Sciencemadness Discussion Board - Dissolution of aluminum in weak acids

Dissolution of aluminum in weak acids

Random - 1-1-2014 at 17:41

As far as I have known aluminum didn't dissolve in citric acid, but I wanted to see if I could form HCl from citric acid and calcium chloride since calcium citrate is insoluble.

So I mixed equal amounts of concentrated solutions and no precipitate formed. I thought, lets add aluminum to see if it's gonna react. No reaction was observed but I left the test tube. After few days I saw little Al dust in the solution along with the main piece of foil. Now after like two weeks it noticeably corroded and there was gas evolved under aluminum. Some gas bubbles were apparent and very small amount of white precipitate was at the bottom.

Now is this white precipitate calcium citrate or aluminum salt?
Does aluminum also dissolve in citric acid alone slowly?
If not, does the calcium citrate solubility drive the equilibrium to slightly favor HCl formation, but it proceeds very slowly because citric acid has a hard time pronating the water (one of the reasons why is a weak acid)?
Do chloride species have any role in this?
If yes, does a calcium chloride only work since it can drive equilibrium to favor calcium citrate precipitation and thus also favor HCl formation which NaCl couldn't do since sodium citrate is soluble?

macckone - 1-1-2014 at 18:49

Aluminum in salt solutions will slowly corrode to
a white precipitate of aluminum oxide. Lowering the
pH will increase the rate. Of course the oxide coating
must be breached first.

There are a lot of articles on corrosion of aluminum.
The following one describes the same experiment with
sodium chloride.
http://faculty.ksu.edu.sa/ljuhaiman/publication/corrosion%20...

Random - 2-1-2014 at 09:35

Thanks for explanation, I'll check the document.

AJKOER - 3-1-2014 at 12:16

A few comments:

Aluminum will fairly well dissolve in aqueous Na2CO3 upon heating. This is the cause of significant corrosion to Aluminum pans and the like placed in a disk washer. Never tried to repeat this experiment using photolysis in place of heating. But, if it proceeds, I would try repeating replacing the Na2CO3 with Citric acid. It would also be interesting if dilute aqueous Citric acid attacks Al on heating. Adding a significant amount of NaCl, making the solution more ionic, may raise the activity level of the Citric acid and the reaction may then proceed on heating.

Now, Al foil may contain Fe (living organisms may also contain Fe, Cu,.., as does some cookware). This is possibly significant as than, in the presence of an electrolyte (like aqueous NaCl), an electrochemical reaction (actually a battery cell) may ensue. Such reactions with Al definitely proceed in aqueous NaCl in the presence of Copper metal and a compound capable of a REDOX reaction (like Na2CO3, NaOCl, HOCl, NaClO3, Acetic acid resulting in CHCl3 see http://www.sciencemadness.org/talk/viewthread.php?tid=27530#... ,.., and my commentary on a so-called 'Bleach battery' at http://www.sciencemadness.org/talk/viewthread.php?tid=24318 and in other in threads).

As such electrochemical reactions readily proceed in even very dilute concentrations (see references I provided previously on these battery cells), to ignore electrochemical induced reactions is probably gross negligence, in my opinion, as these could quite efficiency create Aluminum compounds (like Aluminium chlorohydrate please see the discussion at http://en.wikipedia.org/wiki/Aluminum_chlorohydrate linked to breast cancer...), which in even low doses, may produce serious health concerns.

However, if you are working for an Aluminum foil manufacture or a consulting firms, and have been asked to publish/report 'comforting' experimental results, proceed as you are currently.

[Edited on 3-1-2014 by AJKOER]

Zyklon-A - 3-1-2014 at 12:26

How about boric acid(aq), would Al form H3AlBO3 or something?

blogfast25 - 3-1-2014 at 13:36

Quote: Originally posted by Zyklonb

How about boric acid(aq), would Al form H3AlBO3 or something?

The term 'boric acid' is a bit oxymoronic. Its first pKa is about 9, that makes it about 100,000 times weaker [Bronsted definition] than acetic acid!

[Edited on 3-1-2014 by blogfast25]

Zyklon-A - 3-1-2014 at 14:22

Dang, that is very weak.

bismuthate - 3-1-2014 at 14:50

Well it would most likely react if the oxide coating was removed.

[Edited on 3-1-2014 by bismuthate]

DraconicAcid - 3-1-2014 at 15:14

Is aluminum borate very soluble? I wouldn't expect so, so I would also expect that removing the oxide coating from aluminum in a solution of boric acid would result in a coating of aluminum borate.

bismuthate - 3-1-2014 at 15:45

It is insoluble. However I wonder if by using an Ga or Hg/Al almalgam it would react fully.

blogfast25 - 4-1-2014 at 06:56

Quote: Originally posted by bismuthate

It is insoluble. However I wonder if by using an Ga or Hg/Al almalgam it would react fully.

That pKa value of 9 was for the first deprotonation. Imagine how weak the second and third protonation steps must be!

I'm no expert but imagine that most borates are derivatives of borax, not obtained from boric acid.

[Edited on 4-1-2014 by blogfast25]

Random - 4-1-2014 at 08:56

Boric acid and B2O3 is extremly shady chemistry as i remember dehydrating boric acid will almost never yield pure boron oxide but a sticky polymerised mixture of them. Thats what I remember from golden book of chem experiments.

Best way to make borates is to mix soluble metal salt and sodium borate.

[Edited on 4-1-2014 by Random]

blogfast25 - 4-1-2014 at 10:24

Quote: Originally posted by Random

Boric acid and B2O3 is extremly shady chemistry as i remember dehydrating boric acid will almost never yield pure boron oxide but a sticky polymerised mixture of them. Thats what I remember from golden book of chem experiments.
[Edited on 4-1-2014 by Random]

I doubt if the GBCE is much of an authority on this. Pyrolysis at > 300 C (Wiki) with perhaps further calcination of pure boric acid should get rid of the last traces of water and yield quite a pure B2O3.

[Edited on 4-1-2014 by blogfast25]

Zyklon-A - 4-1-2014 at 11:07

I thought so to but when I tried it, about a week ago, it just gave me a mess like Random said, I heated it up >400C.

blogfast25 - 4-1-2014 at 11:29

I've done this myself. You obtain a hard, glassy, amorphous material that tends to stick to crucibles like mad (at least ceramic ones) and is fairly hard to grind down (a white powder when ground down). But don't let appearances deceive you: from pure B(OH)3 you'll get quite pure B2O3, no matter how 'messy' it looks.

[Edited on 4-1-2014 by blogfast25]