The question of the strongest acid often conjures images of laboratory mainstays like hydrochloric or sulfuric acid. While these conventional acids are powerful, their strength is limited by the solvent in which they are typically measured: water. Acids far exceeding these common chemicals exist, requiring a specialized measurement system designed for extreme chemical environments, moving beyond the familiar pH scale.
Understanding How Acid Strength is Measured
The conventional method for measuring acid strength relies on the concentration of hydrogen ions, expressed on the pH scale. This scale works well for acids dissolved in water, but it is inadequate for extremely powerful substances due to the leveling effect. Water acts as a proton acceptor; any acid stronger than the hydronium ion (\(\text{H}_3\text{O}^+\)) reacts completely with water to form hydronium ions. This reaction makes all very strong acids appear to have the same strength in aqueous solution, effectively “leveling” their true power.
To accurately compare acids stronger than pure sulfuric acid, chemists use the Hammett acidity function, denoted as \(H_0\). This function provides a quantitative measure for concentrated or non-aqueous acidic media. The \(H_0\) value is determined by measuring the acid’s ability to protonate a series of very weak organic indicator bases in a non-leveling solvent. Pure, 100% sulfuric acid has an \(H_0\) value of approximately \(-12\), which serves as the benchmark; any acid with a more negative \(H_0\) value is classified as a superacid.
Identifying the World’s Strongest Superacids
Superacids are defined as any acidic medium with a protonating power greater than that of 100% sulfuric acid. The strongest measurable superacid is Fluoroantimonic Acid (\(\text{HSbF}_6\)), a mixture of hydrogen fluoride (\(\text{HF}\)) and antimony pentafluoride (\(\text{SbF}_5\)). Its \(H_0\) value reaches as low as \(-28\), meaning the mixture is many quintillion times stronger than pure sulfuric acid.
The mechanism behind this extreme acidity involves the stability of the resulting anion, hexafluoroantimonate (\(\text{SbF}_6^-\)). When the components are mixed, antimony pentafluoride, a powerful Lewis acid, bonds tightly with the fluoride ion from hydrogen fluoride. This creates the large and stable \(\text{SbF}_6^-\) anion, which is extremely weakly coordinating. Because the negative charge is delocalized over the six fluorine atoms, this anion has very little tendency to recapture the proton.
This process allows the proton to exist almost entirely free, or solvated as a fluoronium ion (\(\text{H}_2\text{F}^+\)). This ability to donate a proton makes the acid so powerful. Fluoroantimonic acid is significantly stronger than other notable superacids, such as Magic Acid (\(\text{FSO}_3\text{H}:\text{SbF}_5\)), which exhibits an \(H_0\) value around \(-23\). While Fluoroantimonic Acid is the strongest known mixture, carborane acids contain some of the strongest known single-molecule acids.
Industrial Applications and Handling Extremes
Superacids serve a function in modern chemical engineering and organic synthesis. Their ability to protonate molecules resistant to acid, such as hydrocarbons, makes them effective catalysts. In the petrochemical industry, superacids drive processes like alkylation and isomerization, necessary to produce high-octane gasoline. They are also used to stabilize highly reactive chemical intermediates called carbocations, which are then used in the synthesis of complex organic compounds.
Handling these substances requires specialized equipment and rigorous safety protocols, as they can dissolve glass and nearly all organic compounds. Superacids are stored in containers made of fluoropolymers, most commonly Teflon (polytetrafluoroethylene or PTFE). These materials are resistant because their strong carbon-fluorine bonds are not easily attacked by the acid’s free-floating protons. When working with superacids that generate hydrogen fluoride gas, safety measures must include having calcium gluconate gel immediately available to treat severe burns from fluoride ions.