Friday, July 18, 2008

SCIENCE- Last weekend's Tanker Spill on I-40

Over the weekend we had a semi crash a couple dozen miles from here which closed down I-40 for over a day. I thought I’d put on my Mr. Science cap and explain what all the hub-bub was about.

The tanker was full of a substance called Titanium Tetracloride. This is a liquid metal that’s used in an intermediary step in making solid titanium metal of the type you find expensive cell phones, I-devices, and laptop computers (as well as aircraft parts and medical implants). Titanium has the highest strength to weight ratio of any metal and is corrosion resistant, even to hydrochloric acid (more about that later).

A little basic chemistry explains why this is so dangerous that TDOT had to close the interstate down for a day. I’m going to try to keep this as simple as possible because I find chemistry to be a little like algebra- we all had to do it once, but that was long ago and far away. Also because chemistry is one of those things that get real deep pretty fast.

Basically Titanium Tetrachloride or TiCl4 (pronounced “tickle” and a lot easier than spelling it out) is primarily used to make titanium metal and pigments. It used to be used for skywriting and smoke screens but isn’t much anymore for exactly the same reasons you don’t want a tanker of it busting all over your interstate. It’s odd because it’s a liquid metal at room temperature. This is a property of the way it to bonds to the chlorine to form a polymer. Vanadium Tetrachloride is similar. Both these molecules have the same arrangement of 4 (or sometimes 3) Chloride atoms around a metal center. Titanium has 2 electrons in it’s outer shell and Chloride has seven. Now I know what you’re thinking. The most stable arrangement is 8 electrons in the outer shell. This means Ti wants 6 and Cl wants one. Aren't chloride bonds supposed to be more like NaCl, where Sodium has 1 electron and Chlorine wants one? And the answer is yes. That’s what makes this stuff so dangerous. It can lose that chlorine very easily. When you want to make titanium metal out of this stuff you just pass some magnesium (Mg) through it and you wind up with magnesium dichloride and titanium. And Magnesium also has only 2 electrons in it’s outermost shell, so it’s hardly a great match. But give that chloride something that really suits it and it’s going to bug out in a hurry. Like hydrogen, for instance, which has one electron, making it a perfect match. And lets say that hydrogen is attached to some oxygen, which with 6 electrons in it’s outer ring is a perfect match to pair with the titanium and fill both their outer shells with 8 electrons.

Did I mention it was raining Saturday?

This is exactly what they do to make titanium oxide for dyes. In a factory that’s fine. But all at once, out in the open?

The worst case scenerio, I guess, is an explosion with hydrochloric acid clouds and acid rain. Titanium and Chlorine are so anxious to pair with Oxygen and Hydrogen that the result is a chemical bomb. After the explosion what you have is titanium oxide (harmless enough but bad for the enviroment if dumped on the ground in tanker loads), and HCl which is usually incorrectly referred to as hydrochloric acid. HCl has to combine with water to actually form hydrochloric acid. Of course if there were any water in the vicinity the HCl would rapidly dissolve in it.

Did I mention it was raining Saturday?

So there you have the reason, chemically, that having the interstate access from Nashville to Knoxville all day long on Saturday and evacuating 164 homes and businesses and thousands of people was a good idea. All because of some simple chemistry.

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