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Author: Subject: Making Al2(SO4)3 on Monday
daragh8008
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smile.gif posted on 18-9-2009 at 09:45
Making Al2(SO4)3 on Monday


OK Folks I want to make a few grams of Aluminium sulphate. To hand I have foil and lump Al, NaOH, H2SO4 and um... water. I was thinking to start with ..

Barrier Oxide
Al2O3 + 2NaOH + 3H2O -> Na+ + [Al(OH)4]-

Metal
2Al + 6H2O -> 2Al(OH)3 + 3H2

2Al(OH)3 + 2NaOH + 3H2O -> Na+ + [Al(OH)4]-

Neutralise soln with H2SO4 to precipitate Al(OH)3 Filter, wash and dry over H2SO4?

Next Add Al(OH)3 to H2SO4 to yield the sulphate

2 Al(OH)3 + 3 H2SO4 -> Al2(SO4)3ยท6H2O

Any suggestions? Will it work. Should one add something like ammonium chloride to the Initial NaOH soln to reduce the Na content in the precipitate of the hydroxide. Will neutralising the soln produce a precipitate? Also in the final step do I have to use 98% H2SO4 or will a more dilute soln work. If so how would I precipitate the Al2(SO4)3 at the end? Can Al2(SO4)3 be recrystalised from water with out hydrolyzing? As you can see many questions. Anyone one got any thoughts?
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woelen
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[*] posted on 18-9-2009 at 11:17


I expect that Al-metal directly dissolves in dilute H2SO4, giving aluminium sulfate and water. But there may be side reactions, in which sulfate is reduced to lower compounds of sulphur.

I personally would go the following route:
- Dissolve the metal in dilute H2SO4.
- If also sulphur and sulfites are formed, then you need to filter and then reoxidize the liquid. This can be done by adding hydrogen peroxide to the filtered liquid and boiling for a while.
- Add excess ammonia. This precipitates Al(OH)3, but now there is no sodium contamination.
- Rinse a few times with a little distilled water.
- Dissolve in dilute H2SO4. Use slight excess of Al(OH)3 and filter the clear solution. If you use excess H2SO4 then it will be difficult to get pure Al2(S)4)3.
- Let the liquid evaporate. Hydrated Al2(SO4)3 can be crystallized from this liquid without hydrolysis.

The preparation will be more easy if you add potassium ions as well, then you get the more easily crystallized KAl(SO4)2.12H2O.

I expect that if you take the NaOH-route, then you will have more difficulty getting rid of all sodium ions.

[Edited on 18-9-09 by woelen]




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JohnWW
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[*] posted on 18-9-2009 at 11:35


There is not much point in doing that, because technical-grade hydrous aluminium sulfate, which I think is Al2(SO4)3.18H2O, is made and sold in large commercial quantities as the "alum" used in water purification, to remove colloidal substances as a flocculent precipitate. It is probably not made from the metal, but instead more likely by reaction with concentrated H2SO4 from part of the hydrous alumina precipitated in the refining of raw bauxite. (Most of the alumina is used for aluminium smelting including here in New Zealand near Invercargill, after being refined at Gladstone, Queensland, and some of it is also used as a polishing material and in some specialized glazes and glasses).
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bilcksneatff
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[*] posted on 18-9-2009 at 12:04


Quote: Originally posted by woelen  
I expect that Al-metal directly dissolves in dilute H2SO4, giving aluminium sulfate and water. But there may be side reactions, in which sulfate is reduced to lower compounds of sulphur.


Woelen, I just tried dissolving some Al foil in dilute (20%) H2SO4. The acid was still warm because I just diluted some conc H2SO4. After about 10 mins, only one tiny little bubble (most likey hydrogen) had formed on the foil. I'll let it sit overnight, but it looks like it will be really slow.

I know that HCl can dissolve aluminum though. Then you could follow that with your ammonia idea and then react the aluminum hydroxide with dilute H2SO4 (or convert it to potassium alum like you said).
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entropy51
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[*] posted on 18-9-2009 at 12:31


Not exactly your compound, but you can dissolve Al in KOH, then add H2SO4 to form potassium alum KAl(SO4)2, which crystallizes nicely with 12 H2O. This sounds very close to your original procedure.

2Al + 2KOH +6H2O -> 2KAl(OH)4 + 3H2

2KAl(OH)4 + H2SO4 -> 2Al(OH)3 + K2SO4 + 2H2O

2Al(OH)3 + 3H2SO4 -> Al2(SO4)3 + 6H2O

(K)+ + (Al)+++ +2(SO4)-- + 12H2O -> KAl(SO4)2*12H2O

Filtering off the Al(OH)3 to remove K2SO4 and treating with H2SO4 would appear to give Al(SO4)3.

[Edited on 18-9-2009 by entropy51]
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daragh8008
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[*] posted on 19-9-2009 at 05:19


Thanks folks for the replies. It seems from the suggestions that at which ever route one goes, at some stage I'll be dealing with the hydroxide and converting that to the sulphate. I was hoping someone would suggest to just duck the Al into H2SO4, because that is what I hoped would work!! But alas I feared that that would just be too easy. Something about all the talk of the great corrosion resistance of the oxide barrier put me off the idea. I read on wikipedia that when the hydroxide is precipitated it forms a gel type substance which might be a problem to filter, trapping a bucket load of NaOH in the process. Thank fully I have anhydrous EtOH to hand to (as suggested on wikipedia) Would that remove the Sodium or at least reduce it to an acceptable level? Thanks again for all the useful advice.

One other thing a little off topic, has anyone ever made silicon compounds such a teos or the like. Again unfortunately starting off with the basics silicon wafer a couple of acids bases solvents etc. I would prefer not to have to buy something expensive that after trying once may not work for the application that I'm attempting, so where possible I try make a test quantity first (1-2g). I suspect that this one is probably a little to challenging for a newbie like me though.
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bilcksneatff
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[*] posted on 19-9-2009 at 06:22


I would just use ammonia instead of sodium hydroxide (like woelen mentioned), then you have no sodium contamination at at. Either that, or use KOH to get potassium alum.
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daragh8008
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[*] posted on 3-11-2010 at 03:45


Sorry to bring this up again after such a long time. This time I was wondering if Aluminium sulphate could be made by electrolysis. In a similar fashion to coppersulpate. As in Al anode and cathode and a fairly conc soln of H2So4. Any ideas
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hissingnoise
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[*] posted on 3-11-2010 at 05:51


Hot, conc. H2SO4 attacks Al metal;
2Al + 6H2SO4 --->Al2(SO4)3 + 3SO2 + 6H2O
The reaction, as can be seen, is wasteful of acid but if no other means are available and you have a surfeit of acid - try it!
I doubt that the sulphate could be recovered by electrolysis . . .




[Edited on 3-11-2010 by hissingnoise]
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daragh8008
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[*] posted on 4-11-2010 at 04:07


OK so I realise that most people would just buy the stuff, here is what I ended up doing. Firstly I took Al foil and digested it in 37% HCl. Kept adding foil until it stopped dissolving. This mix was a bit of a mess so I filtered it a few times. Resulting in a pale yellow liquid. To this I added an excess of 98% H2SO4 ( based on the Al content) The solution was then heated at 180C, the soln turned from pale yellow into a deep yellow and eventually in to a smooth gel. The heating was increase to 250C and continued until the gel turned into a white powder and H2SO4 fumes stopped evolving. This powder was redissolved in hot water a filtered again. A couple more drops of H2SO4 and the whole lot heated until it was a crunchy white powder. Now to test the product to see if it what I hope it is? Thanks All
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