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cjtune or William: What does vinegar do to Aluminum oxide?

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CalCoolage

Registered
Joined
Jun 30, 2001
Or anyone. What does vinegar do to Aluminum oxide and aluminum?

Tiger reports it bubbling in contact with a HS. What gas is it giving off? What is the
aluminum or Al O being converted to?
 

Tiger

Member
Joined
Apr 6, 2001
Location
UK
Well vinegar is a weak organic acid (acetic acid). Vinegar contains approximately 3%. I used a dilution of 5ml to 3l of water which is 0.005%. The chemical reaction would form Aluminium acetate and water. Two Hydroxyl ions (strong base) of the Aluminium oxide (which converts to Aluminium hydroxide) react with the hydrogen radical of the acid because it is weakly bound, to form H+ OH- ,or water, leaving the 2 acetate radicals(weaker base), which are negatively charged to react with the Al++.
Because acetic acid is a weak acid and also because it is in a very diluted form it generally will not attack the metal. A stronger acid such as a non organic based one like Sulphuric would react to form hydrogen and Aluminium sulphate.
The gas that I saw could have been small particles of Aluminium reacting with the acid or more likely air trapped within the aluminium hydroxide lattuce. Aluminium acetate is water soluble while the hydroxide is not (a commonly used antacid like 'Milk of Magnesia')
I am a pharmacist by profession.
 

Kingslayer

Senior Member
Joined
Jun 12, 2001
Location
Port Charlotte, Florida
Tiger (Jul 31, 2001 02:52 p.m.):
Well vinegar is a weak organic acid (acetic acid). Vinegar contains approximately 3%. I used a dilution of 5ml to 3l of water which is 0.005%. The chemical reaction would form Aluminium acetate and water. Two Hydroxyl ions (strong base) of the Aluminium oxide (which converts to Aluminium hydroxide) react with the hydrogen radical of the acid because it is weakly bound, to form H+ OH- ,or water, leaving the 2 acetate radicals(weaker base), which are negatively charged to react with the Al++.
Because acetic acid is a weak acid and also because it is in a very diluted form it generally will not attack the metal. A stronger acid such as a non organic based one like Sulphuric would react to form hydrogen and Aluminium sulphate.
I am a pharmacist by profession.

Is it just me, or was everything between "Well vinegar is a weak organic acid" and "I'm a pharmacist by profession" a complete blur?
 

Shadow ÒÓ

Mod in Hiding
Joined
Dec 20, 2000
Location
Pensacola, Florida USA
Kingslayer (Jul 31, 2001 02:58 p.m.):
Tiger (Jul 31, 2001 02:52 p.m.):
Well vinegar is a weak organic acid (acetic acid). Vinegar contains approximately 3%. I used a dilution of 5ml to 3l of water which is 0.005%. The chemical reaction would form Aluminium acetate and water. Two Hydroxyl ions (strong base) of the Aluminium oxide (which converts to Aluminium hydroxide) react with the hydrogen radical of the acid because it is weakly bound, to form H+ OH- ,or water, leaving the 2 acetate radicals(weaker base), which are negatively charged to react with the Al++.
Because acetic acid is a weak acid and also because it is in a very diluted form it generally will not attack the metal. A stronger acid such as a non organic based one like Sulphuric would react to form hydrogen and Aluminium sulphate.
I am a pharmacist by profession.

Is it just me, or was everything between "Well vinegar is a weak organic acid" and "I'm a pharmacist by profession" a complete blur?

I'm still going through my dictionary........what would it do again? =)
 
W

William

Guest
well I understood that. Yeah, thats what would happen, but it was bubbling? You had something else on it then. Shouldn't be any gases going off.
 

Fink

Member
Joined
Dec 18, 2000
Well, don't know if I agree with the radical idea for formation of Al(OAc)2. I was thinking more protonation of AlO followed by displacement with an acetate ion to generate Al(OH)(OAc). Again, protonation of the hydroxyl and displacement would then generate the water soluble Al(OAc)2 and two water molecules. I suspect the bubbling is most likely from a redox reaction of micronized Al metal, forming either Al(OAc)2 or Al(OH)2 and H2 via a radical reaction. Just my best guess for the chemistry.
Just my $0.02.
 
W

William

Guest
need I call my chemistry teacher, I think so! Will post the results in a little bit.
 

schmidty

Registered
Joined
Dec 17, 2000
Location
Portland, OR
I'm willing to guess that the off gassing was O2 being formed (from the other reaction). But, really should that weak of an acid be eating away at a heat sink?

Should I be more careful next time I'm using 3M Acetic acid at school? ;-)
 

Richard

Senior Member
Joined
Jan 15, 2001
The guy's a pharmacist. I have a feeling he took more chemistry classes than your teacher did.

William (Jul 31, 2001 06:11 p.m.):
need I call my chemistry teacher, I think so! Will post the results in a little bit.
 
W

William

Guest
Richard999 (Jul 31, 2001 10:15 p.m.):
The guy's a pharmacist. I have a feeling he took more chemistry classes than your teacher did.

William (Jul 31, 2001 06:11 p.m.):
need I call my chemistry teacher, I think so! Will post the results in a little bit.

my chem teacher has a PhD in organic chemistry, I don't go to an ordinary highschool.
 

Richard

Senior Member
Joined
Jan 15, 2001
Nothing against your chem teacher, but I just found is kind of a slap in the face the way people were discounting Tiger's answer. A pharmacy degree is not something that is just handed out you know.
 
W

William

Guest
Richard999 (Jul 31, 2001 10:25 p.m.):
Nothing against your chem teacher, but I just found is kind of a slap in the face the way people were discounting Tiger's answer. A pharmacy degree is not something that is just handed out you know.

I should know, my mom's one! I think he is right, but not too sure. My chem teacher will either give me the answer or give me a whatever depending on her mood. ;D
 
W

William

Guest
schmidty (Jul 31, 2001 06:32 p.m.):
I'm willing to guess that the off gassing was O2 being formed (from the other reaction). But, really should that weak of an acid be eating away at a heat sink?

Should I be more careful next time I'm using 3M Acetic acid at school? ;-)

its not going to be oxygen for sure. No, that acid shouldn't be eating away at it and especially forming air bubles. If anything, the gas would be hydrogen.
 
OP
C

CalCoolage

Registered
Joined
Jun 30, 2001
Fink (Jul 31, 2001 11:03 p.m.):
It is not O2.

I have a Ph.D. in organic chemistry.

All I had was high school chemistry which I now barely recall. I do recall that it was kind of interesting because it touched somewhat on reality, unlike everything else in high school. Anyway, I expected the answer to be that nothing much happens; the equilibrium would be such that small traces of the obvious compounds would form, but that would take a long time.

Although my knowledge of chemistry is very basic, the stability of aluminum oxide is such that getting it to react much is a well known story in the history of chemistry. Plus, water hardly dissolves it at all, so getting radicals to form that way gets you almost nowhere. Early chemists deduced the existence of aluminum in compounds, but the elaborate steps needed to free the metal made it far more expensive than gold. Finding that vinegar readily reacts with aluminum oxide is a surprise to say the least.

Vinegar being an acid, it might attack the aluminum, but because of aluminum's affinity for oxygen it would seem anything that got started that way would end up with the aluminum combining with the oxygen in the air instead. In the process some aluminum might be dissolved from the surface.

Still, real chemistry often proceeds differently than high school theory. My chemistry teacher had a doctors degree and frequently informed us that what we were learning was NOT real chemistry, which was ORGANIC chemistry. One day he became so exasperated with the text book that he decided to demolish the theory with a demonstration. He mixed the usual base and acid together and did not get the chemical salt and water that was predicted. You could tell that because the stuff was a different color than what you were supposed to get. Plus it gave off some noxious gas that the teacher wanted to keep us away from. (chlorine?)

So, I was really wondering what actually happens, rather than what might happen in theory.

If vinegar dissolves the oxide off aluminum, you could save yourself some work by doing that before you lap you HS, because aluminum oxide is very tough, while aluminum is not. Aluminum oxide is the second hardest naturally occurring substance, one tenth the hardness of diamond. Common sandpaper is usually aluminum oxide, although you sometimes see some with flint or garnet. Wet sandpaper is usually silicon carbide, which is nearly as tough as aluminum oxide, I believe.
 

cjtune

Member
Joined
Dec 18, 2000
Here's my 1% contribution to the discussion:

What I know from my Engineering Materials 101 is that aluminium oxide is resistant to pHs between 4 and 9 but is particularly susceptible to hydrochloric acid. But that's for pure aluminium, I think, and may not really apply to the hundreds of aluminium alloy types, especially for the 2xxx series (primary additive: copper) aluminium alloys. Low corrosion resistant alloys are often coated with pure aluminium itself and they are known as alclad aluminiums. Most likely the vinegar solution (weak acid), then, was in contact with pure or high-corrosion resistant alloy aluminium.

One more thing, water (and most liquids) often contains dissolved gases and they tend escape from the main body of the liquid and form bubbles on the surfaces -something (but not exactly) like coke in a glass. I'm not sure of this mechanism of bubble formation but I've observed bubbles forming on a sandpapered aluminium heatsink I left for a month in heavily salinated (stagnant) water without any obvious tarnishing or material erosion -a steel paper clip I also immersed as a control experiment was almost completely 'eaten' up and reduced to brown oxide powder and scum. Maybe it was just something natural and harmless you observed, CalCoolage. I'll check up more on the bubble formation thingy.
 
OP
C

CalCoolage

Registered
Joined
Jun 30, 2001
cjtune (Aug 01, 2001 10:39 a.m.):
I'm not sure of this mechanism of bubble formation but I've observed bubbles forming on a sandpapered aluminium heatsink I left for a month in heavily salinated (stagnant) water without any obvious tarnishing or material erosion -a steel paper clip I also immersed as a control experiment was almost completely 'eaten' up and reduced to brown oxide powder and scum. .

The iron/aluminum contrast is dramatic. How about trying this with vinegar?

Well, while washing the storm windows using vinegar in water, I have also washed the old bare aluminum frames, but I never saw any effect on the discoloration and pitting, which admittedly may have been unrealistic considering the following. They used to sell an aluminum cleaner that was primarily, I think, hydrochloric acid. You were supposed to use rubber gloves and goggles and all that BS. This stuff did cause some bubbling on the aluminum or oxide, although it got bubbles as soon as you poured it out of the container too. Unfortunately, after the frames dried, the aluminum looked about the same after you used the stuff as before.

Hydrochloric acid is easy to find, labeled as muriatic acid for cleaning brick. It might clean heat sinks. It is also dangerous to human flesh. (And if you ever get the urge, don't drink it, kids!)

Acid applied to metal does result in bubbling, I think. What are the bubbles? Hydrogen? Boiling water? Released oxygen from the layer of corrosion?

( I hate to write a message just containing "bump".)
 

Fink

Member
Joined
Dec 18, 2000
With HCl, the bubbles are most certainly hydrogen gas. Most of the oxygen contained in the aluminum oxide will be coverted to the hydroxide and remain bound to the aluminum atom or converted to water by protonation in the reaction with aqueous HCl. However, aqueous HCl will also go after aluminum metal very quickly in a reduciton-oxidation reaction, releasing hydrogen gas and various forms of Al(OH)x(Cl)y (x+y=3). I could write out the stoichometry, but it is not important