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 (dissociates) 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 is in red, hydrogen in white, and carbon in black)
Hammett acidity function (Ho): We’re familiar with the pH scale, commonly used to gauge the acidity or basicity of substances. However, when dealing with superacids, the pH scale becomes irrelevant because their acidity levels exceed those of sulfuric acid and hydrochloric acid by millions of times.
So, in order to scale the superacids based on their acidity levels, researchers came up with the 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 a Hammett acidity function lower than -12. In 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.
Table of Contents
9. 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 and 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.
There are various methods to produce sulfuric acid, but the most commonly used ones are the contact process and the wet sulfuric acid process.
H2SO4 can cause extensive damage 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.
8. 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 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.
7. 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.
It goes without saying that sulfuric acid is extremely hazardous. Even slight contact with the skin can result in severe burns and delayed tissue damage. Inhaling sulfuric acid can lead to pulmonary edema, spasms, and other serious conditions.
6. Hydrobromic Acid
Hydrobromic acid is a powerful acid formed in water by dissolving bromides of hydrogen. Hydrobromic acid is a diatomic molecule with the chemical formula for hydrobromic acid is HBr. Hydrogen bromide is a colorless, suffocating gas, very soluble in water and highly dissociated .. pic.twitter.com/TtBqrbK0AX
— Pascal Scientific Ltd (@DelsonScientifi) July 7, 2023
Chemical Formula: HBr
pKa value: -9
Hydrobromic acid is a strong acid in aqueous solution. It dissociates almost completely into hydrogen ions and bromide ions when dissolved in water. It has a strong and distinctive odor that serves as a warning sign of its presence.
The acid is commonly used in the production of organic compounds containing bromine, such as pharmaceuticals, pesticides, and flame retardants.
It is often used for cleaning and etching metals, especially in industries where precise surface treatments are required. It may be utilized in processes like metal surface preparation or circuit board manufacturing.
The global Hydrobromic Acid market stood at nearly 1.28 million tonnes in 2022, and it is expected to grow at a CAGR of 6.7% from 2023 to 2032.
5. Fluorosulfuric Acid
Chemical Formula: HSO3F
Ho value: -15.1
pKa value: -10
Fluorosulfuric acid, also known as sulfurofluoridic acid, is the second strongest single-component acid available today. Its appearance is yellow, and it’s highly corrosive and 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.
This acid is utilized to alkylate hydrocarbons, typically with alkenes, and to isomerize alkanes. It’s also employed for glass etching, particularly in glass art. Additionally, it serves as a common fluorinating agent in laboratory settings.
4. Perchloric acid
60% Perchloric acid | Image Courtesy: W. Oelen
Chemical Formula: HClO4
pKa value: -10, -15.2
Perchloric acid ranks among the strongest Brønsted–Lowry acids and possesses potent oxidizing properties, making it highly corrosive. Typically, it’s produced by treating sodium perchlorate with hydrochloric acid (HCl), resulting in the formation of both perchloric acid and 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. Some of them have an incredibly low Hammett acidity function value, as low as -18, indicating acidity levels over a million times stronger 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 the strongest Brønsted Acid belonged 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
Fluoroantimonic acid is about a billion times stronger than 100% pure sulfuric acid. It’s too dangerous to handle pure so it’s diluted with hydrofluoric acid (HF).
“Diluted with hydrofluoric acid” is not a phrase you really expect to hear, but that’s fluoroantimonic acid for you pic.twitter.com/dQryCQN8fM
— Ash Jogalekar (@curiouswavefn) April 30, 2023
Chemical Formula: H2FSbF6
Hovalue: -15 (pure form), -28 (with >50 mol%)
Fluoroantimonic acid is arguably the strongest of all known superacids, as indicated by its Hammett acidity function values. It is produced by mixing hydrogen fluoride with antimony pentafluoride, typically 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.
Many of you may have encountered carborane acids, either chlorinated or fluorinated, while researching “the strongest acids in the world.”
Technically, this is accurate because carborane acids are indeed the most potent known single-component acids on Earth, surpassing the acidity of perchloric and triflic acid by far. (It’s worth noting that 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 (found in vinegar), hydrocyanic acid, hydrogen sulfide, formic acid, and trichloroacetic acid are among the most well-known examples of weak acids. These acids are commonly encountered in daily life, present in items such as the vitamins you consume, the food you eat, and the cleaning products 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.
How do strong acids differ from concentrated acids?
While the term “strong acid” refers to the degree of ionization in solution, “concentrated acid” means the amount of acid present in a solution.
A concentrated acid may be strong or weak, depending on its degree of ionization. For example, concentrated hydrochloric acid is both strong and concentrated, whereas concentrated acetic acid is weak but concentrated.
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.