Thursday, December 27, 2007

Nitrous

I don’t call them “pet peeves.” I call them “things that annoy the hell out of me.” One of them is comparing the energy of high energy cosmic rays to a hard hit golf ball, and I'll perhaps explain why sometime. But the one that gets me every time is when someone is talking or writing about nitrous oxide and then says “NO2.”

If you want to go all British, it’s true that the generic Brit for nitrogen oxides is “nitrous oxides,” but that’s not what being referred to, for example, on the TV program Mythbusters, or any reference to nitrous either as a combustion booster (the “poor man’s supercharger”) or as a dental anesthetic / recreational drug. That is N2O, a notably different compound that comes in big blue tanks, because it’s an oxidizer. Also, because nitrous is slightly sweet and has a relatively low vapor pressure (because its critical temperature is about 35 C, it can be liquefied at room temperature) it’s used as a propellant for whipping cream.

If you compress NO2, you wind up with the dimmer N2O4, dinitrogen tetroxide, which dissociates back to NO2 on pressure release, producing toxic levels of NO2. It’s also a fine oxidizer, and will cause a lot of things to burn mighty fast. It is also somewhat self-oxidizing, which means that it can burn itself mighty fast, giving a good replica of an explosion.

Nitrous oxide, N2O, is also a good oxidizing agent, hence its use in auto racing (or in the movie Road Warrior). It just drops the oxygen atom off the nitrogen molecule and away we go. And since the bottle doesn’t need to be at high pressure, it’s safer than using compressed oxygen. Both compressed oxygen and N2O can be a little dangerous if there's anything like grease in your line, however.

Nitrous is a pretty good greenhouse gas, and it’s also a source of nitrogen oxides in the stratosphere. Usually the photolysis of ozone just give what’s called the “triplet state” of the oxygen atom that cleaves off the O3, but if it’s hit with short wave UV (down below about 290 nm), it give a more energetic form of oxygen radical called the “singlet state.” These have electrons in the p and d orbits respectively, so the shorthand is O3p and O1d, pronounced “Oh triplet p” and “Oh singlet d” respectively.

Most of the time, O1d just bounces around until an inelastic collision drops it back to O3p, but not always. In the troposphere, the most common reactive fate of O1d is reaction with water vapor, to give two hydroxyl radicals:

O1d + H2O -> OH + OH

But water is scarce in the stratosphere, and O1d is more plentiful (because there’s more short wave UV. Sometimes the O1d runs into a molecule of N2O and you get nitric oxide:

O1d + N2O -> NO + NO

Whitten once had a very clever idea for measuring the amount of short wave UV in the UNC outdoor smog chamber that involved pumping some 20 pounds of N2O into the chamber along with a lot of acetaldehyde and ozone. The ozone absorbed the shortwave UV, which reacted with the N2O. The resulting NO got sucked up by peroxyacetyl radicals that had been formed from the normal smog chemistry reactions from the acetaldehyde and the amount of PAN that was formed was a quantitative measure of O1d formation. In essence the whole shebang had become a giant actinometer. Very cool, and it happened to match the light models that UNC had been using to estimate UV in their chamber, so everyone went home happy, though a few of them were disappointed that they hadn’t been allowed to sample the nitrous.

The recreational use of nitrous is not exactly illegal, but it is discouraged. Most automotive nitrous is sold “sour,” with added sulfur dioxide to make huffing unpleasant. One the other hand, “whippets,” for use on whipping cream dispensers can be bought in stores all across the land. Whippet nitrous is mighty expensive, but I’ve seen people walking down the streets of the French Quarter in New Orleans with little balloon dispensers that take a whippet charge. Those dispensers used to be sold in “head shops” before somebody figured out how to harass them out of existence.

There are, of course, dangers involved in using nitrous recreationally, not getting enough oxygen being one of them. That was one significant problem at the World Science Fiction Convention in Denver a couple of decades ago (when nitrous abuse was much more common than it is now). People used to sea level air found themselves passing out and falling off their chairs. There’s also a known, long-term danger to nitrous use/abuse, in that it causes a vitamin B12 related “die off” of nerves in the peripheral nervous system. Those do grow back, but over-exposure to nitrous can get ahead of the ability to regenerate. And various neurological problems result. This has mostly been reported in anesthesiologists, who sometimes get a high background dose of nitrous even without an abuse syndrome, though the case I read of the dentist who used to take 2-6 hour “naps” under an N2O/O2 mix probably counts as abuse,

The biggest bang for the buck is nitrous by the tank full, in either medical/dental or metallurgical grade. There was a time when the “typical Bay Area SF Convention party” featured one or more large nitrous tanks. A few time I brought along another tank filled with helium, and those of us who were dispensing gas made people ask for the nitrous by first inhaling a balloon full of helium.

That wasn’t how nitrous came to be called “laughing gas,” but it should have been. Those times are long gone now, but that doesn’t stop me from eyeing the empty can of whipped cream from time to time. It’s out of cream, but sometimes there’s just enough gas left in it for a little whiff of nostalgia.

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