Molecule of the Day

This is one of those molecules that everyone who does chemistry has heard of, but you rarely hear about it outside chemistry and engineering. HF is a member of the acids of group VII - that is, HF, HCl, HBr, and HI. If you paid attention in general chemistry, you know all Bronsted acids dissociate into their conjugate base and a proton, that is, HX -> H⁺ + X^-. This is the definition of a Bronsted acid - I am only saying Bronsted to be technically correct since there is another type of acid that we won’t get into today that doesn’t have to involve protons at all.

To a first approximation (and a very good one beyond which I rarely go), A Bronsted acid’s strength is proportional to how weak its conjugate base is. Cl, Br, and I are all terribly weak bases, and their acids are correspondingly strong. F, however, is actually a decent base. Remember the pH scale that goes from 1-14? A 1M solution of HCl has a pH of about zero! A 10M solution approaches pH negative 1! (Yep, you can do that). A 1M solution of HF, however, has a pH of 1.6 or so. HF is actually a “weak acid” by the Bronsted definition (a strong acid HX will dissociate into all H⁺ + X^- in water; a weak acid will still have some HF left.

Interestingly, then, HF is actually the most dangerous of these four acids. I have gotten drops of corrosive, nasty, stinky concentrated HCl on myself, and this required no more than a trip to the sink. Even a drop or two of HF, however, would likely require a trip to the hospital. Since not all the HF dissociates, some can make it across your skin (as a rule, uncharged, small, nonpolar compounds have the best luck at doing this; HF gets two out of three on this). In your blood, only HF has made its way across, so it dissociates, releasing some protons (the least of your worries right now). It also releases F^-, and here, the interesting and dangerous chemistry of the fluoride ion goes to work.

HF is toxic in a number of ways. The acid will be corrosive to tissue (and in a more pervasive, interesting way than regular acids; the protons and conjugate bases are all charged and cannot penetrate your skin - HF can). Fluoride will interact with serum calcium and cause insoluble fluorite (CaF₂) to precipitate. Calcium homeostasis will be disrupted, here. Bones will release calcium in response to low serum calcium (Calcium fluoride crystals floating around in your blood don’t count as dissolved calcium!). All this can induce cardiac arrest. That, and it actually has a slight anesthetic effect, so pain is delayed. The figure I usually see thrown around by the safety folks is that a five-by-five inch spill on your skin can kill by heart attack, even with treatment. Labs that use HF regularly stock calcium gluconate gel, which can complex the HF and let you keep on keepin’ on after an exposure. Please still visit the hospital though, and go to the largest (trauma certified) one nearby. Another safety person told me a story once: someone went to a hospital after an HF exposure. A small hospital. The Dr. Nick-on-duty apparently figured HF=HCl and sent him on his way after a cursory washing. He lived, but it was actually touch and go for awhile. Scary, eh?
So HF can hurt, bad. In that usual cruel-sense-of-humor way, though, Nature’s given us something pretty useful. Geology as a science wouldn’t exist without HF; it dissolves essentially all mineral oxides. It is an article of commerce; it’s used in the production of freons (and their replacements, since essentially all are fluorinated), etching glass (HF dissolves glass - in fact, you pretty much have to store it in teflon. Not much else does this.), and, of course, in your drinking water (as other fluoride derivatives). Fluoride ion, in low concentrations, isn’t that toxic, and combines with the hydroxyapatite in your bones to form stronger, more acid-resistant fluoroapatite. Fluorine is truly bizzare. The stranger thing is, since it’s an element, we’ll never have anything else like it!

Bye, everybody!