Friday, February 09, 2007

This is why Chemists fear HF acid

All the sciences, well maybe not math, but all the others, have unique fears about certain aspects of the other sciences. I've seen biologists freak when I opened a bottle of toluene ("Don't you know that's a carcinogen!" It's not, but I can't convince them otherwise, and the fact that a number of independent biologists have stated it makes me fear for how wide spread the thought is) and I've talked with nuclear physicists paranoid about acrylamide monomer (as if the it's a worse risk than the radioactive materials that they work with).

Chemists certainly have fears in these other sciences, but they also have a few chemical fears. The most widespread one is of hydrofluoric acid. These pictures give you some idea why. HF is a deceptively nasty acid. The deception is in the actual acid strength of it. The halide acids (HF, HCl, HBr, HI) are stronger as you proceed down the periodic table. This is because the anions increase in size in that manner and are better able to disperse the charge. Look at the pKa's: HF 3.45, HCl -4, HBr -9, HI -10. Not quite as wimpy as acetic acid at 4.75, but pretty close.

Despite this, the acid is properly feared because it is merciless in attacking two important everyday materials: glass and human flesh. Glass is remarkably resistant to most acids, but is readily etched by the tiny HF acid, and in the human body, HF is a systemic poison going right for the calcium in your bones. I've never experienced such a burn, but I understand it gives a new meaning to pain.


Anonymous said...

HF is a systematic poison not because of its pKa but precisely because it has a low pKa. Since it stays together as a neutral molecule, it is surprisingly lipophilic. The reason why calcium gluconate is used to counteract HF is because it's more lipophilic than other forms of calcium.

As for its acidity, it's also highly acidic when neat because HF can dissociate into FHF- and H+, thanks to super-strong F-H hydrogen bonding.

HF is safe in small quantities (< 10 mL) especially when handled in inverse-pressure systems.

Anonymous said...

So qrazyy..