Thursday, 30 April 2015

Dry ice? It's a blast

What could possibly be wrong with carbon dioxide? Our lungs are full of the stuff. I've mentioned the toxicity of carbon dioxide twice before. The first was for the "humane" killing of lab mice, which seemed to this witness to be a pretty desperate brutal way to terminate anything.  Earlier I described the killing of several hundred people by carbon dioxide asphyxia in the surroundings of Lake Nyos in Cameroon in 1986.  In both cases, the carbon dioxide, being heavier than air, displaced all the oxygen upwards. A very similar case almost happened at my place of work about ten years ago. 

A lot of biological reagents are shipped in styrofoam/polystyrene containers containing a shovelful of dry-ice = frozen carbon dioxide.  If scientists were six-year-olds they would be disappointed every delivery day when a large [40x40x40cm] box arrives and after unpacked is found to contain only a tiny tube full of some necessary but frightfully expensive reagent. Dry-ice sublimates (goes directly from solid to gas without becoming liquid] at any temperature above -78oC, so it makes a handy coolant, because doesn't go soggy if delayed in transit and dissipates as vapor into something a lot less obviously poisonous than sarin or cyanide.  Any large biological research institute will have some plan or protocol for recycling the dry-ice which arrives surrounding a delivery - the gal in the lab next door may need to ship a sample out the same day and it would be silly to buy dry-ice for that purpose. The standard practice for dealing with surplus dry-ice is to leave it in the styrofoam shipping container with the lid open or dump it into a sink; a few hours later it has all gone into the circulating air.

You can have some innocent fun with dry-ice.  If you put a knob of the stuff in an old plastic 35mm film canister and put it in your pocket, it will sublimate and build up pressure until the lid blows off with a satisfying >!pop!<.  You can repeat the trick many times by replacing the lid; huge fun at a cocktail party as nobody can tell where the sound is coming from.  You can also do something which is dip-stick dangerous but not obvious-to-all-thinking-people hazardous. This is what happened one evening at work.  Poorly trained researcher, knowing s/he wanted some the following day, put a shipping container full of dry-ice in the walk-in cold room and left for the night. Another researcher, known for favoring an early start, came into the lab before everyone else, walked into the cold-room and was pole-axed by a wall of unbreathable, invisible, ordourless carbon dioxide which had filled the almost air-tight box of the cold-room.  Luckily she fell out and back, luckily the floor area of the lab was large so the CO2 puddle was not thick enough to cover her head, luckily the doors were open, luckily it wasn't in the cellar, luckily someone else came in a good bit early for work, so we didn't lose a colleague. Dry ice in an enclosed space?  Don't do that at work, kids.

The math is: the molecular weight of carbon-dioxide is 12+16+16 = 44, the density of dry-ice is a bit more than water 1.5g/cc.  So a mole (gram molecular weight) of dry-ice occupies about 30cc maybe a tablespoonful. Avogadro's gas law states that a mole of any molecule will, as a gas, fill 22.4 litres of space.  That tablespoon of solid material will make enough gas to fill a couple garden buckets.  This is what builds up the pressure in the film-canister until it gives at the weakest point.  Bigger quantities in tougher containers will make a much louder noise. Blowing up a 2 lt soda-bottle is hazardous enough but imagine if the receptacle was made of two thin glass walls inside a light steel container, like a Thermos vacuum flask.  That would make some serious shrapnel.  This last weekend a chap called minusbat in Islington, N London contrived to have a near-death experience for his family with dry-ice and a vacuum flask.  Nobody died but . . . please . . . don't do that at home, kids.

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