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(This is a post for the #ChemMovieCarnival.  Check out the other entries over at Just Like Cooking!)

There’s a lot of inaccurate science in movies, but one of the most prevalent is the “Hollywood Acid” trope.  One drop of Hollywood acid is enough to melt a fist-sized whole in the floor, with the acid then continuing down to the level beneath [1, 2]. These acids have similar effects on steel and plastic, and only the original, unsealed, glass container is immune.

The toon variant of Hollywood Acid (at 10:00):

Fascinating as Hollywood acids are, you won’t find one at the corner store. Incredibly strong acids have been prepared in the lab, but no acid is going to be able to eat through multiple molar equivalents of concrete.

With that in mind, what is—in real life—the most dangerous acid one is likely to come across? I’m tempted to say Piranha, but given its incredible toxicity Hydrofluoric Acid takes the prize.

Background

HF(aq) is no stranger to fiction, with a prominent (and largely accurate) appearance in the second episode of Breaking Bad [3, 4]. In real life you’re most likely to come across HF in an old-school peptide synthesis lab or a materials chemistry lab (where it can be used to clean aluminum and etch niobium), or in railcars outside the local oil refinery or aluminum smelters.

While it will react violently with silicon compounds (ie. glass), with a pKa of 3.2 at first HF doesn’t seem particularly dangerous (the first pKa of citric acid is 3.09) [5]. However, the relatively low acidity keeps dissociation in water low, allowing the fairly lipophilic acid to deeply penetrate human flesh.

Once in the body the proton half of HF is free to react with tissue, and so unlike stronger acids (HCl, H2SO4, etc.) HF burns from the inside out.  Tissue damage can’t be stopped by simply rinsing the site of contact, and at HF concentrations below about 50% there won’t necessarily be any overt signs of injury until long after the initial exposure (NSFL).  Like many caustic agents HF can also deaden the nerves, for a time blocking pain.

Bad as this sounds, the true danger of HF doesn’t come from acidic burns, but from the simple fluoride ion.  Fluoride binds strongly to calcium, to the extent that it can draw Ca2+ from our bones [6].  If untreated, hypocalcemia is more dangerous than the burns, as it can lead to heart failure.  The insoluble fluoride salts themselves also cause damage wherever they form, clogging vessels and tearing tissue.

As a bit of a bonus, the low pKa allows HF to vaporize easily.  Unlike Hollywood acid the vapors are highly toxic.

Using HF Safely

<disclaimer>I haven’t had the pleasure of working with concentrated HF myself. These recommendations are based on publicly available safety information.</disclaimer>

Any work involving HF should be conducted in the fumehood, especially when working with concentrated (50%+) solutions.  Safety goggles, not safety glasses, should be worn (for the cautious, a face shield is also advisable). Thick, elbow length nitrile gloves are recommended, or properly sealed, thick, wrist-length gloves. A chemically resistant (ie. PVC) sleeved apron should be worn over the standard lab coat. Closed-toe shoes are obviously a must, and never work with HF alone [7].

Calcium gluconate (1-2.5%) should always be kept on hand, even when simply transporting HF containers.

Ready to party?

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First Aid

(Condensed from the CDC’s guide. Look there if you have been exposed to HF)

In all cases of exposure to concentrated HF expert medical attention should be sought. The measures below are only sufficient to prevent further exposure, not treat existing burns/toxicity. Exposure to HF does NOT necessary lead to pain prior to extensive tissue damage.

Immediately separate the victim from the HF spill. In the case of skin contact flush the area with water for 5 minutes, then immerse the affected area in 0.13% benzalkonium chloride.

If BAC is not available, or immersion is impractical, massage 2.5%(+) calcium gluconate gel into the site while wearing nitrile gloves. Wait for the arrival of medical professionals.

At the hospital treatments can include IV administration of both magnesium and calcium, as well as injection of calcium gluconate around the area of injury.

Of the two, I think I’d prefer to work with Hollywood acid.  It’s just safer (provided your lab is on the top floor).

[1] Hollywood acid also gets a nod in this #ChemMovieCarnival piece on Superman III.

[2] Youtube hilarity.

[3] The anhydrous variant, Hydrogen fluoride, is a gas that condenses at 19.5 degC.  It was also the subject of Derek Lowe’s very first “Things I won’t work with.”

[4] No high school science teacher has eight-plus litres of HF just sitting around. It’s far too toxic, and there’s no real need for more than 100 mL or so even if a demonstration required it.

[5] Side note? The person in this video is wearing gloves and goggles, but no coat, face shield, apron, or fume hood.  The fumes of HF are just as dangerous as the liquid.

[6] CaF2 has a solubility in water of about 0.016 g/L. MgF2 hovers around 0.13 g/L, and hypomagnesemia is clinically relevant.

[7] The best shoes are leather/pleather loafers, as mesh running shoes still allow liquids to drip through. Last years hush puppies can usually be found on the cheap.