Published online 16 November 2004 | Nature | doi:10.1038/news041115-5

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Powerful yet gentle compound excels at delicate reactions.

Many ‘superacids’ eat straight through glass, yet the new record holder is stable enough to be stored in a bottle. © Getty

The world's strongest acid, at least a million times more potent than concentrated sulphuric acid, has been made in a lab in California. Perhaps confusingly, it is also one of the least corrosive.

The compound, called a carborane acid, is the first 'superacid' that can be stored in a bottle, say its creators. The previous record-holder, fluorosulphuric acid, is so corrosive that it would eat straight through the glass.

The new acid's gentleness is down to its remarkable chemical stability, say Christopher Reed of the University of California, Riverside, and his colleagues. Like all acids, it reacts with other compounds, donating a charged hydrogen atom to them. But what is left behind, although negatively charged, is so stable that it refuses to react further.

It is this secondary reaction that is essential for corrosion. For example, hydrofluoric acid corrodes glass, which is composed largely of silicon dioxide, because the fluoride ion attacks the silicon as the hydrogen reacts with oxygen.

Acid test

“Our research is driven by making molecules that have never been made before.” Christopher Reed

University of California, Riverside

The new acid, which has the formula H(CHB 11 Cl 11 ), is incredibly good at donating hydrogen ions, or protons, which is how acidic strength is defined. It is over 100 trillion times more acidic than the water in your local swimming pool. But the carborane part of the molecule, which is what remains when a proton is given away, contains a cluster of 11 boron atoms and one carbon arranged into a shape called an icosahedron.

This may be the most stable group of atoms that exists in chemistry, says Reed, and explains why carborane acids refuse to take part in full corrosion. "Carborane acids deliver very clean acidity without ferocity," the researchers say.

Reed insists that the joy of his work comes from simply dreaming up new chemicals. "Our research is driven by making molecules that have never been made before," he says. "Carborane acids are allowing us to do this."

But they may have more practical uses too. They allow the production of 'acidified' organic molecules. These are compounds that have had a hydrogen ion added to them, as in the case of many vitamins in over-the-counter supplements.

Acidified compounds occur fleetingly in the digestion of food, petroleum refinement and drug manufacture, says Reed. Carborane acids could be used to study these elusive chemicals more closely, or even help chemical industries to run their reactions more efficiently.

But the researchers' immediate goal would be less of a money-spinner. They want to use carborane acids to acidify atoms of the inert gas xenon, simply because, they say, "it's never been done before".

University of California, Riverside