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AWLB
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Identifying Zn2+/Al3+ ions
Does anyone know how Zn2+/Al3+ ions can be identified in solution. I recently found a sample of metal which I assumed to be either zinc or aluminum;
which I dissolved in conc. hydrochloric acid- the salt formed is of course ZnCl2 or AlCl3, is there a way to determine which cation (Zn2+/Al3+) is in
solution by a simple test?
Thanks in advance.
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unionised
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Add excess ammonia. Al(OH3) won't dissolve Zn(OH)2 will.
Of course, it could be an alloy.
If there's a precipitate that doesn't dissolve, filter it off or wait for it to settle then pour off the clear solution. Add a dilute acid slowly- if
a precipitate forms whrn the excess ammonia in neutralised then there's probably zinc in there as well as aluminium.
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AWLB
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Thank you very much for replying, that seems like a good way to test for the ions. Why does the Zn(OH)2 dissolve with excess ammonia, does it form a
complex?
Thank you for your help.
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j_sum1
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Quote: Originally posted by unionised  | Add excess ammonia. Al(OH3) won't dissolve Zn(OH)2 will.
Of course, it could be an alloy.
If there's a precipitate that doesn't dissolve, filter it off or wait for it to settle then pour off the clear solution. Add a dilute acid slowly- if
a precipitate forms whrn the excess ammonia in neutralised then there's probably zinc in there as well as aluminium. |
It ain't quite that easy.
Zn and Al behave in a very similar manner.
https://www.youtube.com/watch?v=KNvPfGBnN94
(I only know this because I have been looking at it recently. This is the third time recently I have had occasion to post this video -- which I
believe shows a method for differentiating between these two ions. https://www.youtube.com/watch?v=IFlIi_Xk3FE)
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AWLB
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Thank you for posting those informative videos j_sum1, they are very helpful. I believe I have now identified the cation using ammonia- it appears to
be aluminum, though as you said they are almost indistinguishable(though the precipitate does not dissolve in excess NH4OH); the metal also is also
not dense enough to be zinc. Thank you very much (is that your YouTube channel ? Your videos are very good!)
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j_sum1
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Not my channel.
Agreed, they are presented to a high standard. I have bookmarked it because I have lots to learn.
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AWLB
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Thank you nonetheless for helping me j_sum1, I appreciate it.
I also have a lot to learn about chemistry, but learning is perhaps the greatest enjoyment of this hobby, I am still at school so these videos, this
blog and home chemistry are very helpful.
[Edited on 27-7-2015 by AWLB]
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blogfast25
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j_sum1:
Zinc does form a tetrammine complex and Al does not. But redissolving the firstly formed Zn(OH)2 into an excess of NH3(aq) takes time and agitation.
Another difference between Al and Zn is that when a carbonate solution is added Al(OH)3 precipitates (with CO2 escaping), where as with Zn the
carbonate or basic carbonate precipitates.
[Edited on 27-7-2015 by blogfast25]
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AWLB
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Thank you blogfst25 for providing an alternate way to distinguish between Zn2+ and Al3+ ions in solution.
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unionised
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| Quote: Originally posted by blogfast25 | j_sum1:
Zinc does form a tetrammine complex and Al does not. But redissolving the firstly formed Zn(OH)2 into an excess of NH3(aq) takes time and agitation.
Another difference between Al and Zn is that when a carbonate solution is added Al(OH)3 precipitates (with CO2 escaping), where as with Zn the
carbonate or basic carbonate precipitates.
[Edited on 27-7-2015 by blogfast25] |
Tell me, if the zinc forms a basic carbonate, where does the CO2 go?
Also, if the solution is already acidic 9and it will be- remember -, it was made by dissolving the metal in acid- then the zinc solution and the
aluminium solutions will both fizz and give a white solid.
Not quintessentially specific.
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unionised
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| Quote: Originally posted by j_sum1 | | Quote: Originally posted by unionised | Add excess ammonia. Al(OH3) won't dissolve Zn(OH)2 will.
Of course, it could be an alloy.
If there's a precipitate that doesn't dissolve, filter it off or wait for it to settle then pour off the clear solution. Add a dilute acid slowly- if
a precipitate forms whrn the excess ammonia in neutralised then there's probably zinc in there as well as aluminium. |
It ain't quite that easy.
Zn and Al behave in a very similar manner.
https://www.youtube.com/watch?v=KNvPfGBnN94
(I only know this because I have been looking at it recently. This is the third time recently I have had occasion to post this video -- which I
believe shows a method for differentiating between these two ions. https://www.youtube.com/watch?v=IFlIi_Xk3FE) |
Did you actually watch the video?
At about 5:10 you can see that the Zn(OH)2 dissolves.
At about 4:00 they confirm that Al(OH)3 doesn't dissolve in excess
The reactions are fast ( it takes a few seconds to mix the solutions properly in the tube.)
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blogfast25
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| Quote: Originally posted by unionised |
Did you actually watch the video?
At about 5:10 you can see that the Zn(OH)2 dissolves.
At about 4:00 they confirm that Al(OH)3 doesn't dissolve in excess
The reactions are fast ( it takes a few seconds to mix the solutions properly in the tube.)
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Actually the first caption (a propopos Zn2+) says: 'A white precipitate forms that does not redissolve in excess ammonia'.
Problem is that they've got their proportions all wrong: far too much ZnCl2 solution. With about 1 ml of solution, redissolving the formed Zn(OH)2
into excess NH3(aq) would be quick and conclusive.
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blogfast25
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| Quote: Originally posted by unionised |
Tell me, if the zinc forms a basic carbonate, where does the CO2 go?
Also, if the solution is already acidic 9and it will be- remember -, it was made by dissolving the metal in acid- then the zinc solution and the
aluminium solutions will both fizz and give a white solid.
Not quintessentially specific.
|
I'm not claiming it's 'quintessentially specific'. But one can carefully wash the precipitate and subject it to acid again. The Al precipitate will
not fizz, the Zn one will. That's distinctive: the first one's a hydroxide, the second one's a carbonate.
If a basic zinc carbonate forms then of course CO2 also evades. It's not clear to me in which circumstances 'straight' or basic carbonate forms
though.
A mineral basic zinc carbonate - Hydrozincite:
Zn5(CO3)2(OH)6 = 2ZnCO3.3Zn(OH)2
https://en.wikipedia.org/wiki/Hydrozincite
[Edited on 27-7-2015 by blogfast25]
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unionised
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by unionised |
Did you actually watch the video?
At about 5:10 you can see that the Zn(OH)2 dissolves.
At about 4:00 they confirm that Al(OH)3 doesn't dissolve in excess
The reactions are fast ( it takes a few seconds to mix the solutions properly in the tube.)
|
Actually the first caption (a propopos Zn2+) says: 'A white precipitate forms that does not redissolve in excess ammonia'.
Problem is that they've got their proportions all wrong: far too much ZnCl2 solution. With about 1 ml of solution, redissolving the formed Zn(OH)2
into excess NH3(aq) would be quick and conclusive. |
The first caption is wrong; the first solution is Mg++
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blogfast25
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According to this somewhat fluffy paper synthetic basic zinc carbonates do exist:
http://rruff.info/uploads/CM8_92.pdf
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unionised
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by unionised |
Tell me, if the zinc forms a basic carbonate, where does the CO2 go?
Also, if the solution is already acidic 9and it will be- remember -, it was made by dissolving the metal in acid- then the zinc solution and the
aluminium solutions will both fizz and give a white solid.
Not quintessentially specific.
|
I'm not claiming it's 'quintessentially specific'. But one can carefully wash the precipitate and subject it to acid again. The Al precipitate will
not fizz, the Zn one will. That's distinctive: the first one's a hydroxide, the second one's a carbonate.
If a basic zinc carbonate forms then of course CO2 also evades. It's not clear to me in which circumstances 'straight' or basic carbonate forms
though.
A mineral basic zinc carbonate - Hydrozincite:
Zn5(CO3)2(OH)6 = 2ZnCO3.3Zn(OH)2
https://en.wikipedia.org/wiki/Hydrozincite
[Edited on 27-7-2015 by blogfast25] |
indeed, but that's not what you wrote at first is it?
It's not always possible to produce straight carbonates. Using bicarbonate as the precipitant sometimes helps (slightly lower pH an thus slightly less
OH- about the place) and working under a high pressure of CO2 canalso work.
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blogfast25
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Yep. It's a shame about the caption mix-up: in the third experiment (with Zn2+) the precipitate clearly redissolves quite quickly in NH3(aq).
[Edited on 27-7-2015 by blogfast25]
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blogfast25
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'Pffft' and 'sigh' spring to mind.
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AJKOER
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Here is an interesting way based on the observation that adding Na2CO3 or NaHCO3 to say ZnCl2 creates a precipitate of ZnCO3 that can be partially
redissolved upon the addition of more Na2CO3 or NaHCO3. Reference: see http://pubs.acs.org/doi/abs/10.1021/ic50190a004 and a very dated observational account at https://books.google.com/books?id=FSxGAQAAMAAJ&pg=RA2-PA... of Zinc bicarbonate's stability/solubility in natural streams.
So add a small amount of NaHCO3 to aqueous ZnCl2 in a long thin tube. Shake and poor half of the suspension into an identical tube. Now, add more
solid Na2CO3 to just one tube and stir to dissolve. If the relative height of the precipitate is lower in the treated tube on standing (which is the
best technique but possibly sooner, if comparisons are taken at the same time when both tubes were shaken together to dissolve the added carbonate in
one tube assuming vicosity differences can be adjusted or not too material) , than some of the treated ZnCO3 is now a soluble complex (perhaps as
Zn(HCO3)2 ). Add more Na2CO3 and see if a further relative decline is visible. This property is not shared by Aluminum unless the Na2CO3 /Al(OH)3 mix
is boiled, at which point an aluminate is formed.
I am assuming we known the weight of the mystery metal and max % Zn so that a suitable amount of Na2CO3 can be added to precipitate all the Zinc as
ZnCO3.
[Edit] Note, this test works in alloy situations as more ZnCO3 relatively dissolves. Also, boiling the carbonate rich tube reveals the presence of
Aluminum.
[Edited on 27-7-2015 by AJKOER]
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blogfast25
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| Quote: Originally posted by AJKOER |
So add a small amount of NaHCO3 to aqueous ZnCl2 in a long thin tube. Shake and poor half of the suspension into an identical tube. Now, add more
solid Na2CO3 to just one tube and stir to dissolve. If the relative height of the precipitate is lower in the treated tube on standing (which is the
best technique but possibly sooner, if comparisons are taken at the same time when both tubes were shaken together to dissolve the added carbonate in
one tube assuming vicosity differences can be adjusted or not too material) , than some of the treated ZnCO3 is now a soluble complex (perhaps as
Zn(HCO3)2 ). Add more Na2CO3 and see if a further relative decline is visible.
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Interesting as all this sounds, but have you tested this?
And this bit:
| Quote: | | This property is not shared by Aluminum unless the Na2CO3 /Al(OH)3 mix is boiled, at which point an aluminate is formed. |
... is, despite your past announcements, NOT true. Carbonate solutions are not alkaline enough to form aluminate in any significant amounts. Instead,
carbonate solutions can be used to precipitate Al(OH)3 (although NH3 is preferred).
[Edited on 27-7-2015 by blogfast25]
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Brain&Force
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Really easy test:
Run a current through the solution. Zinc plates out as the metal, aluminum makes a gummy gunk.
At the end of the day, simulating atoms doesn't beat working with the real things...
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j_sum1
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Glad to have provoked some discussion here.
@unionised -- Yes I did watch the video. But did not immediately review before posting. I was actually recalling in my mind Al behaviour with NaOH
in which it does redissolve with excess. My bad. In my defense I had been watching a gazillion YT vids and... well, details blur.
@zts16 -- Of course. If one has narrowed down to Al or Zn, then I think that is by far the easiest test.
[edit] [smacks forehead]
@brain and force -- Of course. If one has narrowed down to Al or Zn, then I think that is by far the easiest test.
(Thanks blogfast)
[Edited on 28-7-2015 by j_sum1]
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blogfast25
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| Quote: Originally posted by j_sum1 |
@zts16 -- Of course. If one has narrowed down to Al or Zn, then I think that is by far the easiest test. |
I didn't see zts16 comment on this thread?
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AJKOER
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Blogfast:
You may be right that a fresh precipitate of Al(OH)3 does not dissolve in boiling concentrated aqueous Na2CO3.
Honestly, I am not completely sure, but please explain how Aluminum metal (naturally coated with Al2O3, certainly less reactive than Al(OH)3 ) manages
to entirely dissolve, nevertheless, in boiling concentrated Na2CO3 (which I have performed on several occasions in a microwave) and previously
reported on SM as a path to sodium aluminate.
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blogfast25
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AJ:
Firstly, you're talking small amounts of Al with CONCENTRATED, BOILING Na2CO3. Chances are that on cooling Al(OH)3 dropped out anyway. Carbonate
solutions are ORDERS of MAGNITUDE less alkaline than strong (even dilute) alkali (NaOH e.g.). Aluminate is only stable at very high
pH, although at say 11 very limited amounts of Al(OH)3 can stay in solution.
In the context of what is being discussed here, you're peddling misinformation.
If you still believe Al(OH)3 can be dissolved in boiling, concentrated Na2CO3, then prove it but I'll remind you that once not so long ago you
believed NH3 can do that too.
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