The Great SO2 Canister Case Or, how to stop worrying and dispose of a bomb P. E. Spargo Science Education Unit. University of Cape Town, Rondebosch 7700, South Africa
One of the services that the Science Education Unit a t the University of Cape Town offers to local schools is the disposal of old laboratory chemicals. Some time ago we were somewhat taken aback to receive for disposal a canister of liquid sulphur dioxide. Made by BDH, this aluminium canister, holding 500g of liquid SOz when full, was half full but t h e metal t a p was heavily corroded a n d jammed fast. I t was somewhat like being given a bomb to dispose of! What were we to do? Various possibilities were considered. Place i t in the middle of a football field and fire a bullet through it? No, that would release a highly toxic gas into t h e atmosphere-and also seemed somewhat too much like the Wild West. Hand i t over to one of the local companies t h a t disposes of hazardous wastes? No, that would not only be a n admission of defeat but would also cost money. The high solubility of SOz, and the fact that swimming pools, which are chlorinated using calcium hypochlorite ("HTH"), need periodic additions of acid to bring down the pH, provided inspiration for a n attack on the problem. So, donning a swimsuit I jumped into our home pool holding the canister i n one hand and a strong spanner in the other. Working under water (and watched with more than a little apprehension by my wife but with great interest by Jessica the dog!) I tried to open the corroded tap. No success-hut I did notice that the canister floated when released. More inspiration: SOz h a s a relatively high melting point: -73'C, so why not freeze it, pierce the can and allow the gas in i t to dissolve i n the pool? Liquid nitrogen, with a boiling point of -196C, was clearly the stuff for the job.
1134
Journal of Chemical Education
But i t might take quite a lot of liquid nitrogen, so perhaps i t would be wise to cool the canister as much a s possible first. The obvious place to do this was to place the canister in the deep-freeze unit i n the kitchen, which has an interior temperature of -20°C. In i t went, to the accompaniment of more strange looks from my wife, for its overnight stay. The next day I brought home from the University 2 L of liquid nitrogen in a vacuum flask with a mouth wide enough to accept the SOz canister. After tying a brick to the canister to ensure that it would not float, the latter was cooled in the liquid nitrogen for -5 minutes, when the contents were tested and found to be solid. Watched from a safe distance hv. mv . wife. but close-UP . bv. Jessica. I took a d w p h i i t h and drove n scrcu d r ~ v e rthrough the side of the c;inistcr. \Vould ihe l a w of ohvsical chernisrrv hold'! (Even a lifelong scientist has occasional fleeting doubts.) Yes, they did and no wisp of gas emerged from the hole. Can and brick were dropped into the pool, where the brick rested on the bottom and the can floated halfway up the pool, tethered by the string. After a short while gas bubbles began to appear from the hole in the canister but on account of the high solubility of SOz they disappeared after a n ascent of -25 em. After some 10 minutes the emission of gas ceased, no whiffof SOz being detectable in the vicinity of the pool. The pH of the pool had dropped from "Add Acid" to "Normal" on the pool pH indicator. The canister was now safely empty, no money had changed hands and a good time had been had by a l l a e s s i c a in particular!
."
