What Is Stronger Than Concentrated Sulfuric Acid? Quite a Lot, Actually
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A handful of substances are more acidic than pure sulfuric acid, some by an enormous factor, and they can force a proton onto molecules that normally refuse to take one.
How the category is defined
The conventional scale of acidity runs out in concentrated solutions, because it measures the behaviour of water rather than of the acid, so a different measure is used for very strong acids. On that scale, any substance more acidic than pure sulfuric acid counts. The strongest known are more acidic than sulfuric by factors reaching ten thousand million million, which is a difference so large that the ordinary language of strong and weak stops being useful and the comparison has to be stated numerically.
What makes one so strong
Strength depends on what is left behind after the proton departs:
- •An acid gives up a proton and becomes a negative ion
- •The more stable and unreactive that ion is, the stronger the acid
- •Spreading the charge over many atoms stabilises it
- •Fluorine attached to carbon does this extremely well
- •The strongest known involve a cage of boron and carbon with fluorines
- •That leftover ion is so unreactive it barely interacts with anything
What they do that ordinary acids cannot
The practical interest is in forcing protons onto molecules that have no interest in accepting one. Saturated hydrocarbons such as methane are chemically inert and can be protonated by these substances, producing species that had only been theoretical. George Olah worked out how to generate and observe short-lived positively charged carbon species using them, which explained a great many reaction mechanisms and won a Nobel Prize in 1994. Petroleum refining uses related chemistry to rearrange hydrocarbon molecules into shapes suitable for fuel, which is an industrial process of enormous scale.
The other end of the scale
The mirror image exists and receives far less attention. Substances more basic than pure sodium hydroxide are described by the corresponding term, and the strongest of them will pull a proton off molecules normally considered to have none available at all, including hydrogen gas and simple hydrocarbons. They are prepared from alkali metals with certain organic compounds and are violently reactive with air, water and most containers. The two extremes together define the practical range of proton chemistry, which is far wider than the ordinary scale running from zero to fourteen suggests.
The dangerous ones and the surprising ones
The reputation of these substances is uneven and mostly deserved. A mixture of hydrogen fluoride and antimony pentafluoride is extraordinarily aggressive, dissolves glass and is handled only in containers made of fluorinated plastic, and hydrogen fluoride is separately dangerous because it penetrates skin and attacks bone. Against that, the strongest known acid of all is comparatively safe to handle, because the ion it leaves behind is so unreactive that it does not attack anything, which is a striking demonstration that acidity and corrosiveness are two different properties frequently confused.
The takeaway
Acidity beyond pure sulfuric acid is measured on a separate scale and the strongest examples exceed it by factors around ten thousand million million. Strength comes from leaving behind an unreactive ion with its charge spread out. They can protonate inert hydrocarbons, which explained reaction mechanisms and underlies petroleum refining. The strongest of all is comparatively safe, since acidity and corrosiveness differ.