Cecil is mistaken, coke does not disolve aluminum

Cecil's Columns/Staff Reports
1

http://www.straightdope.com/columns/read/1158/why-doesnt-coke-dissolve-the-can

The column either says or implies that Coke will dissolve aluminum. Whether Coke lines their cans with food grade plastic is irrelevant to this question. When I read this load of crap this morning, I immediately obtained a bottle of genuine Coca-Cola, lined a bowl with Reynolds brand aluminum foil and filled the bowl. Upon return from my days burdens, the bowl lined with aluminum and filled with Coke was still there. The bowl was dry, and upon draining and inspection, there wasn’t the slightest damage to the aluminum foil.

I can also say that in the past I have deposited various metal items, nail, galvanized nail and penny into a can of Coke. Nothing that I recall. Might be interesting to retry with a very shiny penny though.

2

(Caveat: it’s been years since I took Chemistry.)

I think that this column from 1982 is the original article that the writer is referring to, and here is the relevant paragraph:

First off, Coca Cola has a pH around 2.6 or so, and it contains small amounts of citric and phosphoric acids, along with carbonic acid from the dissolved CO[sub]2[/sub]. All the same, Coke is pretty weak stuff, and I don’t think that the acids mentioned have an affinity for aluminum oxide or aluminum (Personally, I’d use hydrofluoric acid for that, and you won’t see much of it in any food product.)

More to the point, Cecil used a fair amount of poetic license in the 1970s and 80s. I am sure that the comment about Coke being able to rot out the hull of a ship was meant entirely as entertainment and wasn’t meant to be taken seriously.

3

The writer linked to this column from 1997 which appears to be the intended reference.

I don’t know if it was poetic license or not but it was 1997 and Cecil wrote: “This prevents the highly corrosive contents from dissolving the containers. The unanswered question is, what keeps them from dissolving you?”

4

As far as dissolving you goes, since stomach acid has a pH value of 1.4 and Coke is only 2.6, the main effect of drinking Coke, dissolvant-wise, is to dilute your stomach acid.

5

If I remember correctly, aluminium will oxidise in air fairly readily. So putting aluminium foil in coke isn’t a fair test. The foil is coated in a thin but pretty complete lair of aluminium oxide, a much more robust chemical.

6

Yes, aluminum builds a very thin, very strong passivation layer. However if coke indeed cannot effectively corrode the oxide away, why add the plastic liner in the first place?

7

IME, vinegar–which has a pH similar to Coke–will dissolve aluminum foil. I’ve placed foil-covered bowls of leftovers that contain vinegar in the fridge and the next day found the foil full of holes where it had been in contact with the food.

8

Fair question. I was thinking the same thing.

My guess would be because the oxide molecules have relatively low attraction to each other and will flake off like a powder if agitated. The Plastic isn’t to easy to disturb.

9

I thought a soapy layer of mucus kept your stomach safe from acid. That would explain why acid reflux hurts. The acid burns your esophagus because it lacks the mucus shield.

10

I think you’re…completely correct, actually. I used to spend some time with aluminum anodizing (a paintball thing, mostly) and while the alumina is tough as, ah, nails (:D) it’s also very brittle. Plastic coating makes perfect sense for this application. Ignorance fought, thank you!

There’s got to be more to it than simple pH, though. Coke’s a lot more acidic than, say, tomato sauce, but the latter will eat through aluminum foil in short order.

11

At last, a use for the penny.
Guess you can tell, I’m with the “ditch the penny” crowd.
But I’ve tried to disolve most metals, and many other materials, in Coke and Pepsi and Dr Pepper and even Squirt. Boringly nothing.
I once left the penny in the Coke fore a week, hoping for something to happen. Nothing. I even tasted the Coke. It was flat, but that’s all. No coppery taste.
I gave up on chemistry as way too boring and joined the Navy. :wink:
Peace,
mangeorge

12

Aren’t all foodstuff cans lined with plastic, or is it just steel?
I’m going to try vinegar (acetic acid) on aluminum.

13

Here’s a thought, which I haven’t had time to follow up on.

You say Coke is more acidic than tomato sauce. But, …is coke, opened and left for hours, still as acidic as tomato sauce?

14

Coke evaporates away, tomato sauce doesn’t. At least not quickly. It bonds with (sticks to) the foil. Maybe something in the sauce (anaerobic bacteria?) gobbles up the aluminum molecules.

15

This is a good question- most of the acid in Coke is carbonic, which can escape into the atmosphere as CO[sub]2[/sub]. Flat Coke should be less acidic than fresh Coke.

Anyone feel like a titration?

16

But this shouldn’t effect the action of Coke upon aluminum left in a closed container.

17

How airtight is the “closed container?”

18

Airtight enough to make a Pfffttt sound when I opened it.

19

You’ve already had the answer to why Coke in a can doesn’t dissolve the aluminum. It’s the lining.

I was asking how the pH of Coke changes from the instant that it comes out of a freshly opened can, to 1 hour later, 2 hours later, a day later, etc.

My guess is that it rapidly goes neutral as far a pH is concerned.

20

You misundrstood my reply. I put the aluminum foil in a ball jar with the Coke. When I loosened the lid, several days later, it made the noise I so aptly described. I’m not sure which way the air was going, though.
My method:
I put a strip of aluminum foil, crumpled up on one end and flat on the other, and dropped it into the ball jar. I then pulled the tab on the Coke and quickly but gently poured it into the jar to within 1/2 inch of the top and quickly screwed the lid down tight. The jar went into the reefer. I should have been typing “Ball” jar, I guess. :wink:
Anyway, some days later, the Coke was still pretty gassy but nothing apparent had happened to the foil.
I don’t have the means to test pH. Nor do I have any coke. I don’t drink the stuff, but I do experiment with it.