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Author: Subject: Aluminium and NaOH: what's the gray stuff?
ave369
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[*] posted on 21-3-2023 at 09:25
Aluminium and NaOH: what's the gray stuff?


Today I was experimenting with a hydrogen flame torch, and made hydrogen by reacting aluminium and sodium hydroxide. Then I decided to examine the products closely: it was a gray precipitate. I filtered out the gray precipitate using a Buchner set and tested the resulting clear solution with hydrochloric acid. The result was a gel of Al(OH)3. The clear solution is, clearly, a solution of sodium hydroxoaluminate. Then what's the gray stuff?

I tried to dissolve the gray stuff in hydrochloric acid. It failed to dissolve clearly and produced a yellowish milky suspension. When the suspension separated, the precipitate turned white. So I guess it's not Al(OH)3. Then what it is?



[Edited on 21-3-2023 by ave369]




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[*] posted on 21-3-2023 at 10:34


I generally assume is silicon impurities in the aluminum, especially if you're using foil.



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[*] posted on 21-3-2023 at 21:53


It was wire, cut into short pieces.



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[*] posted on 22-3-2023 at 00:23


Silicon? Or carbon? Many metals and alloys contain little amounts of carbon. Even tiny amounts of carbon produce grey impurities, when the metal or alloy is dissolved. I myself once bough iron, which was said to be very pure (99.9% or so), but when I dissolved this in acid, then the solution also went grey and remained turbid. It took a very long time before the grey material settled. Maybe you have a similar issue with your aluminium.



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[*] posted on 22-3-2023 at 01:43


If you used a kitchen alluminium foil as a source of the metal be aware that it has a mica-like "shiny" layer which after dissolving Al forms a grey precipitate.

[Edited on 22-3-2023 by teodor]
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[*] posted on 22-3-2023 at 02:00


Quote: Originally posted by teodor  
If you used a kitchen alluminium foil as a source of the metal be aware that it has a mica-like "shiny" layer which after dissolving Al forms a grey precipitate.

[Edited on 22-3-2023 by teodor]


I have read that in the Czech Republic,and it may be elsewhere in the world,aluminium foil is coated with a thin layer of enamel,at least on one side.I don't know if this is true or just a legend. Anyway,a friend of mine worked in a factory that produced this foil,so I asked her to bring me the chemical composition of the foil from work and I was amazed at how many elements were listed.Of course silicon,iron,but also manganese and other metals,it was varied...I guess carbon was on the list too...



[Edited on 22-3-2023 by Admagistr]
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[*] posted on 22-3-2023 at 03:38


There is another thread on this topic here:
http://www.sciencemadness.org/talk/viewthread.php?tid=16337


I think, as others have said, it is mostly silicon?
I actually thought initially it was silica, I.e. SiO2, but it seems like maybe it is just elemental silicon...

I once dissolved aluminium foil in hydrochloric acid, and a similar grey precipitate remained undissolved.
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[*] posted on 22-3-2023 at 06:29


According to this: https://cometmetals.com/aluminum-foils-supplier/alloy-1235-a... , alloy 1235 with 0,65% silicon is the most common alloy for foil. Carbon is pretty specific to iron alloys.




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[*] posted on 23-3-2023 at 17:11


I have also observed the emergence of a non-reactive gray precipitate while trying to produce aluminum salts. Seemed like more than 0.65%, but maybe not.

It's worth noting that any unreacted aluminum powdered may also appear a very dark gray color.

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[*] posted on 24-3-2023 at 07:51
Powdered Aluminum


Pyro grade aluminum powder especially the dark, German and Indian, contain some
carbon because of the production process. One method is to glue very thin strips
of paper to strips of aluminum. Once dry a machine stamps it very hard to cause
"tearing" of the foil into tiny particles. This is then charred in an inert atmosphere.
Ball milling is the final step. So expect some carbon in these aluminum powders.




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[*] posted on 24-3-2023 at 10:47


Quote: Originally posted by Fulmen  
According to this: https://cometmetals.com/aluminum-foils-supplier/alloy-1235-a... , alloy 1235 with 0,65% silicon is the most common alloy for foil. Carbon is pretty specific to iron alloys.


I found silicon when I acid-dissolved quite a bit of foil. Of course that would dissolve by alkali dissolution.

[Edited on 24-3-2023 by blogfast25]




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