mrjeffy321
Hazard to Others
Posts: 149
Registered: 11-6-2005
Member Is Offline
|
|
Zinc / Aluminum Salt Crystals
My goal is to form a Zinc metal layer on an Aluminum substrate (Aluminum foil).
To do this I planned on employing the fact that Aluminum is a more reactive metal than Zinc (just barely) and suspending the Aluminum foil in a Zn+2
(aq) solution. The Aluminum should displace the Zinc ions from solution and in the process Zinc metal should plate out onto the surface of the
Aluminum foil.
I have attempted my experiment twice with poor results.
On my first try I used a 1 M solution of Zinc(II) Nitrate Hexahydrate and suspended a piece of Aluminum foil. Before submersion, I cleaned the
Aluminum foil thoroughly, which included a brief dip in a Sodium Hydroxide solution to remove the oxide layer on its surface. After over 1 day of
waiting, nothing much happened. At best, I can (maybe) detect a very faint hint of something which might be Zinc on the Aluminum foil near
where the water level was when it was submerged…other than this nothing happened.
On my second attempt I wanted to user a much more concentrated Zinc salt solution to see if I could get any better results. This time I prepared a 5
M Zn(NO3)2 * 6H2O solution and suspended a thoroughly cleaned piece of Aluminum foil in it.
After a few hours I noticed some crystals forming near the bottom of the beaker I had the Aluminum suspended in and on the Aluminum itself it seemed.
After a few more hours (and over night) the crystals continue to grow and are now quite large. The crystals are white and (when they were much
smaller) seemed to form a wide, flat, crystalline structure.
I do not know what these crystals are.
My first guess was that, perhaps, they were Zn(NO3)2 crystals precipitating out of solution. You see, I prepared my 5 M Zn(NO3)2 solution and it was
a little below 5 M so I slowly heat it and evaporated off a little of the water to get it to the right volume. I thought that maybe I evaporated too
much water and as it cooled some of the excess Zn(NO3)2 precipitated out. But I do not think this is the case because crystals only formed in the
beaker with the Aluminum, no crystals formed in my excess Zn(NO3)2 solution I prepared.
The crystals might be Aluminum Nitrate, maybe, but Al(NO3)3 is quite soluble too (although not as much as Zn(NO3)2).
Maybe the single replacement reaction occurred, somewhat, and the presence of the Al(NO3)3 caused the already concentrated Zn(NO3)2 to precipitate
out.
Or maybe the Zinc and Aluminum ions are forming some type of double cation Nitrate salt. I don’t know.
I see no evidence of any Zinc metal plating out on to the surface of the Aluminum foil, the only evidence I see of a reaction is the white crystals.
As of right now, I still have the Aluminum suspended in the Zn(NO3)2 solution and, in theory, the crystals are still growing. I plan on do some quick
experiments soon to try and determine just what those crystals are.
Do you have any idea what is causing the crystal formation and why Zinc metal is not plating out onto the Aluminum?
I guess I could try to drive the Zinc plating reaction forward with an electric current, but I should not need to…the reaction should be spontaneous
already.
|
|
|
Mr. Wizard
International Hazard
Posts: 1042
Registered: 30-3-2003
Member Is Offline
Mood: No Mood
|
|
Doesn't the oxide layer form on the aluminum immediately? It is so strong and thick that it will stop any reaction that depends on a clean surface
to work. Electrolytic capacitors take advantage of it to form layers of oxide that will withstand hundreds of volts. Even Nitric Acid is not
supposed to attack aluminum because of this quickly formed oxide layer.
Is this zinc layer to be used for something or just to demonstrate it can be done?
|
|
|
mrjeffy321
Hazard to Others
Posts: 149
Registered: 11-6-2005
Member Is Offline
|
|
It sounds like you are thinking along the lines of one of my original concerns.
Aluminum is a little strange in how it behaves due to that Oxide layer it forms, making it resistant to reactions which one would ordinarily expect to
occur (for example, with Nitric acid).
I was originally somewhat concerned that (even if I obtained a fresh Aluminum layer) the Aluminum Nitrate would somehow impede the reaction. But I
really do not have much of a choice when it comes to metals that can displace Zinc out of solution. I guess I could use Magnesium (and I have tried
but I was not getting the quality of results I would have liked with my Mg ribbon), but after that the metals begin to react violently with the water
itself as opposed to just the Zn+2 ions in solution.
What I did was clean the Aluminum foil with DI water, then with acetone, then with DI water again. I then dipped the foil into a NaOH solution for a
few seconds until I saw a healthy number of bubbles form. I then lifted the Aluminum up into the air atmosphere, give it a quick rinse with DI water
and then immediately placed it into the Zn(NO3)2 solution.
There was very little time for an Oxide layer to form on the Aluminum, not to say that it did not occur.
Yes, I have a very specific use for the Zinc in mind.
It actually does not matter a great deal to me what the Zinc layer forms on, but I thought using Aluminum would be easy.
|
|
|
mrjeffy321
Hazard to Others
Posts: 149
Registered: 11-6-2005
Member Is Offline
|
|
If this helps,
Here is a rather poor picture of the crystals forming. If you look very closely you can see the crystal structure…at least you can get a sense of
the color from the picture.
The crystals have grown so much so that the Aluminum foil is trapped by the crystals and does not easily lift out.
http://i62.photobucket.com/albums/h104/mrjeffy321/Aluminum_a...
|
|
|
Eclectic
National Hazard
Posts: 899
Registered: 14-11-2004
Member Is Offline
Mood: Obsessive
|
|
Zincating Aluminum
|
|
|
mrjeffy321
Hazard to Others
Posts: 149
Registered: 11-6-2005
Member Is Offline
|
|
Of course…..
I remember a process which is designed to turn a Copper penny into a “gold” penny by depositing Zinc onto the penny and then using heat to turn it
into brass (which looks gold).
What I did was place some Zinc granules into a warm solution of Sodium Hydroxide, and then I placed in a piece of Copper. Not to long thereafter
(only a minute or two), one can easily see a color change on the Copper strip due to the Zn metal being deposited.
I think this will work very nicely for my need to get a thin Zinc metal coating on something.
But that still leaves to question the reason why the Aluminum would not work and what those crystals were.
The electronegativities of Zinc and Aluminum are VERY close together, so maybe this will explain why the Zn depositing does not appear to proceed on
the Aluminum. Although Al is technically above Zn on the activity series….Aluminum is only barely above Zinc.
As for the crystals, my best guess would be that they are Zinc Nitrate which has precipitated out of solution. Perhaps more water than I thought
evaporated from the beaker, thus causing the Zn(NO3)2 to precipitate out, but when the crystal formed they displaced more water than the ions took up
while in solution so it appeared as if the water level stayed the same.
|
|
|
12AX7
Post Harlot
Posts: 4803
Registered: 8-3-2005
Location: oscillating
Member Is Offline
Mood: informative
|
|
Well, it's obviously a salt. 5M isn't very strong as zinc nitrate goes (unless wikipedia is right, which doesn't seem likely for a nitrate!). How
much has your metal plate changed? Likely, zinc metal will grow as dendritic crystals, unless you have the surfactants and levelers to make a smooth
deposit- which, even then, is iffy on aluminum's sort-of-metal surface.
Tim
|
|
|
Eclectic
National Hazard
Posts: 899
Registered: 14-11-2004
Member Is Offline
Mood: Obsessive
|
|
Zincating is done in alkaline solution to keep the Al2O3 dissolved until a uniform zinc coating forms. You would need to amalgamate the aluminum to
have it work reliably in acidic conditions to reduce Zn++. Even then, it would reduce the NO3- to NH3, grabbing oxygen whenever possible.
|
|
|
chemoleo
Biochemicus Energeticus
Posts: 3005
Registered: 23-7-2003
Location: England Germany
Member Is Offline
Mood: crystalline
|
|
To be honest, this looks to me like an over-saturated solution. Just allow your other 5M stock to stand, on air... I bet you'll have the same thing
happening.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
|
|
|
mrjeffy321
Hazard to Others
Posts: 149
Registered: 11-6-2005
Member Is Offline
|
|
I think the solubility of Zinc(II) Nitrate Hexahydrate (which is what I am using) is something like 200 g per 100 cm^3 of water, which would come out
to a molarity of about well over 6 M. So my 5 M solution was a decent ways below the saturation point. And Zinc Nitrate is hygroscopic, so I would
not expect its solution to loose so much water so readily to cause all those crystals to form (the crystals started forming after only a few hours or
less).
Zinc Nitrate solutions in water should naturally have an acidic pH since Zinc Hydroxide is insoluble in water. I did not want to raise to pH any
since I did not want to cause any Zn(OH)2 to precipitate out…which is really easy to do with only a few drops of NaOH solution.
I will try taking some of my left over Zn(NO3)2 solution and leave it uncovered for a day to see what happens.
And if nothing significant changed with the Zn(NO3)2 solution I already have the Aluminum sitting in by tomorrow I will add some Ammonia to raise the
pH.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
