Please explain a chemical reaction

September 10, 2009 7:43 PM

A ten percent solution of water and molasses is an effective, thorough, but slow way of removing rust from ferrous metals. When the rust/molasses/water has set for a while, you can burn your skin if you are not using gloves. What reaction turns mild mannered molasses, the soul of my baked beans, so nasty?
posted by Raybun to science & nature (6 comments total)

Well, it's possible that it's getting a bunch of iron oxide-hydroxide (rust) in some sort of hydrated form. Hydroxides are bases. I don't know if that'll burn you or not.
posted by floam at 8:07 PM on September 10

After poking around some on the web, it looks like the fermentation of the molasses/water mixture produces acids as by-products. (I've seen oxalic, citric, and acetic mentioned in different places, but couldn't find a definite reaction)
posted by Green Eyed Monster at 8:07 PM on September 10

So molasses has iron it it, and the solution restores rust because the non-oxidized iron in the molasses displaces the oxidized iron in the metal. however, it works in the other direction too, and whatever it was in molasses that contained regular iron now becomes an acid when it deal with iron oxide.

(Also, this post needs the 'science' tag)
posted by Jon_Evil at 8:16 PM on September 10

I know that my dad worked in a plant that fermented molasses to produce commercial food-grade citric acid powder.
posted by blue_wardrobe at 8:37 PM on September 10

High concentration acetic acid is strong enough to burn skin, but I've played around with high concentration citric acid and it didn't do anything.

I wonder if it might be the iron which is the villain here. When I was in middle school, my older brother used to use a solution of ferric chloride to etch the copper off of circuit boards after he'd put resist on them. And he used to wear rubber gloves when handling the stuff because he said it would burn your skin.

Wikipedia says that it is highly acidic. I wonder if that's because the iron is trying to drop to valence 2, thus liberating a lot of chlorine radicals?

Anyway, whatever it is, maybe the same thing happens in this case, but with different kinds of acid radicals. I'm no chemist but I'm under the impression that radicals are far more reactive than ions.
posted by Chocolate Pickle at 9:22 PM on September 10

IANAChemist...Some molasses can have sulphur added as a preservative. Could that cause the reaction in this application? I buy unsulphured molasses, myself, so I have no idea just how common sulphured molasses actually is anymore.
posted by Thorzdad at 3:29 AM on September 11