Sulfuric acid

Sulfuric acid, also known as oil of vitriol, is a very important mineral acid made from the elements sulfur, oxygen, and hydrogen. Its chemical formula is H₂SO₄. It is a clear, odorless liquid that mixes easily with water.

Pure sulfuric acid does not exist naturally because it quickly absorbs water from the air. It is very powerful and can break down many materials, including metals and rocks. When added to water, it releases a lot of heat, so it is important to always add the acid to water, not the other way around, to avoid dangerous splashing.

Sulfuric acid is made using several methods, such as the contact process, the wet sulfuric acid process, and the lead chamber process. It is used in many industries, especially for making fertilizer. It is also important for processing minerals, refining oil, treating wastewater, and creating many different chemicals. You can find it in products like drain cleaners and in the lead-acid batteries found in cars.

Physical properties

Sulfuric acid can be made in concentrations close to 100%, but when heated, it loses some of its components and settles at about 98.3%. This is the most common form used and stores better than very pure acid. Different strengths of sulfuric acid are used for different jobs.

Sulfuric acid looks like a clear, oily liquid. It mixes well with water and can carry electricity because its molecules can shift around easily, allowing it to conduct electric current. This makes it useful in many chemical processes.

Chemical properties

The mixing of sulfuric acid with water creates a lot of heat. Sulfuric acid is a strong acid, meaning it easily gives away its particles when mixed with water.

Concentrated sulfuric acid can take water away from other substances. For example, when mixed with table sugar, it turns the sugar black and creates a porous, carbon-like material. This happens because the acid removes water from the sugar.

Sulfuric acid also reacts with many metals, producing hydrogen gas and a salt. For example, when mixed with iron, it creates hydrogen gas and iron sulfate.

When mixed with certain salts, sulfuric acid can create new acids. For example, mixing it with sodium acetate creates acetic acid and sodium bisulfate.

Sulfuric acid can also be used to make hydrogen from water in a process called the sulfur–iodine cycle. This process needs a lot of heat to work and has been suggested as a way to produce hydrogen for energy use.

Occurrence

Sulfuric acid is rarely found in its pure form on Earth because it easily mixes with water. However, it can be found in small amounts in acid rain, which forms when sulfur dioxide from burning fuels mixes with water in the air.

Sulfuric acid also forms naturally when certain minerals, like pyrite, break down. This process can create very acidic water, called acid mine drainage. Some sea creatures, like a type of algae, even use sulfuric acid as a way to protect themselves.

In the stratosphere, the layer of air high above Earth, sulfuric acid forms from volcanic gases and helps create tiny particles that make up the stratospheric aerosol layer. On other planets, like Venus, clouds are made of sulfuric acid, and it has also been found on Jupiter's moon Europa.

Production

Main articles: Contact process, Wet sulfuric acid process, and Lead chamber process

Sulfuric acid is made from sulfur, oxygen, and water using two main methods: the contact process and the wet sulfuric acid process.

Contact process

Main article: Contact process

First, sulfur is burned to make sulfur dioxide. This gas is then changed into sulfur trioxide using oxygen and a special helper material. The sulfur trioxide is mixed with a strong sulfuric acid solution to make a substance called oleum. Adding water to oleum creates concentrated sulfuric acid.

Wet sulfuric acid process

Main article: Wet sulfuric acid process

In this method, sulfur is burned to produce sulfur dioxide, or hydrogen sulfide gas can be burned to make the same gas. The sulfur dioxide is changed into sulfur trioxide using oxygen and a helper material. The sulfur trioxide is then mixed with water to make sulfuric acid, which is collected as a liquid.

Other methods

Before 1900, most sulfuric acid was made using a different method called the lead chamber process. There are also special ways to make small amounts of sulfuric acid in a lab, often starting from sulfur dioxide. Some of these methods use electricity or special chemicals to create the acid.

Uses

Sulfuric acid is a very important chemical used in many industries. In 2022, the world made about 260 million tonnes of it. Most of it—about 60%—is used to make fertilizers, which help plants grow. Another 20% is used to make other chemicals like cleaning products, paints, and medicines. Some is also used to clean metals and make batteries work.

Sulfuric acid is especially important for making phosphoric acid, which is used in fertilizers. It is also used to clean steel and to help make certain plastics. In batteries, like car batteries, sulfuric acid helps store and release energy. It can also be found in some drain cleaners at home, but it must be used carefully because it can react strongly with water.

History

Vitriols

People have studied substances called vitriols — hydrated sulfates of different metals — since ancient times. The Sumerians listed types of vitriol by their color. Early Greek and Roman writers like Dioscorides and Pliny the Elder wrote about vitriol, and later, Galen discussed its medical uses. During the Middle Ages, Islamic scholars such as Jabir ibn Hayyan, Abu Bakr al-Razi, and Ibn Sina included vitriol in their studies of minerals.

Jabir ibn Hayyan, Abu Bakr al-Razi, Ibn Sina, et al.

Jabir ibn Hayyan and Abu Bakr al-Razi experimented with distilling different substances, including vitriols. One of al-Razi’s recipes might have created sulfuric acid without him realizing it. Later, an anonymous Latin work described a method to obtain an "oil" by distilling iron(II) sulfate, which was likely sulfuric acid. This work referenced Jabir ibn Hayyan’s writings.

Vincent of Beauvais, Albertus Magnus, and pseudo-Geber

Medieval European alchemists called sulfuric acid "oil of vitriol" because they made it by heating iron(II) sulfate. Early European mentions of it appear in the 13th century in works by Vincent of Beauvais, Albertus Magnus, and pseudo-Geber.

Producing sulfuric acid from sulfur

By the 16th century, people discovered a way to make sulfuric acid by burning sulfur under a glass bell. However, this method was inefficient and produced a less pure acid. In the 17th century, Johann Rudolf Glauber improved this by using saltpeter, which helped create more acid. In 1736, Joshua Ward began large-scale production of sulfuric acid using this improved method.

Lead chamber process

In 1746, John Roebuck developed a way to produce sulfuric acid in lead-lined chambers, making the process stronger, cheaper, and larger in scale. This method, called the lead chamber process, became the standard for almost two centuries.

Distillation of pyrite

Later improvements allowed the concentration of sulfuric acid to increase, but an even more concentrated form was needed for certain chemical processes. One way to achieve this was by heating pyrite to create iron(II) sulfate, which was then heated further to produce sulfuric acid.

Contact process

In 1831, Peregrine Phillips invented the contact process, a more economical way to produce sulfuric acid. Today, this is the most common method used worldwide. In the 19th century, vitriol was used in places like Prestonpans in Scotland and County Antrim in Northern Ireland to bleach linen, speeding up the process compared to using lactic acid from sour milk.

Safety

Sulfuric acid can cause very serious burns, especially when it is very strong. It can harm skin and eyes very quickly and might even make someone very sick if swallowed. It’s important to always use special protective gear when working with it.

When making weaker versions of sulfuric acid, you must always add the strong acid to water, not water to the acid. Adding water to strong acid can cause dangerous splashes or even an explosion because of the heat created. Always be very careful and follow safety steps to stay safe.

Legal restrictions

The trading of sulfuric acid internationally is monitored under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances, 1988. This agreement includes sulfuric acid in a list of chemicals that are often used to make illegal drugs.