Aluminum Oxide in High School - Mixed Blessings

Today was the last day of general chemistry lectures...it was about galvanic and electrolytic cells. I spent it thinking about aluminum oxide. Unconstructive, I know (could've done some calculus instead...)

(Note for pedants: "aluminum" - US/acceptable variant spelling - and "aluminium" - UK/official spelling - shall be used interchangeably)

Also - apologies for the lack of pictures! I don't have any pure aluminum/aluminum oxide on hand. Add an apology for any mistakes that I probably made, too...mostly wrote this off the top of my head.

Aluminum oxide (commonly known as "alumina") is an extremely useful compound. Aside from being the most common ore for aluminum metal (in the form of bauxite), and the major constituient in various gemstones (corundum with transition metal traces e.g. Cr, Fe)^[1], it also finds alot of uses in chromatography. I mean, setting up a column is always fun^[2], even if setting up a TLC plate isn't (of course, I would suspect that most people use fancier stationary phases with fluorescent markers and other cooler things these days). Furthermore, it protects all our shiny aluminum equipment from oxidation, which, considering how many things are made of aluminum, and that the standard reduction potential of Al is -1.66 V^[3], is a good thing.

Unfortunately, a very common high-school lab experiment consists of making a battery of the highest possible voltage using the given solutions and corresponding metals, which often include iron(III), nickel, zinc, copper, and of course, aluminum. For obvious reasons, nobody gives high school students interesting metals like, say, potassium. (E⁰ = -2.92 V though...)

Now, since the person/pair who generates the highest voltage using given materials often gains bonus marks, everybody goes home and looks up the standard reduction potentials yielding the highest voltage before coming to the lab.

In my particular scenario, the combination that yields the highest voltage is reduction of CuSO₄ with Al (I forgot what the aluminum salt was), E⁰ = 0.34 + 1.66 = 2.00 V.

I begged for a hot plate and boiled my 0.1 M CuSO₄ to increase [Cu²⁺], while diluting the aluminum solution to decrease [Al³⁺]. I scrubbed my aluminum metal with steel wool, dipped it in 0.1 M HCl, and rinsed with water, drying it before use because I don't want to dilute my copper sulfate (rather silly rationale, true). After connecting, the voltmeter reads...0.62 V^[4]. Yep. While, ironically, some pair who randomly hooked up solutions with no extra work (I think it was iron with nickel) got 0.82 V, at the very least.

The question now becomes, why?

Answer: aluminum oxide, which I have been praising not five minutes ago, passivates the metal! It's hard to oxidize aluminum metal through an impermeable^[5], very stable, already-oxidized oxide layer. I didn't figure it out until I read a particularly enlightening forum entry. It is so obvious in retrospect, too! Should've asked for concentrated acid or base to dissolve the oxide layer (aluminum oxide is amphoteric), avoid letting the metal sit for any period of time (avoiding aerial oxidation is hard) and somehow get rid of dissolved oxygen in the aluminum salt solution.

Despite my inability to get decent voltages, I still like electrochemistry. Electrolytic production of sodium from molten salts, anyone? (Dangerous, yes, but the idea is so fun!) Or even (*gasp*) electrolytic production of organics?

No idea why I think of tangential topics during general chemistry. At least it is partially relevant (unlike last time - did math instead of chemistry). I think writing entries like these distracts me from other issues (let's just say that I found out my Putnam score today, and yes, it's a 0 through and through.)

^[1] Transition metals are well-known for their colourful nature, but it'd be interesting to look up why do the impurities change the light absorption/transmission of corundum that way. Something to do after finals...
^[2] Of course, I have set up exactly one chromatography column in my entire life, so it probably gets tedious to tend the column after awhile (what with the banging to get rid of air bubbles during setup and the careful refilling of solvent).
^[3] The potentials come from Zumdahl, S. Chemical Principles, Second Edition. I love this book, even if it's outdated and nowhere nearly as handsome as the 9th edition of Petrucci.
^[4] Or thereabouts. This is from memory.
^[5] Bad choice of wording...anyone knows the proper term?