Sulfuric acid (British English: sulphuric acid), H2SO4, is a strong mineral acid. It is soluble in water at all concentrations. The old name for sulfuric acid was oil of vitriol. When high concentrations of SO3(g) are added to sulfuric acid, H2S2O7 forms. This is called fuming sulfuric acid or oleum or, less commonly, Nordhausen acid.
Sulfuric acid has many applications, including in many chemical reactions and production processes. It is the most widely used industrial chemical. Principal uses include fertilizer manufacturing, ore processing, chemical synthesis, wastewater processing and oil refining.
The hydration reaction of sulfuric acid is highly exothermic. If water is added to concentrated sulfuric acid, it can boil. Always add the acid to the water rather than the water to the acid. (One mnenomic, which depends on a joking mispronounciation, goes: "Do as you oughta: add acid to water.") Note that part of this problem is due to the relative densities of the two liquids. Water is less dense than sulfuric acid and will tend to float above the acid.
Because the hydration of sulfuric acid is thermodynamically favorable, sulfuric acid is an excellent dehydration agent, and is used to prepare many dried fruits.
The affinity of sulfuric acid for water is sufficiently strong that it will take hydrogen and oxygen molecules out of other compounds; for example, mixing glucose (C6H12O6) and concentrated sulfuric acid will give elemental carbon and water which is absorbed by the sulfuric acid (which becomes slightly diluted): C6H12O6 → 6C + 6H2O.
When in the atmosphere it is part of many chemicals which make up acid rain.
History of sulfuric acid
The discovery of sulfuric acid is credited to the 9th century Islamic physician and alchemist Ibn Zakariya al-Razi (Rhases), who obtained the subtance by dry distillation of minerals including iron (II) sulfate heptahydrate, FeSO4 • 7H2O, called green vitriol, and copper(II) sulfate pentahydrate, CuSO4 • 5H2O, called blue vitriol. When heated, these compounds decompose to iron(II) oxide and copper(II) oxide, respectively, giving off water and sulfur trioxide, which combine to produce a dilute solution of sulfuric acid. This method was popularized in Europe through translations of Islamic treatises and books by European alchemists, such as the 13th-century German Albertus Magnus. For this reason, sulfuric acid was known to medieval European alchemists as oil of vitriol and spirit of vitriol, among other names.
In the 17th century, the German-Dutch chemist Johann Glauber prepared sulfuric acid by burning sulfur together with saltpeter (potassium nitrate, KNO3), in the presence of steam. As the saltpeter decomposes, it oxidizes the sulfur to SO3, which combines with water to produce sulfuric acid. In 1736, Joshua Ward, a London pharmacist, used this method to begin the first large-scale production of sulfuric acid.
In 1746 in Birmingham, John Roebuck began producing sulfuric acid this way in lead-lined chambers, which were stronger, less expensive, and could be made larger than the glass containers which had been used previously. This lead chamber process allowed the effective industrialization of sulfuric acid production, and with several refinements remained the standard method of production for almost two centuries.
John Roebuck's sulfuric acid was only about 35–40% sulfuric acid. Later refinements in the lead-chamber process by the French chemist Joseph-Louis Gay-Lussac and the British chemist John Glover improved this to 78%. However, the manufacture of some dyes and other chemical processes require a more concentrated product, and throughout the 18th century, this could only be made by dry distilling minerals in a technique similar to the original alchemical processes. Pyrite (iron disulfide, FeS2) was heated in air to yield iron (II) sulfate, FeSO4, which was oxidized by further heating in air to form iron(III) sulfate , Fe2(SO4)3, which when heated to 480 °C decomposed to iron(III) oxide and sulfur trioxide, which could be passed through water to yield sulfuric acid in any concentration. The expense of this process prevented the large-scale use of concentrated sulfuric acid.
Replacement by the Contact Process
In 1831, the British vinegar merchant Peregrine Phillips patented a far more economical process for producing sulfur trioxide and concentrated sulfuric acid, now known as the Contact Process. Most of the world's supply of sulfuric acid is produced by it.
Sulfuric acid is one of the few chemicals whose formula is widely known by the lay public, at least in the United States—thanks to this jingle:
- Little Johnny took a drink
- but he shall drink no more.
- For what he thought was H2O
- Was H2SO4.
Another (similar) one goes:
- Little Johnny was a chemist
- Little Johnny is no more
- For what he thought was H2O
- Was really H2SO4.