is unstable and rapidly decomposes 2CuI2 --> 2CuI + I2

This technically isn't true. CuI2 cannot exist because Cu2+ oxidises I- to iodine (look at the SRPs), and so the two ionic species cannot co-exist (as both are kinetically fast reactants). It is a redox reaction rather than a decomposition.

unionised - 10-11-2014 at 03:08

jamit, you hear that? So now we at least know which compound you do NOT have.

I think it was a given that it wasn't CuI₂; it was CuI that was considered a possibility.

It wasn't necessarily a given, since CuI2 is indeed yellow.

How can it be yellow when it doesn't exist?

DJF90 - 10-11-2014 at 04:16

Another indication is that silver iodide photolyses in light...

DrMario - 10-11-2014 at 07:53

Quote: Originally posted by DJF90

The elemental iodine is due to the fact that you didn't synthesize CuI2, you synthesized CuI. CuI2 is unstable and rapidly decomposes 2CuI2 --> 2CuI + I2

It clearly exists for a while - the reaction where 2CuI2 --> 2CuI + 2I is relatively slow. You should try it, it's educative.

Now the fact that the OP does not have CuI2 because of its instability, is another question. This instability is new information that we learned in this thread (or from some Wikipedia page).

DrMario - 10-11-2014 at 07:56

jamit, you hear that? So now we at least know which compound you do NOT have.

I think it was a given that it wasn't CuI₂; it was CuI that was considered a possibility.

It wasn't necessarily a given, since CuI2 is indeed yellow.

How can it be yellow when it doesn't exist?

unionised - 10-11-2014 at 08:29

I see no evidence of any CuI2
The colour of I2 forms immediately
http://www.youtube.com/watch?v=6SQus2Ikqqs
about 03:35

DraconicAcid - 10-11-2014 at 09:35

Is it just my imagination, or is this a rather long thread to see if someone has silver iodide (which is yellow) or copper iodide (which is white)?

CuI is only white when it's pure. Some of us often get off-white products which were supposed to be white.

DraconicAcid - 10-11-2014 at 09:37

How can it be yellow when it doesn't exist?

I have done the reaction, and while there is a transient yellow colour, that could easily be due to a CuI⁺ complex or other species. Or dilute triiodide ion, or....

DrMario - 10-11-2014 at 10:15

How can it be yellow when it doesn't exist?

I have done the reaction, and while there is a transient yellow colour, that could easily be due to a CuI⁺ complex or other species. Or dilute triiodide ion, or....

Okay.

BTW, this text (with no actual corroboration except for "yeah, it makes some sense, why not...." states that cupric iodide can be stabilized with ammonium complexation:
http://copper.atomistry.com/cupric_iodide.html

DraconicAcid - 10-11-2014 at 10:20

BTW, this text (with no actual corroboration except for "yeah, it makes some sense, why not...." states that cupric iodide can be stabilized with ammonium complexation:
http://copper.atomistry.com/cupric_iodide.html

Well, what do you think the blue colour we've been talking about is? The blue colour is the tetramminecopper(II) ion, and the iodide counterions are still there in solution with it.

DrMario - 10-11-2014 at 11:46