Geologists are used to being in mortal peril. Just ask Pierce Brosnan. Whether it’s contending with exploding volcanoes, braving the high seas on on a research cruise or melting rock in furnaces at 1400ºC, danger is part and parcel of being a rock-lover. But after much deliberation, I’ve come to the conclusion that the most dangerous thing Earth Science PhD students have to contend with is the big daddy of chemicals – hydrofluoric acid.
For the uninitiated, hydrofluoric acid (HF) is a highly corrosive chemical that can dissolve glass, which means that it is feted by geochemists and experimental petrologists to dissolve rocks and clean metal capsules. It can also penetrate skin and dissolve bone, something which has been utilised by TV criminals to dispose of bodies.
As well as being quite nasty, HF is a bit of an unusual compound. A hydrogen fluoride molecule consists of a covalently bonded pair of hydrogen and fluorine atoms. If you can cast your mind back as far as GCSE Chemistry, you’ll remember that a covalent bond occurs where two atoms share a pair of electrons to in order to fill their outer electron shells. In this case, both fluorine and hydrogen are missing one electron and so combine to form a single covalent bond.
Electronegativity is a measure of the tendency of an atom to attract electrons. It turns out that fluorine is the most electronegative of all the elements. This is because its outer orbital is only a short distance from its proton-rich nucleus and there is less “screening” by inner shell electrons than in other elements, thus the positively charged protons are able to exert a strong attractive force on nearby electrons. Poor old hydrogen and its one puny proton just cannot compete, and so the “shared” electrons in the covalent bond actually spend most of their time nearer to the greedy fluorine, causing a charge imbalance (dipole) in the molecule.
Each molecule of HF has a positive and a negative pole. These poles are attracted to their opposite charged pole, forming a “hydrogen bond”. This dipole-dipole force increases the energy required to disrupt the intermolecular structure and explains why HF (and also H2O) is liquid at room temperature.
HF does not actually become an acid until it is dissolved in water and dissociates into H+ (actually H3O+) and F– ions. And believe it or not, HF is technically a weak acid. Those hydrogen bonds are so strong that it does not dissociate completely, thus limiting in the H+ ions produced. What makes it so effective at eating glass (and rocks) is the fluorine, which reacts with silica to produce a water soluble compound:
SiO2 (s) + 6 HF(aq) → H2SiF6 (aq) + 2 H2O(l)
Even if you’re not made of glass, HF is certainly not to be trifled with. Spills of less than 100mL conc. HF have been fatal (if you want to send some shivers up your spine, read this). Most acids only cause surface burns (from H+ ions), but this is where HF gets nasty. Fluoride ions readily penetrate through skin and can enter the bloodstream. The fluoride ions then react with calcium (and magnesium) in the blood and in bones. Calcium ions in the blood are vital for regulating heartbeat and low serum calcium levels can cause cardiac arrest, and ultimately death. Just to top it off, HF burns from dilute solutions aren’t always immediately painful, so theoretically it could be several hours before you notice something is awry.
Oh, one final thing…HF boils at around 20ºC and has a vapor pressure of 110 mm Hg, meaning it will vaporize quickly (about four times faster than water, as a comparison). And in case you hadn’t realised, you wouldn’t want to breathe this stuff in.
When dealing with HF in our lab, the standard personal protective equipment (PPE) is a face visor, lab coat, double gloves and a PVC apron, and all tasks are carried out a in fume hood. Any lab using HF should provide calcium gluconate gel – applying this can help to neutralize the fluoride ions before they can penetrate the skin and is the standard first treatment for HF burns; however, nothing seems to be effective at stopping your brain from making your hands tingle for a good half an hour after you’ve touched, or even just looked at, the bottle!