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Comment by dnautics | Hacker News Reader

The irony is that HF is probably worse in lower concentrations partly due to it's dissociative properties, which I won't get into a technical discussion about, and general human stupidity. If you're using 100% HF it's safer, because you're using not very much (we're talking 5-10 mL at a time), the equipment you're using is by nature safer, and you're more vigilant. It's also much easier to mechanically handle 10 mL than 100 (or more mL).

But hey I worked with 100% HF for seven years (BOC protected amino acid synthesis), so what do I know...

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Only if we ignore that HF is volatile, meaning that you're dealing with much more (not dissociated) HF evolving from your reagents. Even with a fume hood, you've got much more HF in your face versus a 5% concentration. Calcium gluconate gel will do nothing for HF in your lungs.

I disagree. If something happens with an apparatus, you fucking shut the hood quick. There is very fast positive pressure air blowing in through the sash opening, the apparatus is bolted into the wall, sashed above the nonglass barrier, and there is negative pressure inside the apparatus, so the tendency is to not leak out but leak in.

If you have a 5% aqueous solution most of that HF is still binary undissociated HF, which will leak into the atmosphere so you could very easily unawaredly be breathing it in if you are working with it in any open air situation below the sash line (which is what the tendency is when you're working with aqueous concoctions).

I fail to see how quickly shutting the hood in cases of catastrophic failure address my concerns. When you're dealing with pressure differentials and compressed gasses, explosions and implosions are a very real risk, and shutting the fume hood, even quickly, will not help in the fraction of a second these failures would take to fill your lab with aerosolized HF and shrapnel.

Even working at negative pressures, you can easily end up with a positive-pressure manifold if your sample is unexpectedly evolving gas at a higher than usual rate, or your valves aren't configured as intended. To be clear, these aren't academic exercises; I've actually seen all of these happen. With closed systems, you have to anticipate explosive pressures and plan accordingly. HF is incompatible with that kind of failure.

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