Can attack acidic glass

Berlin : Not only corrosive, also dangerous

It sounds like a contradiction at first: "Hydrofluoric acid is a weak acid, but very dangerous," says Matthias Driess, Professor of Inorganic Chemistry at the Technical University (TU) Berlin. Actually, the strength of an acid is considered a measure of the risk associated with it. Hydrochloric acid, for example, eats holes in clothing and burns the skin as soon as you come into contact with it. Glass, however, is not attacked by hydrochloric or sulfuric acid. That's why you can keep them in glass bottles. Hydrofluoric acid, on the other hand, eats up glass and must therefore be stored in plastic containers.

Hydrofluoric acid is made up of positively charged hydrogen ions and negatively charged fluoride ions. Ions are electrically charged particles. The fluoride parts often combine with positively charged silicate ions, which are the main constituent of glass. This is why hydrofluoric acid is particularly suitable for damaging glass. "Silicon is washed out, the glass becomes cloudy," says Matthias Driess. The electronics industry also takes advantage of the affinity for silicon. This is the only reason why computer chips can be processed or solar cells formed. The hydrofluoric acid, which etches metal, also performs well in electroplating.

Within the hydrofluoric acid molecule, the bond between the hydrogen and fluoride ions is quite strong. That is why hydrofluoric acid is considered a weak acid. The strength of an acid depends on the amount of free hydrogen ions in the solution.

This high affinity for positively charged ions also makes hydrofluoric acid so dangerous for living things. "Hydrofluoric acid acts as a Trojan horse," says the TU chemist. It can easily get into the body through the skin. The acid fooled the organism into thinking that it was water. Because water is also a liquid with few free hydrogen ions. This allows hydrofluoric acid to get into the deeper layers of the body. Magnesium or calcium ions are bound there. Vital enzymes fail. Metabolism disorders and damage to the liver and kidneys are the result.

"It can also lead to cardiac arrhythmias," says Elke Goldbach, a doctor at the Berlin poison control center. The hydrofluoric acid can even penetrate to the bones, says Driess. Contact with hydrofluoric acid can cause wounds that are very purulent and difficult to heal.

However, here too the effect depends on the concentration. Severe chemical burns, which can even lead to death, are only known from industry so far. Hydrofluoric acid is not readily available in the shop around the corner, but it can be found in rust removers or stone cleaners. Perhaps the sprayers who burned windows in the Berlin subway or S-Bahn received the substance via the Internet, speculates Driess. Sprayed onto the glass pane, the hydrofluoric acid quickly dissolves the silicate. What remains is harmless water.

Driess sees the greatest danger for bystanders who inhale the substance so that it gets into the lungs and causes coughing or breathing difficulties. It is possible that bystanders or the sprayers themselves come into contact with the acid before it is used up. It can then penetrate the skin so quickly that the person concerned does not even notice it at first. “This can be the case when it comes into contact with liquids that contain up to 20 percent hydrofluoric acid,” says Goldbach. Redness, blisters or painful burns to the skin may only appear after hours. Washing off with a calcium-containing solution would be an immediate measure. Then you should see a doctor quickly.

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