The scientific definition of acid strength focuses on a chemical’s ability to donate a proton, or a positively charged hydrogen ion. This proton donation ability changes dramatically depending on the environment in which the acid is placed. To find the most powerful acid, chemists must look beyond standard measurements used in water and explore the extreme edge of chemical reactivity.
Defining Acid Strength
In conventional chemistry, acid strength is determined by how completely it dissociates, or breaks apart, when dissolved in a solvent like water. This dissociation releases a proton into the solution, which then forms the hydronium ion. The pH scale, which measures the concentration of hydronium ions, is the most familiar metric for this strength, where a lower number indicates greater acidity.
However, the pH scale is limited to dilute aqueous solutions, and a value called pKa is used to compare the intrinsic strength of acids. The pKa value is the negative logarithm of the acid dissociation constant, where a smaller, or more negative, pKa means a stronger acid. This system works well for most common acids, but it hits a barrier when acids become extremely strong.
This barrier is known as the “leveling effect,” which is imposed by the solvent itself. Water is a weak base, and it reacts completely with any acid that is stronger than the hydronium ion. Because of this, acids like hydrochloric acid and nitric acid appear to have the same strength in water, even though their inherent ability to donate a proton is different. To measure the true power of the world’s strongest acids, chemists must use non-aqueous, non-basic solvents that do not “level” their strength.
The Realm of Superacids
Superacids are defined as any acid with an acidity greater than that of 100% pure sulfuric acid. Pure sulfuric acid has a Hammett acidity function value of approximately -12. This category of superacids allows chemists to explore a level of chemical reactivity that is inaccessible with common mineral acids.
Superacids are typically composed of a mixture of two components: a strong Brønsted acid and a strong Lewis acid. The Lewis acid component acts as an electron pair acceptor that binds to and stabilizes the anion formed when the Brønsted acid donates its proton. This stabilization prevents the anion from accepting the proton back, dramatically increasing the overall proton-donating power of the medium.
This combination creates a medium with a high chemical potential for the proton. Commercially available superacids like fluorosulfuric acid are already a thousand times stronger than sulfuric acid. The most potent superacids are created by mixing components like antimony pentafluoride with an already strong acid to achieve a high level of acidity.
Identifying the Most Powerful Acid
The acid widely accepted as the strongest measurable acid is Fluoroantimonic Acid, which is created by mixing hydrogen fluoride and antimony pentafluoride. The Hammett Acidity Function (\(H_0\)) is the specific scale used to measure the strength of these superacids, as the pKa and pH scales are insufficient for such extremes. Fluoroantimonic acid has an \(H_0\) value that can reach as low as -31.3, making it millions to billions of times stronger than pure sulfuric acid.
The source of this extreme strength lies in the stability and low nucleophilicity of its conjugate base, the hexafluoroantimonate anion. The antimony pentafluoride accepts a fluoride ion from the hydrogen fluoride, resulting in the formation of the stable hexafluoroantimonate anion. Because this anion is so stable, it has virtually no tendency to accept a proton back from the solution.
This stable anion allows the proton to exist in a reactive state, ready to be donated to almost any substance. The mixture is so potent that it can protonate, or add a proton to, even substances normally considered non-basic, such as hydrocarbons. Because it attacks glass, Fluoroantimonic Acid must be stored in containers made of polytetrafluoroethylene.
Practical Applications of Superacids
Superacids are used in certain areas of science and industry. Their ability to donate a proton is exploited in chemical synthesis to stabilize highly reactive chemical intermediates. Specifically, superacids are used to generate and stabilize carbocations, which are positively charged carbon species important in many organic reactions.
By creating a stable medium for these carbocations, chemists can study their structure and reactivity, allowing for the design of new synthetic pathways. In industrial catalysis, this protonating power facilitates reactions that are impossible with weaker acids. Superacids are used in the petrochemical industry for processes like the isomerization and alkylation of hydrocarbons, which are necessary for the production of high-octane gasoline and other petroleum products.