Thanks to the big screen, many of us think of acids as dangerous, burn-through-anything substances. Think of those scenes in the Alien movies, where the alien’s blood drips through solid metal, destroying everything in its path.
Of course the vast majority of acids are much more boring. Vinegar (which contains ethanoic acid) and citric acid (found in, guess what, citrus fruits) are common acids that we eat all the time, and they don’t burn holes in your mouth. There’s an even stronger acid, hydrocholoric acid (HCl), in your stomach and not only does it not burn you from the inside out (usually), it actually helps you to digest your food and keeps you safe from nasty bacteria.
But there is an acid that’s really, properly scary. And its name is hydrofluoric acid.
Hydrofluoric acid has the chemical formula HF, but unlike HCl you won’t find this one in a school laboratory, and if it turns up in your stomach you’re in very big trouble. In true movie-acid style it’s capable of dissolving many materials, and is particularly well-known for its ability to dissolve glass (which is mainly silicon dioxide). It will also dissolve most ceramics (which contain aluminosilicates: compounds made of chemically-bonded aluminium, silicon and oxygen). And, like many other acids, it also reacts with metals, so storing it is a bit tricky. Where do you put something that eats through its container? Well, these days it’s stored in special plastic bottles, but in the 17th century when it was first discovered chemists had to use glass bottles coated inside with wax, and hope the coating was a good one.
HF has been an important industrial chemical for centuries. It’s used to etch patterns into, and clean, glass and ceramics, and also to dissolve rock samples, for example to extract chemicals or fossils from rocks. It’s also used to clean stainless steel and, in more recent times, to prepare silicon wafers (used to make silicon chips) in the electronics industries.
The chemist Carl Wilhelm Scheele (him again - he just keeps turning up doesn’t he?) was the first person to produce HF in large quantities in 1771. Scheele is particularly famous for his bad habit of sniffing and tasting any new substances he discovered. Cumulative exposure to mercury, arsenic, lead, their compounds, hydrofluoric acid, and other substances took their toll on him and he died on 21 May 1786 at the age of just 43. And that’s why your science teacher was endlessly telling you not to eat or drink in the laboratory.
So why is hydrogen fluoride so nasty? For starters the gas is a severe poison that immediately and permanently damages the lungs and the corneas of the eyes – lovely. Hydrofluoric acid solution is a contact-poison that causes deep, initially painless burns which result in permanent tissue death. It also interferes with calcium metabolism, which means that exposure to it can and does cause cardiac arrest (heart attack) and death. Contact with as little as 160 square centimeters (25 square inches) of skin can kill – that’s about the area of the palm of your hand.
And now for a gruesome and tragic tale: in 1995 a chemist working in Australia was sitting working at a fume cupboard and knocked over a small quantity (100-230 millilitres, about the equivalent of a drinking glass full of water) of hydrofluoric acid onto his lap, splashing both thighs. He immediately washed his legs with water, jumped into a chlorinated swimming pool at the rear of the workplace, and stayed there for about 40 minutes before an ambulance arrived. (Should you ever need to know, the proper treatment for HF exposure is calcium gluconate gel: calcium gluconate reacts very quickly with hydrofluoric acid to form non-toxic calcium fluoride, rendering it harmless.) Sadly, his condition deteriorated in hospital and, despite having his right leg amputated 7 days after the accident, he died from multi-organ failure 15 days after hydrofluoric acid spill. Remember, that was a spill the size of a glass of water.
Because hydrofluoric acid interferes with nerve function, burns from it often aren’t painful to begin with. Small accidental exposures can go unnoticed, which means that people don’t seek treatment straight away, making the whole thing worse. Do a Google image search on ‘hydrogen fluoride burns’ and you’ll see some images that will really turn your stomach.
So which would you rather meet? An alien with acid blood and a habit of laying eggs in your stomach or an invisible gas that destroys your tissues and leaves you, if not dead from multiple organ failure, then suffering with horribly disfiguring burns? You might stand a better chance against the alien…