What makes an acid strong or weak? To answer this question, we first need to look at the definition of an acid. It is a chemical compound that accepts electrons and/or donates (dissociate) hydrogen ions, also known as protons.
Therefore, the acidity levels of an acid depend on its ability to disassociate hydrogen ions, i.e., the higher the number of hydrogen ions produced by acid in a solution, the more acidic it is.
Now, before we get on with the list of the strongest acids on Earth, there are specific terms and definitions you first need to get familiar with.
Acid dissociation constant (Ka): Sometimes referred to as the acid-ionization constant or simply acid constant, Ka is the quantifiable strength of an acid in an aqueous solution. While pH (power of Hydrogen) specifies the level of basicity, the acidity of any solution (acid dissociation constant) tells us about the concentration of hydrogen ions [H+] or hydronium ions [H3O+] in a solution.
This brings us to another related and important acidity indicator pKa. It is basically a negative integer logarithm of Ka
pKa = -log10Ka.
The stronger the acid, the lower the pKa values.
Acetic acid donates a proton (in green) to water to produce hydronium ion and acetate ion. (Oxygen in red, hydrogen is white and carbon in black)
Hammett acidity function (Ho): We all are aware of the pH scale, which is widely used to measure acidity or basicity levels of chemicals, but when it comes to superacids, it simply becomes useless, since their acidity levels are million times more than sulfuric acid and hydrochloric acid.
So in order to scale the superacids based on their acidity levels, researchers came up with Hammett acidity function. It was initially suggested by American physical chemist Louis Plack Hammett.
But what exactly is superacid?
A superacid is simply an acid with an acidity level of more than that of 100% sulfuric acid with Hammett acidity function lower than -12. In more technical terms, it can be defined as a medium in which the chemical potential of the proton is higher than that found in pure sulfuric acid.
8. Sulfuric Acid
Sulfuric acid (98%) on a piece of paper
Chemical Formula: H2SO4
pKa value: -3
Ho value: 12
Sulfuric acid or vitriol doesn’t need any formal introduction. It is odorless, colorless, and produces an intense exothermic reaction when mixed with water. It is essential for multiple industries like agriculture, wastewater treatment, and oil refining. Sulfuric acid is also used in battery acids and cleaning agents.
It also has a vital role in the study of acids as a whole. Sulfuric acid serves as a base reference to compare the acidity levels of superacids or acids. While there are several methods to produce sulfuric acid, the contact process, and wet sulfuric acid process are the ones that are generally used.
H2SO4 can cause extensive damages to human skin when it comes to direct contact. It is also highly corrosive to many metals. The chemical is much more corrosive and dangerous when present in high concentrations due to its superior oxidizing and dehydrating properties.
7. Hydrochloric acid
Chemical Formula: HCl
pKa value: -5.9
Similar to sulfuric acid, hydrochloric acid is also an important chemical that is widely used in labs and various industries. Hydrochloric acid was discovered sometime around 800 AD by an Iranian polymath named Jabir ibn Hayyan.
Those who are wondering why hydrochloric acid is stronger than sulfuric acid despite the latter being a reference point for superacids, this is because the sulfuric acid is a diprotic acid, which usually doesn’t dissociate completely.
In other words, HCl is stronger than sulfuric acid since its (HCl) hydrogen ions are easily separated from chloride as compared to the sulfate ion from sulfuric acid.
Anyway, hydrochloric is largely used in heavy industries to remove rust from iron and steel before further processing. Moreover, it is a vital component in the production of organic (vinyl chloride used for PVC) and many inorganic compounds.
6. Triflic Acid
Chemical Formula: CF3SO3H
pKa value: -14.7
Trifluoromethanesulfonic acid, most commonly known as triflic acid, was first synthesized/discovered by Robert Haszeldine, a British chemist back in 1954. It is known for its remarkable chemical and thermal stability. While other strong acids like nitric and perchloric acids are susceptible to oxidation, triflic acid is not.
Triflic acid is used in many protonations and titrations (quantitative analysis of a chemical’s composition). An important reason why triflic acid is preferred in certain cases is that it doesn’t sulfonate other substances, which is common with chlorosulfonic acid and sulfuric acid.
Needless to say, it’s extremely dangerous. Any skin contact with the acid can cause severe burns and can have slightly delayed tissue damage. It can also cause pulmonary edema and spasms and other critical conditions when inhaled.
5. Fluorosulfuric Acid
Chemical Formula: HSO3F
Ho value: -15.1
pKa value: -10
Fluorosulfuric acid or sulfurofluoridic acid (official name) is the second strongest single-component acid available today. It’s yellow in appearance and of course highly corrosive/toxic. HSO3F is generally produced by reacting hydrogen fluoride with sulfur trioxide, and when combined with antimony pentafluoride, it produces “Magic acid,” a far stronger acid and protonating agent.
The acid can be used to alkylate hydrocarbons (with alkenes) and isomerize alkanes, and for glass etching (glass art). It’s a common fluorinating agent in laboratories.
4. Perchloric acid
60% Perchloric acid | Image Courtesy: W. Oelen
Chemical Formula: HClO4
pKa value: -10, -15.2
Perchloric acid is among the strongest known Brønsted–Lowry acids, which have powerful oxidizing properties and is highly corrosive. Traditionally, it’s produced by treating sodium perchlorate with hydrochloric acid (HCl), which also creates sodium chloride.
NaClO4 + HCl → NaCl + HClO4
Unlike other acids, perchloric acid is not vulnerable to hydrolysis. It is also one of the most regulated acids in the world. Back in 1947, in Los Angeles, California, about 150 people were injured, and 17 died due to a chemical explosion that contained nearly 75% perchloric acid (by volume) and 25% acetic anhydride. It also damaged more than 250 nearby buildings and vehicles.
Despite its explosive nature, perchloric acid is widely used and even preferred in certain types of syntheses. It is also an important component of Ammonium perchlorate, which is used in modern rocket fuel.
3. Fluorinated Carborane acid
The generic structure of a carborane acid
Chemical Formula: H(CHB11F11)
Ho value: -18
pKa value: -20
Carborane acids are one of the strongest groups of superacids known to humans, few of which are considered to have a Hammett acidity function value as low as -18, that’s more than a million times stronger acidity level than pure (100%) sulfuric acid.
One such member of this group is Fluorinated Carborane acid. While the existence of such a chemical was initially reported in 2007, researchers were able to study the full extent of its nature only in 2013. Before its discovery, the crown of strongest Brønsted Acid went to a highly chlorinated version of this superacid family.
Fluorinated Carborane is the only known acid that can protonate (transfer of hydrogen ion) carbon dioxide to produce Hydrogen-bridged cations. In contrast, CO2 doesn’t go through any remarkable protonation when treated with other superacids like Magic acid and HF-SbF5.
2. Magic Acid
Fluorosulfuric acid+antimony pentafluoride (1:1)
Chemical Formula: FSO3H·SbF5
Ho value: -23
FSO3H·SbF5, most commonly known as Magic acid, is produced by mixing fluorosulfuric acid and antimony pentafluoride in a molar ratio of 1:1. This superacid system was first developed in 1966 by researchers at the George Olah lab, Case Western Reserve University in Ohio.
Its rather fancy name was established after a formal event in 1966, where a member of the Olah lab demonstrated a hydrocarbon protonation in which a paraffin candle got “magically” dissolved and changed into a tert-butyl cation solution after it was placed on top of what is now known as the magic acid.
While Magic acid is generally used to stabilize carbonium ions in solutions, it has few other important industrial applications. For example, it can accelerate the isomerization of saturated hydrocarbons and even protonate methane, xenon, and halogens, all of which are weak bases.
1. Fluoroantimonic acid
The molecular structure of Fluoroantimonic acid
Chemical Formula: H2FSbF6
Hovalue: -15 (pure form), -28 (with >50 mol%)
Fluoroantimonic acid is perhaps the strongest of all known superacids based upon the Hammett acidity function values. It’s produced by mixing hydrogen fluoride with antimony pentafluoride generally in a 2:1 ratio. This reaction is exothermic in nature.
This superacid has several essential applications in the chemical engineering and petrochemical industry. For example, it can be used to separate methane and H2 from neopentane and isobutane (both alkane), respectively.
Unsurprisingly, H2FSbF6 is extremely corrosive and can go through violent hydrolysis, when in contact with water. Like most superacids, Fluoroantimonic acid can eat right through the glass; therefore, it must be stored in PTFE (polytetrafluoroethylene) containers.
Now, most of you might have stumbled upon carborane acids (either chlorinated carborane acid or fluorinated Carborane acid) when searching for “the strongest acids in the world.”
Well, technically, they are correct since carborane acids are the strongest known single-component acids on Earth, far more acidic than the likes of perchloric and triflic acid. (fluoroantimonic acid is actually a mixed acid).
Frequently Asked Questions
What are weak acids?
Weak acids do not completely dissociate into their constituent ions in water or aqueous solution. They have higher pH values (between 2 and 7) and are much more common than strong acids.
They feel sticky, taste sour, and sometimes burn the nostrils when smelled. While they are mostly encountered in low concentrations, the high concentration of weak acids can be corrosive and even dangerous.
Acetic acid (vinegar), hydrocyanic acid, hydrogen sulfide, formic acid, and trichloracetic acid are some of the most common examples of weak acids. In fact, they are a common substance in everyday life — they may be in the vitamins you take, the food you eat, or in the cleaning supplies you use.
What determines the strength of an acid?
The two major factors that determine the strength of acid are:
- The inductive effect: arises due to the difference in electronegativity of atoms bonded together. In organic carboxylic acids, the electronegative substituent pulls the electron density of the acidic bond through this effect, resulting in a smaller pKa value.
- Effect of oxidation state: In oxygen-containing acids, the pKa value reduces with the oxidation state of the element.
Why do aqueous solutions of acid conduct electricity, and pure acids do not?
Acids quickly dissociate into positive and negative ions when dissolved in water or aqueous solution. HCl acid, for example, dissociates into H+ can Cl- ions. The charged particles in the solution (H+ ions) make it much easier to transmit a current through a medium.
HCl(aq) –> H+(aq) + Cl–(aq)
More specifically, when electricity is passed through the aqueous solution, each H+ ion picks up one electron from the cathode to form H2 gas. That’s how the aqueous solution of an acid allows electricity to flow through it.
Undissolved or pure acids, on the other hand, are similar to pure water. They can transmit a charge but the resistance is quite high, especially in the solid-state.