I recently watched one of the better known chemistry demonstration shows, which, I confess left me in part underwhelmed, but also rather appalled. Part way through the lecture the famous SF6 floating boat demo was carried out. If you haven’t seen it then look on YouTube. It is astonishing. No question. But should we be doing it?
I’m sure you are aware of the problem. SF6 is one of the most robust molecules ever discovered/invented. Its six S-F bonds are tightly packed around the central sulfur in such a way that virtually nothing can get in there to interfere with the sulfur – it’s a molecular armadillo – and that makes it brilliant as a blanketing gas for smelting magnesium – it’s astonishing that not even hot magnesium will bite it. But it also means that if you release it into the environment it will hang around forever and a day – the atmospheric lifetime is 3200 years according to the EPA.
But at the same time, the S-F bonds are actually highly polarized – there is a lot more negative charge on the fluorine than there is on the sulfur. That means that when the atoms move they can couple very strongly with infrared radiation. In other words, it’s one hell of a greenhouse gas.
Coupled with its atmospheric lifetime that gives it a global warming potential of 34000 compared to CO2 (which has a value of 1).
So why the hell is anyone doing this demo any more? But even worse, how can one do this demo without talking about the chemistry of SF6 and all the implications of using it? Sure you only use a few litres, which is nothing compared to what industry does, but does that make it right? By that argument then it’s OK to pour mercury waste down the sink “because it’s not much really”. There are some demos that may pass our current obsession with Health and Safety and yet which I just don’t think should be done.
Did I go and tell the guy what I thought? I’m afraid I didn’t. And that was partly because the last thing you need after giving a talk is some arrogant nutter out of the audience giving you grief. But when (not if) I’m in the bar with him one of these days, I will tell him. And I hope that if I do something that you feel I shouldn’t, come and tell me. Just be gentle. I’m not an armadillo.