Sulfuric acid
Sulfuric (or sulphuric) acid, H2SO4, is a strong mineral acid. It is soluble in water at all concentrations. It was once known as oil of
vitriol, coined by the 8th-century Muslim alchemist Jabir ibn Hayyan (Geber) after his discovery of the chemical. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in
2001 was 165 million tonnes, with an approximate value of US$8 billion. Principal uses include ore processing, fertilizer manufacturing, oil refining, wastewater processing, and chemical synthesis.
Many proteins are made of
sulfur-containing amino acids (such as cysteine and methionine) which produce sulfuric acid when metabolized by the body.
Manufacture
Sulfuric acid is produced from sulfur, oxygen and water via the contact process.
Physical properties
Forms of sulfuric acid
Although nearly 100% sulfuric acid can be made, this loses SO3 at the boiling point to produce 98.3% acid. The
98% grade is more stable in storage, and is the usual form of what is described as concentrated sulfuric acid. Other concentrations are used for different purposes. Some common concentrations are
* 10%, dilute sulfuric acid for laboratory use,
* 33.5%, battery acid (used in lead-acid batteries),
* 62.18%, chamber or fertilizer acid,
* 77.67%, tower or Glover acid,
* 98%, concentrated acid.
Different purities are also available. Technical grade H2SO4 is impure and often colored, but is suitable for making
fertilizer. Pure grades such as US Pharmacopoeia (USP) grade are used for making pharmaceuticals and dyestuffs.
Uses
Sulfuric acid production in 2000
Sulfuric acid is a very important commodity chemical, and indeed, a nation's sulfuric
acid production is a good indicator of its industrial strength. The major use (60% of total production worldwide) for sulfuric acid is in the "wet method" for the production of phosphoric acid, used for manufacture of
phosphate fertilizers as well as trisodium phosphate for detergents. In this method, phosphate rock is used, and more than 100 million tonnes are processed annually. This raw material is shown below as fluorapatite, though the
exact composition may vary. This is treated with 93% sulfuric acid to produce calcium sulfate, hydrogen fluoride (HF) and phosphoric acid. The HF is removed as hydrofluoric acid. The overall process can be represented as:
Ca5F(PO4)3 + 5 H2SO4 + 10 H2O ¥ 5 CaSO4•2 H2O + HF + 3 H3PO4.
Sulfuric acid is used in large quantities by the iron and steelmaking industry to remove oxidation, rust and scale from rolled sheet and
billets prior to sale to the automobile and white-goods industry. Used acid is often recycled using a Spent Acid Regeneration (SAR) plant. These plants combust spent acid with natural gas, refinery gas, fuel oil or other fuel
sources. This combustion process produces gaseous sulfur dioxide (SO2) and sulfur trioxide (SO3) which are then used to manufacture "new" sulfuric acid. SAR plants are common additions to metal smelting plants, oil
refineries, and other industries where sulfuric acid is consumed in bulk, as operating a SAR plant is much cheaper than the recurring costs of spent acid disposal and new acid purchases.
Ammonium sulfate, an important
nitrogen fertilizer, is most commonly produced as a byproduct from coking plants supplying the iron and steel making plants. Reacting the ammonia produced in the thermal decomposition of coal with waste sulfuric acid allows the
ammonia to be crystallized out as a salt (often brown because of iron contamination) and sold into the agro-chemicals industry.
Another important use for sulfuric acid is for the manufacture of aluminum sulfate, also known
as paper maker's alum. This can react with small amounts of soap on paper pulp fibers to give gelatinous aluminum carboxylates, which help to coagulate the pulp fibers into a hard paper surface. It is also used for making aluminum
hydroxide, which is used at water treatment plants to filter out impurities, as well as to improve the taste of the water. Aluminum sulfate is made by reacting bauxite with sulfuric acid:
Al2O3 + 3 H2SO4 ¥ Al2(SO4)3 + 3 H2O.
Sulfuric acid is used for a variety of other purposes in the chemical industry. For example, it is the usual acid catalyst for the conversion of cyclohexanoneoxime to
caprolactam, used for making nylon. It is used for making hydrochloric acid from salt via the Mannheim process. Much H2SO4 is used in petroleum refining, for example as a catalyst for the reaction of isobutane with isobutylene to
give isooctane, a compound that raises the octane rating of gasoline (petrol). Sulfuric acid is also important in the manufacture of dyestuffs solutions and is the "acid" in lead-acid (car) batteries.
Sulfuric acid is also used as a general dehydrating agent in its concentrated form. See Reaction with water.
Safety
Laboratory hazards
The corrosive properties of sulfuric acid are accentuated by its highly exothermic reaction with water. Hence
burns from sulfuric acid are potentially more serious than those of comparable strong acids (e.g. hydrochloric acid), as there is additional tissue damage due to dehydration and particularly due to the heat liberated by the
reaction with water; i.e. secondary thermal damage. The danger is obviously greater with more concentrated preparations of sulfuric acid, but it should be remembered that even the normal laboratory "dilute" grade (approx.
1 M, 10%) will char paper by dehydration if left in contact for a sufficient while. Solutions equal to or stronger than 1.5 M should be labeled CORROSIVE, while solutions greater than 0.5 M but less than 1.5 M should be labeled
IRRITANT. Fuming sulfuric acid (oleum) is not recommended for use in schools due to it being quite hazardous. The standard first aid treatment for acid spills on the skin is, as for other corrosive agents, irrigation with large
quantities of water: Washing should be continued for at least ten to fifteen minutes in order to cool the tissue surrounding the acid burn and to prevent secondary damage. Contaminated clothing must be removed immediately and the
underlying skin washed thoroughly.
Preparation of the diluted acid can also be dangerous due to the heat released in the dilution process. It is essential that the concentrated acid is added to water and not the other way
round, to take advantage of the relatively high heat capacity of water. Addition of water to concentrated sulfuric acid leads at best to the dispersal of a sulfuric acid aerosol, at worst to an explosion. Preparation of solutions
greater than 6 M (35%) in concentration is the most dangerous, as the heat produced can be sufficient to boil the diluted acid: efficient mechanical stirring and external cooling (e.g. an ice bath) are essential.
Industrial hazards
Although sulfuric acid is non-flammable, contact with metals in the event of a spillage can lead to the liberation of hydrogen gas. The dispersal of acid aerosols and gaseous sulfur dioxide is an additional hazard of fires
involving sulfuric acid.
Sulfuric acid is not considered toxic besides its obvious corrosive hazard, and the main occupational risks are skin contact leading to burns (see above) and the inhalation of aerosols. Exposure to
aerosols at high concentrations leads to immediate and severe irritation of the eyes, respiratory tract and mucous membranes: this ceases rapidly after exposure, although there is a risk of subsequent pulmonary edema if tissue
damage has been more severe. At lower concentrations, the most commonly reported symptom of chronic exposure to sulfuric acid aerosols is erosion of the teeth, found in virtually all studies: indications of possible chronic damage
to the respiratory tract are inconclusive as of 1997. In the United States, the permissible exposure limit (PEL) for sulfuric acid is fixed at 1 mg/m³: limits in other countries are similar. Interestingly there have been reports of
sulfuric acid ingestion leading to vitamin B12 deficiency with subacute combined degeneration. The spinal cord is most often affected in such cases, but the optic nerves may show demyelination, loss of axons and gliosis.
Legal controls and regulation
International commerce of sulfuric acid is controlled under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances, 1988, which lists
sulfuric acid under Table II of the convention as a chemical frequently used in the illicit manufacture of narcotic drugs or psychotropic substances.
In the United States of America, sulfuric acid is included in List II of
the list of essential or precursor chemicals established pursuant to the Chemical Diversion and Trafficking Act. Accordingly, transactions of sulfuric acid—such as sales, transfers, exports from and imports to the United States—are
subject to regulation and monitoring by the Drug Enforcement Administration. |
