Oleum

Oleum
Identifiers
CAS number	[8014-95-7]
UN number	1831
RTECS	WS5605000
ChemSpider	23079
Properties
Chemical formula	H2SO4·xSO3
Molar mass	variable
Melting point	2 ºC (20% SO3) 21 ºC (30% SO3) 5 ºC (65% SO3)
Boiling point	138 ºC (20% SO3) 116 ºC (30% SO3) 60 ºC (65% SO3)
Hazards
Material safety data sheet (MSDS)	ICSC 1447
EU index number	016-019-00-2
GHS pictograms
GHS signal word	DANGER
GHS hazard statements	H314, H335
Flash point	non-flammable
Related compounds
Other compounds	Sulfuric acid Disulfuric acid Sulfur trioxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Oleum (Latin oleum = "oil"), or fuming sulfuric acid refers to a solution of various compositions of sulfur trioxide in sulfuric acid or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid).

Oleums can be described by the formula ySO₃·H₂O where y is the total molar sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H₂SO₄·xSO₃ where x is now defined as the molar free sulfur trioxide content. Oleum is generally assayed according to the free SO₃ content by weight. A value of x = 1 gives the empirical formula H₂S₂O₇ for disulfuric acid. Pure disulfuric acid itself is a solid at room temperature, melting at 35 °C, and rarely used either in the laboratory or industrial processes.

Production

Oleum is produced in the contact process, where sulfur is oxidized to sulfur trioxide which is subsequently dissolved in concentrated sulfuric acid. Sulfuric acid itself is regenerated by dilution of part of the oleum.

The lead chamber process for sulfuric acid production was abandoned partly because it could not produce sulfur trioxide or concentrated sulfuric acid directly due to corrosion of the lead, and absorption of SO₂ gas. Until this process was made obsolete by the contact process, oleum had to be obtained through indirect methods. Historically, the biggest production of oleum came from the distillation of iron sulfates at Nordhausen in the Harz Mountains in central Germany, from which the historical name Nordhausen sulfuric acid is derived.

Applications

Sulfuric acid production

Oleum is an important intermediate in the manufacture of sulfuric acid due to its high enthalpy of hydration. When SO₃ is added to water, rather than simply dissolving, it tends to form a fine mist of sulfuric acid, which is difficult to manage. However, SO₃ added to concentrated sulfuric acid readily dissolves, forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid.

As an intermediate for transportation

Oleum is a useful form for transporting sulfuric acid compounds, typically in rail tankcars, between oil refineries (which produce various sulfur compounds as a byproduct of refining) and industrial consumers.

Certain compositions of oleum are solid at room temperature, and thus is safer to ship than when liquid. Solid oleum can then be converted into liquid at the destination through steam heating or dilution or concentration. This requires some care to prevent overheating and evaporation of sulfur trioxide. To extract it from a tank car requires careful heating using steam conduits within the tank car. Great care must be taken to avoid overheating, as this can increase the internal pressure within the tank car to a value exceeding the limit of the tank's safety valve. In Richmond, California in 1993 a significant release occurred due to overheating, causing a release of sulfur trioxide that absorbed moisture from the atmosphere, creating a mist of micrometre-sized sulfuric acid particles that formed an inhalation health hazard. This mist produced adverse health effects on residents and workers over a wide area.

Organic chemistry

Oleum is a harsh reagent, and is highly corrosive. One important use of oleum as a reagent is the secondary nitration of nitrobenzene. The first nitration can occur with nitric acid in sulfuric acid, but this deactivates the ring towards further electrophilic substitution. A stronger reagent, oleum, is needed to introduce the second nitro group onto the aromatic ring.

Explosives manufacture

Oleum is used in the manufacture of many explosives, with the notable exception of nitrocellulose. The chemical requirements for explosives manufacture often call for anhydrous mixtures containing nitric acid and sulfuric acid. Ordinary commercial grade nitric acid consists of the constant boiling azeotrope of nitric acid and water, and contains 68% nitric acid. Mixtures of ordinary nitric acid in sulfuric acid therefore contain substantial amounts of water and are unsuitable for processes such as occur in the manufacture of trinitrotoluene.

Anhydrous nitric acid, referred to as white fuming nitric acid, can be used to prepare water-free nitration mixtures, and this method is used in laboratory scale operations where the cost of material is not of primary importance. Fuming nitric acid is very hazardous to handle and transport, because it is extremely corrosive and volatile. For industrial use, such strong nitration mixtures are prepared by mixing oleum with ordinary commercial nitric acid so that the free sulfur trioxide in the oleum consumes the water in the nitric acid.

Reactions

Like concentrated sulfuric acid, oleum is such a strong dehydrating agent that if poured onto powdered glucose, or virtually any other sugar, it will draw the elements of water out of the sugar in an exothermic reaction, leaving nearly pure carbon as a solid. This carbon expands outward, hardening as a solid black substance with gas bubbles in it.