[Dr. Poliakoff] Most chemists are fascinated by what they’ve always been told, that HF, hydrofluoric acid, eats human flesh. We thought that we should try it. Now, obviously, we can’t try human flesh, and Brady decided that a chicken’s leg, that’s an uncooked leg from a dead chicken, is a reasonable approximation to a finger. We decided that we should try and do a demonstration, but with some controls so we could compare them. And what we wanted to know was, what was special about HF? Was it the acid, or was it the fluorine? So we thought if we took two other acids, HCl, hydrogen chloride, and sulfuric acid, both of those have the H, the acid part, but they don’t have the fluorine. I have been told that HF attacked your nerves so quickly that if you stuck your finger in it, and none of us, obviously, would try this, that it destroyed the nerves really quickly so you could hardly feel anything. There are stories of people whose gloves have leaked, and they’ve taken out their hands and bits are missing. And of course, HF features in the TV show, Breaking Bad, which I’m glad to say I’ve never seen. [Brady] Why are you glad to say that? It’s really good! Well, perhaps… perhaps one day I might watch it, when I’m old. After 5 minutes, nothing! [Brady] Fred; this is 5 minute, 5 minute mark. After half an hour, nothing! [Brady] This is half an hour. At the end of the day, something slightly might have happened. We decided to call it a day, but just in case, to leave it overnight in the acid. In the morning, it was really interesting. The two chicken legs in the control acids, HCl, sulfuric acid, had made quite strong color in the liquid. And the bits of the chicken leg sticking out still looked quite healthy. You could almost imagine cooking them! But the leg in the HF was pale, looked decidedly dead. So we lifted them out, where the ones in HCl and sulfuric acid had turned rather jelly-like… [lab technician] Eww. [Brady] Yeah… she’s done. …and really looked almost like an advertisement for being a vegetarian. Really yucky. But, with the HF, it was quite different. It was as if a surgeon had chopped the bottom off. It was moderately interesting what had happened below the surface, but really what was much more interesting to me is why the chicken leg in HF looked as if it was dead. And, clearly, what had happened is that some of the pigment, some of the red color of the meat, had been bleached away. The reason that meat is red is because it contains a molecule called myoglobin. This is related to hemoglobin, which we have in our blood, and myoglobin stores oxygen in the muscles, and myoglobin is a very similar color to hemoglobin. If you cook beef, roast beef, so it’s rare, and red juice comes out, the red juice is myoglobin. Well, in fact, it’s myoglobin that’s reacted with carbon monoxide, which makes it such bright red. My theory is that the HF is bleaching away, that’s, attacking the structure of the myoglobin, in particular the so-called “heme group”, the iron-porphyrin group in the middle, and breaking some of the bonds, so that it no longer absorbs light and is colorless. And that’s why the solution that was left of HF was completely colorless! It looked as if the stuff had just disappeared, there wasn’t any sludge in the HF beaker. [Brady] From school, I thought acid was acid, and we always learned that H+ is the big deal here and doing all the work, whether the H+ is coming from hydrochloric acid or sulfuric acid, or hydrofluoric acid. Why do they- why do the three all do their work so differently? The reason is that the chicken leg is made up of a mixture of organic compounds. Proteins, a bit of fat, and also bone, which is a mixture of mineral. Now if you break these organic molecules, you are breaking carbon-hydrogen, carbon-carbon bonds. And you can’t just break them, you have to, if you like, cap them off with some other group. And carbon-fluorine bonds are very strong. In fact, HF is not a very strong acid. If you dissolve HF in water, especially when it’s dilute, it is not as acid as HCl, because the bond doesn’t dissociate, the HF bond, quite so much. But hydrogen fluoride, HF, is very reactive, because although the bond’s strong, the bonds in the products that you make are even stronger. [Brady] So in terms of HF having a reputation for being a bad boy, that’s really got nothing to do with its acidity or its hydrogen, it’s just the fact it’s got fluorine. [Dr. Poliakoff] It is because it has got fluorine and it will react very easily. [Brady] Is there anything with fluorine? No, because if you take something with, say, carbon-fluorine bonds, the bonds are so strong that it’s very difficult to break them. They are very inert. They use this refrigerant, for example, because they’re very stable. [Brady] So it’s a bit like fluorine has come to the dance with hydrogen, but *really* wants to dance with someone else. Precisely. Well, it wants not so much to dance, but to bond for life. [Dr. Poliakoff] The reason why I think my explanation is correct is because we then went the whole hog, cut the strings, and dunked the chicken legs completely in the acid.