Iron

Definition

Purpose

Description

Precautions

Interactions

Complications

Parental concerns

Resources

Definition

Iron (Fe) is a metal essential to almost all bacteria, plants, and animals. In humans, iron is a compnent of the red pigment hemoglobin that gives red blood cells their color and affects the transport of oxygen throughout the body, conversion of nutrients into energy, production of new deoxyribonucleic acid (DNA, genetic material), and regulation of cell growth and cell differentiation. Without iron, life on Earth would not exist. Humans must acquire all the iron they need from diet.

Iron

(Illustration by GGS Information Services/Thomson Gale.)

Purpose

Most iron in the body is used to transport oxygen. Oxygen is carried in red blood cells through the circulatory system to all cells in the body. Hemoglobin is the protein within red blood cells that makes this possible, and iron is at the center of the hemoglobin molecule. An average-size adult man has about 4 grams of iron in his body, and an adult woman has about 3.5 grams. Approximately two-thirds of this iron is in hemoglobin. Myoglobin, a protein in muscle, also contains iron. Myoglobin provides short-term storage for oxygen. When muscles do work, this oxygen is released to meet the increased metabolic needs of muscle cells.

Iron is found in every cell in the body, including brain cells. It is needed to synthesize adenosine tri-phosphate (ATP), the compound that supplies most of the energy to drive cellular metabolism . Iron is also used in enzyme reactions that create new DNA, and in this way it affects cell division and differentiation. Iron is also essential to other enzyme reactions that break down potentially harmful molecules formed when immune system cells attack bacteria.

Description

Plants absorb iron from the earth, and humans acquire iron through eating both plants and animals. In the stomach, acid in gastric juice acts on iron and changes it into a form that the body can absorb. Absorption takes place mainly in the first part of the small intestine (the duodenum). Once iron is absorbed into the bloodstream, it binds to a protein called trans-ferrin and is carried to all parts of the body, including the bone marrow where new red blood cells are made. Once in the cells, some iron is transferred to ferritin, a protein that holds the iron in reserve. When too much iron is absorbed, there is not enough transferrin to bind all of it. Free iron can build up in cells and trigger activities that cause damage and create health problems. Too little iron interferes with the body’s ability to get enough oxygen.

Sources of iron

The body has complex mechanisms to achieve iron balance by regulating iron absorption, reuse, and storage processes. Red blood cells live about 120 days. When they die, most of the iron in hemoglobin is recycled in the liver and sent to the bone marrow where it reused in new red blood cells. As a result, humans lose only a small amount of iron daily.

Only about 10–20% of the iron in food, or 1–2 mg for every 10 mg eaten, is absorbed into the bloodstream. Under normal conditions, when iron stores in the body are low, more iron is automatically absorbed. When they are high, less is absorbed. Iron that is not absorbed enters cells that line the intestine. As these cells fill up with iron, they fall into the intestine and leave the body in waste.

Both plant and animal foods provide humans with iron, but that iron comes in two forms, heme and nonheme, that are not equally available to the body. Heme iron comes from hemoglobin. It is found mainly in animal tissue. Red meat is an especially rich source of heme iron. Only trace amounts of heme iron are found in plants. Heme iron is in a form that is easier for humans to use. It is absorbed at a higher rate than

KEY TERMS

Cell differentiation —The process by which stem cells develop into different types of specialized cells such as skin, heart, muscle, and blood cells.

Dietary supplement —A product, such as a vitamin, mineral, herb, amino acid, or enzyme, that is intended to be consumed in addition to an individual’s diet with the expectation that it will improve health.

Enzyme —A protein that change the rate of a chemical reaction within the body without themselves being used up in the reaction.

Mineral —An inorganic substance found in the earth that is necessary in small quantities for the body to maintain a health. Examples: zinc, copper, iron.

nonheme iron, and its rate of absorption is less influenced by other foods that simultaneously are present in the digestive system.

The following list gives the approximate iron content for some common sources of heme iron:

• chicken liver, cooked, 3 ounces: 12.8 mg
• beef, cooked, 3 ounces: 3.2 mg
• turkey light meat, cooked, 3 ounces: 2.3 mg
• chicken dark meat, cooked, 3 ounces: 1.13 mg
• pork loin, cooked, 3 ounces: 0.8 mg
• oysters, 6 medium: 5.04 mg
• shrimp, cooked, 8 large: 1.36 mg
• tuna, light, canned, 3 ounces: 1.3 mg
• halibut, cooked, 3 ounces: 0.9 mg
• crab, cooked, 3 ounces: 0.8 mg

About 40–45% of iron in animal tissue and functionally all the iron in plants is nonheme iron. Nonheme iron is also the type of iron found in dietary supplements and added to iron-fortified foods. Nonheme iron is less easily used by humans; it must be changed in the digestive system before it can be absorbed. Only about 2-10% of nonheme iron in food is absorbed compared to 20-25% of heme iron. In addition, the absorption of nonheme iron is strongly influenced by other substances present in the digestive system. The ability of the body to absorb nonheme iron is decreased by the simultaneous presence of tea, coffee, dairy products, phytic acid (a substance found in grains, dried beans and rice), eggs, soy protein, and some chocolates. Absorption of nonheme iron is increased by the simultaneous presence of vitamin C, certain organic acids, and a small amount of meat, fish, or poultry, which boosts the absorption of nonheme iron as well as providing heme iron. Vegetarians and vegans should take into consideration the influence of other foods on iron absorption when planning meals.

The following list gives the approximate iron content for some common foods that contain nonheme iron:

• cereal, 100% iron fortified, 1 cup: 18 mg
• soybeans, boiled, 1 cup: 8.8 mg
• tofu, firm, 1/2 cup: 6.22 mg
• kidney beans, cooked, 1 cup: 5.2 mg
• lima beans, cooked, 1 cup: 4.5
• pinto beans, cooked, 1 cup: 3.6 mg
• blackstrap molasses, 1 tablespoon: 3.5 mg
• raisins, small box, 1.5 ounces: .89 mg
• potato, medium with skin: 2.75 mg
• cashew nuts, 1 ounce: 1.70 mg
• whole wheat bread, 1 slice: 0.9 mg

Normal iron requirements

The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals . The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.

Iron requirements vary substantially at different ages. Periods of rapid growth in children increase the need for iron. Women who menstruate need more iron because of blood loss during menstruation. Pregnancy puts high demands on the iron supply in the body because of increased production of red blood cells to supply the developing fetus. In 2001, the IOM set RDAs for iron based on preventing iron deficiency at each age. Iron passes into breast milk, and infants can meet their iron needs through breast milk or iron-fortified formula. RDAs and ULs for iron are measured in milligrams (mg).

The following list gives the daily RDAs and IAs and ULs for vitamin C for healthy individuals as established by the IOM.

• children birth-6 months: RDA 0.27 mg; UL not established
• children 7-12 months: RDA 11 mg; UL not established
• children 1-3 years: RDA 7 mg; UL 40 mg
• children 4-8 years: RDA 10 mg; UL 40 mg
• children 9-13 years: RDA 9 mg; UL 40 mg
• boys 14-18 years: RDA 11 mg; UL 45 mg
• girls 14-18 years: RDA 55 mg; UL 45 mg
• men age 19-50: RDA 8 mg; UL 45 mg
• women age 19-50: RDA 18 mg; UL 45 mg
• men who smoke: RDA 125 mg; UL 45 mg
• pregnant women: RDA 27 mg; UL 45 mg
• breastfeeding women 18 years and younger: RDA 10 mg; UL 45 mg
• breastfeeding women 19 years and older: RDA 9 mg; 45 mg

Precautions

Pregnant women should consult their healthcare provider before the fifteenth week of pregnancy about the need for iron supplementation. They should not start taking an iron supplement on their own.

Men and women over age 55 are not at risk for iron deficiency and should take a multivitamin containing iron only on instructions from their healthcare provider.

People with kidney disease, liver damage, alcoholism, or ulcers should consult a healthcare professional before taking a supplement containing iron.

Interactions

Iron interacts with many drugs and nutritional supplements. General categories of substances that may increase or decrease the amount of iron that is absorbed include medications that decrease stomach acidity (e.g. antacids, Tagamet, Zantac), pancreatic enzyme supplements, calcium supplements and dairy products, vitamin C, citric, malic, tartaric, and lactic acids, and copper.

The presence of iron also increases or decreases the effectiveness of many prescription drugs. Individuals should review their medications with a doctor or pharmacist when they begin taking an iron supplement to see if their other medications need adjustment.

Complications

Iron deficiency

The World Health Organization (WHO) considers iron deficiency to be the most widespread dietary disorder in the world. WHO estimates that up to 80% of the world’s population is iron deficient and up to 30% have iron deficiency anemia. The two main causes of iron deficiency are low dietary intake and excessive blood loss. In the United States, women of childbearing age, young children, people with diseases that interfere with the absorption of iron (e.g. Crohn’s disease, celiac disease ), and people receiving kidney dialysis are most likely to seriously be iron deficient. American men rarely have low levels of iron because the tend to eat more meat than women and do not lose blood through menstruation.

At first, the body is able to use stored iron to make up for an iron deficit, but over time, the amount of hemoglobin decreases and a condition called iron deficiency anemia develops. (This is only one type of anemia; other anemias have other causes.) Iron deficiency anemia decreases the amount of oxygen reaching cells in the body. Symptoms of iron deficiency anemia include:

• lack of energy
• feelings of weakness
• frequently feeling cold
• increased infections
• irritability
• decreased work or school performance
• sore swollen tongue
• drive to eat dirt, clay or other non-food substances (pica)

The preferred way to treat mild iron deficiency is through changes in diet. If these changes are ineffective, iron supplements may be used. Dietary supplements contain different formulations such as ferrous fumarate, ferrous sulfate, and ferrous gluconate. Iron in these different formulations is absorbed at differing rates. Because too much iron can cause serious health problems, iron supplements should be taken under the supervision of a healthcare professional.

Iron excess

Iron overload caused by an inherited disorder is called hereditary hemochromatosis. This disorder affects as many as one of every 200 people of northern European descent. These people have a genetic mutation that causes them to absorb iron from the intestine at a rate far higher than normal. Hereditary hemochromatosis is treated by avoiding iron-rich foods and removing blood (usually through blood donation) from the individual on a regular basis.

People who have many blood transfusions can also develop iron overload, but by far the most common cause of excess iron is accidental poisoning. Over 20,000 American children accidentally ingest iron— usually in the form of dietary supplements—each year. Iron poisoning is the leading cause of poisoning deaths in children under age 6 in the United States. Iron overdose is a medical emergency. Symptoms occurring within the first 12 hours include nausea, vomiting, abdominal pain, black stool, weakness, rapid pulse, low blood pressure, fever, difficulty breathing, and coma. If death does not occur within the first 12 hours, damage to the kidney liver damage, cardiovascular system and nervous system may appear within two days. Long-term damage to survivors of iron poisoning include cirrhosis (liver damage), permanent central nervous system damage, and stomach problems.

Parental concerns

Parents should be aware that the RDA and UL for vitamins and minerals are much lower for children than for adults. Accidental overdose may occur if children are give adult vitamins or dietary supplements. Accidental iron overdose is a leading cause of poisoning deaths in young children. Parents should keep all dietary supplements away from children, just as they would other medicines.

Resources

BOOKS

DiSilvestro, Robert. Handbook of Minerals as Nutritional Supplements. Boca Raton, FL: CRC Press, 2005. .

Fragakis, Allison. The Health Professional’s Guide to Popular Dietary Supplement Chicago: American Dietetic Association, 2003 .

Garrison, Cheryl D., ed. The Iron Disorders Institute Guide to Anemia. Nashville, TN: Cumberland House, 2003. .

Garrison, Cheryl D., ed. The Iron Disorders Institute Guide to Hemochromatosis. Nashville, TN: Cumberland House, 2001. .

Lieberman, Shari and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program, 4th ed. New York: Avery, 2007. .

Pressman, Alan H. and Sheila Buff. The Complete Idiot’s Guide to Vitamins and Minerals, 3rd ed. Indianapolis, IN: Alpha Books, 2007.

PERIODICALS

Iannotti, Lora L, James M. Tielsch, Maureen M. Black, et al. “Iron Supplementation in Early Childhood: Health Benefits and Risks.” American Journal of Clinical Nutrition, 84 (2006):1261-76.

ORGANIZATIONS

American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, Illinois 60606-6995. Telephone: (800) 877-1600. Website: <http://www.eatright.org>.

International Food Information Council. 1100 Connecticut Avenue, NW Suite 430, Washington, DC 20036. Telephone: 202-296-6540. Fax: 202-296-6547. Website: <http://ific.org>.

Iron Disorders Institute. 2722 Wade Hampton Blvd., Suite A, Greenville, SC 29615. Telephone: (864) 292-1175. Fax: (864) 292-1878. Website: <http://www.irondisorders.org>.

Linus Pauling Institute. Oregon State University, 571.

Weniger Hall, Corvallis, OR 97331-6512. Telephone: (541) 717-5075. Fax: (541) 737-5077. Website: <http://lpi.oregonstate.edu>.

Office of Dietary Supplements, National Institutes of Health. 6100 Executive Blvd., Room 3B01, MSC 7517, Bethesda, MD 20892-7517 Telephone: (301)435-2920. Fax: (301) 480-1845. Website: <http://dietary-supplements.info.nih.gov>.

OTHER

Harvard School of Public Health. “Vitamins.” Harvard University, November 10, 2006. <http://www.hsph .harvard.edu/nutritionsource/vitamins.html> .

Higdon, Jane. “Iron.” Linus Pauling Institute-Oregon State University, January 6, 2006. <http://lpi.oregonstate.edu/infocenter/minerals/iron> .

Iron Disorders Institute. “About Iron.” November 3, 2006. <http://www.irondisorders.org/Disofders/about.asp> .

Mangels, Reed. “Iron in the Vegan Diet.” Vegetarian Resource Group, April 26, 2006. <http://www.vrg.org/ nutrition/iron.htm> .

Medline Plus. “Iron.” U. S. National Library of Medicine, August 1, 2006. <http://www.nlm.nih/gov/medlineplus /druginfo/natural/patient-iron.html> .

Office of Dietary Supplements. “Dietary Supplement Fact Sheet: Iron.” National Institutes of Health, July 26, 2004. <http://ods.od.nih.gov/factsheets/iron.asp>.

Tish Davidson, A.M.

Iron

General properties

Sources of iron

How iron is obtained

How iron is used

Biochemical applications

Chemistry and compounds

Resources

Iron is a metallic chemical element of atomic number 26. Its symbol is Fe, atomic weight is 55.847, specific gravity is 7.874, melting point is 2,795°F (1,535°C), and boiling point is 4,982°F (2,750°C).

Iron is one of the transition metals, occurring in group 8 of the periodic table. Four naturally occurring isotopes exist with atomic weights of 54 (5.8%), 56 (91.7%), 57 (2.2%), and 58 (0.3%). In addition, six radioactive isotopes have been prepared, with atomic weights of 52, 53, 55, 59, 60, and 61. The element was originally known by its Latin name ferrum, from which its chemical symbol is derived.

General properties

Iron is a silver-white or gray metal that is malleable and ductile. In a pure form, it is relatively soft and slightly magnetic. When hardened, it becomes much more magnetic. Iron is the most widely used of all metals. Prior to its use, however, it must be treated in some way to improve its properties or it must be combined with one or more other elements to form an alloy. By far the most common alloy of iron is steel.

One of the most common forms of iron is pig iron, produced by smelting iron ore with coke and limestone in a blast furnace. Pig iron is approximately 90% pure iron and is used primarily in the production of cast iron and steel.

Cast iron is a term used to describe various forms of iron that also contain carbon and silicon ranging in concentrations from 0.5 to 4.2% of the former and 0.2 to 3.5% of the latter. Cast iron has a vast array of uses ranging from thin rings to massive turbine bodies. Wrought iron contains small amounts of a number of other elements including carbon, silicon, phosphorus, sulfur, chromium, nickel, cobalt, copper, and molybdenum. Wrought iron can be fabricated into a number of forms and is widely used because of its resistance to corrosion.

Sources of iron

Iron is the fourth most abundant element in Earth’s crust and the second most abundant metal, after aluminum. It makes up about 6.2% of the crust by weight. In addition, iron is thought to be the primary constituent of Earth’s core as well as of siderite meteorites. Soil samples taken from the moon indicate that about 0.5% of lunar soil consists of iron.

The primary ores of iron are hematite (Fe₂O₃), magnetite (Fe₃O₄), limonite (FeO[OH] • nH₂O), and siderite (FeCO₃). The element also occurs as the sulfide, iron pyrite (FeS), but this compound is not used commercially as a source of iron because of the difficulty in reducing the sulfide to the pure element. Iron pyrite has a beautiful golden appearance and it is sometimes mistaken for elemental gold. This appearance explains its common name of fool’s gold. Taconite is a low-grade ore of iron that contains no more than about 30% of the metal.

In nature, oxides, sulfides, and silicates of iron are often converted to other forms by the action of water. Iron(II) sulfate (FeSO₄) and iron(II) bicarbonate (Fe(HCO₃)₂) are the most commonly found of these.

How iron is obtained

Iron is one of the handful of elements that was known to ancient civilizations. Originally, it was prepared by heating a naturally occurring ore of iron with charcoal in a very hot flame. The charcoal was obtained by heating wood in the absence of air. There is some evidence that this method of preparation was known as early as 3,000 BC, but the secret of ore smelting was carefully guarded within the Hittite civilization of the Near East for almost two more millennia.

When the Hittite civilization fell in about 1200 BC, the process of iron ore smelting spread throughout eastern and southern Europe. Ironsmiths were soon making ornamental objects, simple tools, and weapons from iron. So dramatic was the impact of this new technology on human societies that the period following 1200 BC is generally known as the Iron Age.

A major change in the technique for producing iron from its ores occurred in about 1773. As trees (and therefore the charcoal made from them) grew increasingly scarce in Great Britain, English inventor Abraham Darby (c. 1678-1717) discovered a method for making coke from soft coal. Since coal was abundant in the British Isles, Darby’s technique insured a constant supply of coal for the conversion of iron ores to the pure metal. The modern production of iron involves heating iron ore with coke and limestone in a blast furnace, where temperatures range from 392° F (200° C) at the top of the furnace to 3,632°F (2,000°C) at the bottom. Some blast furnaces are as tall as 15-story buildings and can produce 2,400 tons of iron per day.

Inside a blast furnace, a number of chemical reactions occur. One of these involves the reaction between coke (nearly pure carbon) with oxygen to form carbon monoxide. This carbon monoxide then reacts with iron ore to form pure iron and carbon dioxide. Limestone is added to the reaction mixture to remove impurities in the iron ore. The product of this reaction, known as slag, consists primarily of calcium silicate. The iron formed in a blast furnace exists in a molten form known as pig iron that can be drawn off at the bottom of the furnace. The slag is also molten but less dense than the iron. It is drawn off from taps just above the outlet from which the molten iron is removed.

Efforts to use pig iron for commercial and industrial applications were not successful. The material was quite brittle and objects of which it was made tended to break easily. Cannons made of pig iron, for example, were likely to blow apart when they fired a shell. By 1760, inventors had begun to find ways of toughening pig iron. These methods involved re-melting the pig iron and then burning off the carbon that remained mixed with the product. The most successful early device for accomplishing this step was the Bessemer converter, named after its English inventor Henry Bessemer (1813–1898). In the Bessemer converter, a blast of hot air is blown through molten pig iron. The process results in the formation of stronger forms of iron, cast and wrought iron. More importantly, when additional elements, such as manganese and chromium, are added to the converter, a new product—steel—is formed.

Later inventions improved on the production of steel by the Bessemer converter. In the open hearth process, for example, a charge of molten pig iron, hematite, scrap iron, and limestone is placed into a large brick container. A blast of hot air or oxygen is, then, blown across the surface of the molten mixture. Chemical reactions within the molten mixture result in the formation of either pure iron or, with the addition of alloying metals such as manganese or chromium, a high grade of steel.

An even more recent variation on the Bessemer converter concept is the basic oxygen process (BOP). In the BOP, a mixture of pig iron, scrap iron, and scrap steel is melted in a large steel container and a blast of pure oxygen is blown through the container. The introduction of alloying metals makes possible the production of various types of steel with many different properties.

How iron is used

Alloyed with other metals, iron is the most widely used of all metallic elements. The way in which it is alloyed determines the uses to which the final product is put. Steel, for example, is a general term used to describe iron alloyed with carbon and, in some cases, with other elements. The American Iron and Steel Institute recognizes 27 standard types of steel. Three of these are designated as carbon steels that may contain, in addition to carbon, small amounts of phosphorus and/or sulfur. Another 20 types of steel are made of iron alloyed with one or more of the following elements: chromium, manganese, molybdenum, nickel, silicon, and vanadium. Finally, four types of stainless and heat-resisting steels contain some combination of chromium, nickel, and manganese alloyed with iron.

The number of commercial products made of iron and steel is very large indeed. The uses of these two materials can generally be classified into about eight large groups, including (1) automotive; (2) construction; (3) containers, packaging, and shipping; (4) machinery and industrial equipment; (5) rail transportation; (6) oil and gas industries; (7) electrical equipment; and (8) appliances and utensils. For example, steel is widely used in many types of construction. It has at least six times the strength of concrete, another traditional building material, and about three times the strength of special forms of high-strength concrete. A combination of these two materials, reinforced concrete, is one of the strongest of all building materials available to architects. The strength of steel has made possible some remarkable feats of construction, including very tall buildings (skyscrapers) and bridges with very wide spans. It has also been used in the manufacture of automobile bodies, ship hulls, and heavy machinery and machine parts.

Metallurgists have also invented special iron alloys to meet very specific needs. Alloys of cobalt and iron (both magnetic materials themselves) can be used in the manufacture of very powerful permanent magnets. Steels that contain the element niobium (originally called columbium) have unusually great strength and have been used, among other places, in the construction of nuclear reactors. Tungsten steels are also very strong and have been used in the production of high-speed metal cutting tools and drills. The alloying of aluminum with iron produces a material that can be used in AC (alternating current) magnetic circuits since it can gain and lose magnetism very quickly.

Metallic iron also has other applications. Its natural magnetic properties make it suitable for both permanent magnets and electromagnets. It is also used in the production of various types of dyes, including blueprint paper and a variety of inks, and in the manufacture of abrasives.

Biochemical applications

Iron is essential to the survival of all vertebrates. Hemoglobin, the molecule in blood that transports oxygen from the lungs to an organism’s cells, contains a single iron atom buried deep within its complex structure. When humans do not take in sufficient amounts of iron in their daily diets, they may develop a disorder known as anemia. Anemia is characterized by a loss of skin color, a weakness and tendency to faint, palpitation of the heart, and a general sense of exhaustion.

Iron is also important to the good health of plants. It is found in a group of compounds known as porphyrins that play an important role in the growth and development of plant cells. Plants that lack iron have a tendency to lose their color, become weak, and die.

Chemistry and compounds

Iron typically displays one of two valences in forming compounds, 2⁺ and 3⁺. According to the older system of chemical nomenclature, these classes of compounds are known as the ferrous and ferric salts, of iron respectively. Because of the abundance of oxygen in the atmosphere, most naturally occurring iron compounds tend to be in the higher (3⁺) oxidation state.

One of the most widely used of iron compounds is iron(III) (or ferric) chloride, FeCl₃. When added to water, it reacts with water molecules forming a thick, gelatinous precipitate of iron(III) hydroxide. The compound is used in the early steps of water purification since, as the precipitate settles out of solution, it traps and carries with it organic and inorganic particles suspended in the water. Iron(III) chloride is also used as a mordant, a substance used in dyeing that binds a dye to a textile. In gaseous form, the compound has still another use. It attacks and dissolves metal and can be used, therefore, for etching. Printed circuits, for example, are often first etched with iron(III) chloride.

Iron(II) (ferrous) compounds tend to oxidize rather easily and are, therefore, less widely used than their 3⁺ cousins. Iron(II) (ferrous) sulfate is an important exception. In solid form, the compound tends not to oxidize as readily as other Fe²⁺ compounds and is used as an additive for animal feeds, in water purification, in the manufacture of inks and pigments, and in water and sewage treatment operations.

From a commercial standpoint, probably the most important chemical reaction of iron is its tendency to oxidize. When alloys of iron (such as the steels) are used in construction, a major concern is that they tend to react with oxygen in the air, forming a coating or iron oxide, or rust. The rusting process is actually a somewhat complex process in which both

KEY TERMS

Blast furnace —A structure in which a metallic ore (often, iron ore) is reduced to the elemental state.

Ductile —Capable of being drawn or stretched into a thin wire.

Isotopes —Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Malleable —Capable of being rolled or hammered into thin sheets.

Transition metal —An element found between groups IIA and IIIA in the periodic table.

oxygen and water are involved. If one or the other of these materials can be prevented from coming into contact with iron, oxidation will not occur. But if both are present, an electrochemical reaction is initiated, and iron is converted to iron oxide.

Each year, billions of dollars are lost when iron-containing structural elements degrade or disintegrate as a result of oxidation (rusting). It is hardly surprising, therefore, that a number of techniques have been developed for reducing or preventing rusting. These techniques include painting, varnishing, galvanizing, tinning, and enameling.

Resources

BOOKS

Byars, Mel. Design in Steel. London, UK: Laurence King, 2003.

Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford, UK: Oxford University Press, 2003.

Ghosh, Ahindra. Secondary Steelmaking: Principles and Applications. Boca Raton, FL: CRC Press, 2001.

Siekierski, Slawomir. Concise Chemistry of the Elements. Chichester, UK: Horwood Publishing, 2002.

David E. Newton

IRON (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

The period in human history beginning in about 1200 B.C. is called the Iron Age. It was at about this time that humans first learned how to use iron metal. But in some ways, one could refer to the current era as the New Iron Age. Iron is probably the most widely used and most important metal today. No other metal is available to replace iron in all its many applications.

Iron is a transition metal. The transition metals are the elements that make up Groups 3 through 12 in the periodic table. The periodic table is a chart that shows how elements are related to one another. The transition metals are typical metals in that they tend to be bright, shiny, silvery solids. They all tend to conduct heat and electricity well. And they usually have high melting points.

SYMBOL
Fe

ATOMIC NUMBER
26

ATOMIC MASS
55.847

FAMILY
Group 8 (VIIIB)
Transition metal

PRONUNCIATION
EYE-um

Iron normally does not occur as a free element in the earth. In fact, iron was not of much value to humans until they learned how to free iron from its compounds. Once they could do that, humans were able to make tools, weapons, household implements, and other objects out of iron. This step marked the beginning of the Iron Age.

Iron is most valuable not as a pure metal, but in alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. The best known and most widely used alloy of iron is steel. Steel contains iron and at least one other element. Today, specialized steels of all kinds are available for many different applications.

Discovery and naming

Ancient Egyptians had learned how to use iron before the First Dynasty, which began in about 3400 B.C. The Egyptians probably found the iron in meteorites. Meteorites are chunks of rock and metal that fall from the sky. Some meteorites are very rich in iron. The Egyptians made tools and jewelry out of iron.

Iron is probably the most widely used and most important metal today.

Iron was also known to early Asian civilizations. In Delhi, India, for example, a pillar made out of iron built in A.D. 415 still stands. It weighs 6.5 metric tons and remains in good condition after nearly 1,600 years.

Early Chinese civilizations also knew about iron. Workers learned to produce iron as early as 200 B.C. A number of iron objects, including cannons, remain from the Han period (202 B.C. to A.D. 221).

The Bible also includes many mentions of iron. For example, a long passage in the book of Job describes the mining of iron. Other passages tell about the processing of iron ore to obtain iron metal.

By the time of the Roman civilization, iron had become an essential metal. The historian Pliny (A.D. 23-79) described the role of iron in Rome:

It is by the aid of iron that we construct houses, cleave rocks, and perform so many other useful offices of life. But it is with iron also that wars, murders, and robberies are effected, and this, not only hand to hand, but from a distance even, by the aid of weapons and winged weapons, now launched from engines, now hurled by the human arm, and now furnished with feathery wings.

Even from the earliest days, humans probably seldom used iron in a pure form. It was difficult to make iron that was free of impurities, such as carbon (charcoal) and other metals. More important, however, it became obvious that iron with impurities was a stronger metal that iron without impurities.

It was not until 1786, however, that scientists learned what it was in steel that made it a more useful metal than iron. Three researchers, Gaspard Monge (1746-1818), C. A. Vandermonde, and Claude Louis Berthollet (1748-1822) solved the puzzLe. They found that a small amount of carbon mixed with iron produced a strong alloy. That alloy was steel. Today, the vast amount of iron used in so many applications is used in the form of steel, not pure iron.

Ancient Egyptians had learned how to use iron before the First Dynasty, which began in about 3400 B.C.

The chemical symbol for iron is Fe. That symbol comes from the Latin name for iron, ferrum.

Physical properties

Iron is a silvery-white or grayish metal. It is ductile and malleable. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. It is one of only three naturally occurring magnetic elements. The other two are nickel and cobalt

Iron has a very high tensile strength. Tensile means it can be stretched without breaking. Iron is also very workable. Workability is the ability to bend, roll, hammer, cut, shape, form, and otherwise work with a metal to get it into a desired shape or thickness.

The melting point of pure iron is 1,536°C (2,797°F) and its boiling point is about 3,000°C (5,400°F). Its density is 7.87 grams per cubic centimeter. The melting point, boiling point, and other physical properties of steel alloys may be quite different from those of pure iron.

Chemical properties

Iron is a very active metal. It readily combines with oxygen in moist air. The product of this reaction, iron oxide (Fe₂O₃), is known as rust. Iron also reacts with very hot water and steam to produce hydrogen gas. It also dissolves in most acids and reacts with many other elements.

Occurrence in nature

Iron is the fourth most abundant element in the Earth's crust. Its abundance is estimated to be about 5 percent. Most scientists believe that the Earth's core consists largely of iron. Iron is also found in the Sun, asteroids, and stars outside the solar system.

The most common ores of iron are hematite, or ferric oxide (Fe₂O₃); limonite, or ferric oxide (Fe₂O₃); magnetite, or iron oxide (Fe₃O₄); and siderite, or iron carbonate (FeCO₃). An increasingly important source of iron is taconite. Taconite is a mixture of hematite and silica (sand). It contains about 25 percent iron.

The largest iron resources in the world are in China, Russia, Brazil, Canada, Australia, and India. The largest producers of iron from ore in the world are China, Japan, the United States, Russia, Germany, and Brazil.

Isotopes

There are four naturally occurring isotopes of iron, iron-54, iron-56, iron-57, and iron-58. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Six radioactive isotopes of iron are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Two radioactive isotopes of iron are used in medical and scientific research. They are iron-55 and iron-59. These isotopes are used primarily as tracers in studies on blood. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system. Inside the system, the isotope gives off radiation. That radiation can be followed by detectors placed around the system. Iron-55 and iron-59 are used to study the way in which red blood cells develop in the body. These studies can be used to tell if a person's blood is healthy.

Extraction

Iron goes through a number of stages between ore and final steel product. In the first stage, iron ore is heated with limestone and coke (pure carbon) in a blast furnace. A blast furnace is a very large oven in which the temperature may reach 1,500°C (2,700°F). In the blast furnace, coke removes oxygen from iron ore:

The limestone removes impurities in the iron ore.

Iron produced by this method is about 91 to 92 percent pure. The main impurity left is carbon from the coke used in the furnace. This form of iron is known as pig iron. Pig iron is generally too brittle (it breaks too easily) to be used in most products.

Most scientists believe that the Earth's core consists largely of iron.

A number of methods have been developed for purifying pig iron. A common method used today is called the basic oxygen process. In this process, pig iron is melted in a large oven. Then pure oxygen gas is blown through the molten pig iron. The oxygen burns off much of the carbon in the pig iron:

A small amount of carbon remains in the iron. The iron produced in this reaction is known as steel.

The term "steel" actually refers to a wide variety of products. The various forms of steel all contain iron and carbon. They also contain one or more other elements, such as silicon, titanium, vanadium, chromium, manganese, cobalt, nickel, zirconium, molybdenum, and tungsten. Two other steel-like products are cast iron and wrought iron. Cast iron is an alloy of iron, carbon, and silicon. Wrought iron contains iron and any one or more of many other elements. In general, however, wrought iron tends to contain very little carbon.

Uses

It would be impossible to list all uses of iron and steel products. In general, those products can be classified into categories: (1) automotive; (2) construction; (3) containers, packaging, and shipping; (4) machinery and industrial equipment; (5) rail transportation; (6) oil and gas industries; (7) electrical equipment; and (8) appliances and utensils. (For more information on specific kinds of steel alloys, see individual elements, such as titanium, vanadium, chromium, manganese, molybdenum, and tungsten.)

Compounds

Some iron is made into compounds. The amount is very small compared to the amount used in steel and other iron alloys. Probably the fastest growing use of iron compounds is in water treatment systems. The terms ferric and ferrous refer to two different forms in which iron occurs in compounds. Some of the important iron compounds are:

The U.S. Recommended Daily Allowance (USRDA) for iron is 18 milligrams.

ferric acetate (Fe(C₂H₃O₂)₃): used in the dyeing of cloth

ferric ammonium oxalate(Fe(NH₄)₃(C₂O₄)₄): blueprints

ferric arsenate (FeAsO₄): insecticide

ferric chloride (FeCl₃): water purification and sewage treatment systems; dyeing of cloth; coloring agent in paints; additive for animal feed; etching material for engraving, photography, and printed circuits

ferric chromate (Fe₂(CrO₄)₃): yellow pigment (coloring) for paints and ceramics

ferric hydroxide (Fe(OH)₃): brown pigment for coloring rubber; water purification systems

ferric phosphate (FePO₄): fertilizer; additive for animal and human foods

ferrous acetate (Fe(C₂H₃O₂)₂): dyeing of fabrics and leather; wood preservative

ferrous gluconate (Fe(C₆H₁₁O₇)₂): dietary supplement in "iron pills"

ferrous oxalate (FeC₂O₄): yellow pigment for paints, plastics, glass, and ceramics; photographic developer

ferrous sulfate (FeSO₄): water purification and sewage treatment systems; catalyst in production of ammonia; fertilizer; herbicide; additive for animal feed; wood preservative; additive to flour to increase iron levels

Health effects

Iron is of critical importance to plants, humans, and animals. It occurs in hemoglobin, a molecule that carries oxygen in the blood. Hemoglobin picks up oxygen in the lungs, and carries it to the cells. In the cells, oxygen is used to produce energy the body needs to survive, grow, and stay healthy.

The U.S. Recommended Daily Allowance (USRDA) for iron is 18 milligrams. The USRDA is the amount of an element that a person needs to stay healthy. Iron is available in a number of foods, including meat, eggs, and raisins.

An iron deficiency (lack of iron) can cause serious health problems in humans. For instance, hemoglobin molecules may not form in sufficient numbers. Or they may lose the ability to carry oxygen. If this occurs, a person develops a condition known as anemia. Anemia results in fatigue. Severe anemia can result in a lowered resistance to disease and an increase in heart and respiratory (breathing) problems. Some forms of anemia can even cause death.

Iron

Description

Iron is a mineral that the human body uses to produce the red blood cells (hemoglobin) that carry oxygen throughout the body. It is also stored in myoglobin, an oxygen-carrying protein in the muscles that fuels cell growth.

General use

Iron is abundant in red meats, vegetables, and other foods, and a well-balanced diet can usually provide an adequate supply of the mineral. But when there is insufficient iron from dietary sources, or as a result of blood loss in the body, the amount of hemoglobin in the bloodstream is reduced and oxygen cannot be efficiently transported to tissues and organs throughout the body. The resulting condition is known as iron-deficiency anemia , and is characterized by fatigue , shortness of breath, pale skin, concentration problems, dizziness , a weakened immune system, and energy loss.

Iron-deficiency anemia can be caused by a number of factors, including poor diet, heavy menstrual cycles, pregnancy , kidney disease, burns , and gastrointestinal disorders. Individuals with iron-deficiency anemia should always undergo a thorough evaluation by a physician to determine the cause.

Children two years old and under also need adequate iron in their diets to promote proper mental and physical development. Children under two who are not breastfed should eat iron-fortified formulas and cereals. Women who breastfeed need at least 15 mg of dietary or supplementary iron a day in order to pass along adequate amounts of the mineral to their child in breast milk. Parents should consult a pediatrician or other healthcare professional for guidance on iron supplementation in children.

It has been theorized that excess stored iron can lead to atherosclerosis and ischemic heart disease . Phlebotomy, or blood removal, has been used to reduce stored iron in patients with iron overload with some success. Iron chelation with drugs such as desferrioxamine (Desferal) that help patients excrete excess stores of iron can be helpful in treating iron overload caused by multiple blood transfusions.

Iron levels in the body are measured by both hemoglobin and serum ferritin blood tests.

Normal total hemoglobin levels are:

• neonates: 17-22 g/dl
• one week: 15-20 g/dl
• one month: 11-15 g/dl
• children: 11-13 g/dl
• adult males: 14-18 g/dl (12.4-14.9 g/dl after age 50)
• adult females: 12-16 g/dl (11.7-13.8 g/dl after menopause)

Normal serum ferritin levels are:

• neonates: 25-200 ng/ml
• one month: 200-600 ng/ml
• two to five months: 50-200 ng/ml
• six months to 15 years: 7-140 ng/ml
• adult males: 20-300 ng/ml
• adult females: 20-120 ng/ml

Preparations

Iron can be found in a number of dietary sources, including:

• pumpkin seeds
• dried fruits (apricots)
• lean meats (beef and liver)
• fortified cereals
• turkey (dark meat)
• green vegetables (spinach, kale, and broccoli)
• beans, peas, and lentils
• enriched and whole grain breads
• molasses
• sea vegetables (blue-green algae and kelp)

Eating iron-rich foods in conjunction with foods rich in vitamin C (such as citrus fruits) and lactic acid (sauerkraut and yogurt) can increase absorption of dietary iron. Cooking food in cast-iron pots can also add to their iron content.

The recommended dietary allowances (RDA) of iron as outlined by the United States Department of Agriculture (USDA) are as follows:

• Children 0–3: 6-10 mg/day
• children 4–10: 10 mg/day
• adolescent–adult males: 10 mg/day
• adolescent–adult females: 10-15 mg/day
• pregnant females: 30 mg/day
• breastfeeding females: 15 mg/day

A number of herbal remedies contain iron, and can be useful as a natural supplement. The juice of the herb stinging nettle (Urtica dioica ) is rich in both iron and vitamin C (which is thought to promote the absorption of iron). It can be taken daily as a dietary supplement. Dandelion (Taraxacum officinale ), curled dock (Rumex crispus ), and parsley (Petroselinum crispum ) also have high iron content, and can be prepared in tea or syrup form.

In Chinese medicine, dang gui (dong quai ), or Angelica sinensis, the root of the angelica plant, is said to both stimulate the circulatory system and aid the digestive system. It can be administered as a decoction or tincture, and should be taken in conjunction with an iron-rich diet. Other Chinese remedies include foxglove root (Rehmannia glutinosa ), Korean ginseng (Panax ginseng ), and astragalus (Astragalus membranaceus ).

Ferrum phosphoricum (iron phosphate), is used in homeopathic medicine to treat anemia. The remedy is produced by mixing iron sulfate, phosphate, and sodium acetate, which is administered in a highly diluted form to the patient. Other homeopathic remedies for anemia include Natrum muriaticum, Chinchona officinalis, Cyclamen europaeum, Ferrum metallicum, and Manganum aceticum. As with all homeopathic remedies, the type of remedy prescribed for iron deficiency depends on the individual's overall symptom picture, mood, and temperament. Patients should speak with their homeopathic professional or physician, or healthcare professional before taking any of these remedies.

Iron is also available in a number of over-the-counter supplements (i.e., ferrous fumerate, ferrous sulfate, ferrous gluconate, iron dextran). Both heme iron and nonheme iron supplements are available. Heme iron is more efficiently absorbed by the body, but non-heme iron can also be effective if used in conjunction with vitamin C and other dietary sources of heme iron. Some multivitamins also contain supplementary iron. Ingesting excessive iron can be toxic, and may have long-term negative effects. For this reason, iron supplements should be taken only under the recommendation and supervision of a doctor.

Precautions

Iron deficiency can be a sign of a more serious problem, such as internal bleeding. Anyone suffering from iron-deficiency anemia should always undergo a thorough evaluation by a healthcare professional to determine the cause.

Iron overdose in children can be fatal, and is a leading cause of poisoning in children. Children should never take supplements intended for adults, and should receive iron supplementation only under the guidance of a physician.

Individuals with chronic or acute health conditions, including kidney infection, alcoholism , liver disease, rheumatoid arthritis, asthma , heart disease, colitis, and stomach ulcer should consult a physician before taking herbal or pharmaceutical iron supplements.

If individuals taking homeopathic dilutions of ferrum phosphoricum experience worsening of their symptoms (known as a homeopathic aggravation), they should stop taking the remedy and contact their healthcare professional. A homeopathic aggravation can be an early indication that a remedy is working properly, but it can also be a sign that a different remedy is needed.

Patients diagnosed with hemochromatosis, a genetic condition in which the body absorbs too much iron and stores the excess in organs and tissues, should never take iron supplements.

Side effects

Taking herbal or pharmaceutical iron supplements on an empty stomach may cause nausea . Iron supplementation may cause hard, dark stools, and individuals who take iron frequently experience constipation . Patients who experience dark bowel movements accompanied by stomach pains should check with their doctor, as this can also indicate bleeding in the digestive tract.

Other reported side effects include stomach cramps and chest pain . These symptoms should be evaluated by a physician if they occur.

Some iron supplements, particularly those taken in liquid form, may stain the teeth. Taking these through a straw, or with a dropper placed towards the back of the throat, may be helpful in preventing staining. Toothpaste containing baking soda and/or hydrogen peroxide can be useful in removing iron stains from teeth.

Signs of iron overdose include severe vomiting , racing heart, bloody diarrhea , stomach cramps, bluish lips and fingernails, pale skin, and weakness. If overdose is suspected, the patient should contact poison control and/or seek emergency medical attention immediately.

Interactions

Iron supplements may react with certain medications, including antacids, acetohydroxamic acid (Lithostat), dimercaprol, etidronate, fluoroquinolones. In addition, they can decrease the effectiveness of certain tetracyclines (antibiotics). Individuals taking these or any other medications should consult their healthcare professional before starting iron supplements.

Certain foods decrease the absorption of iron, including some soy-based foods, foods with large concentrations of calcium , and beverages containing caffeine and tannin (a substance found in black tea). These should not be taken within two hours of using an iron supplement. Some herbs also contain tannic acid, and should be avoided during treatment with iron supplements. These include allspice (Pimenta dioica ) and bayberry (Myrica cerifera, also called wax myrtle).

Individuals considering treatment with homeopathic remedies should also consult their healthcare professional about possible interactions with certain foods, beverages, prescription medications, aromatic compounds, and other environmental elements—factors known in homeopathy as remedy antidotes —that could counteract the efficacy of treatment for iron deficiency.

Resources

BOOKS

Medical Economics Company. PDR 2000 Physicians' Desk Reference. Montvale, NJ: Medical Economics Company, 1998.

Medical Economics Company. PDR for Herbal Medicines. Montvale, NJ: Medical Economics Company, 1998.

Ody, Penelope. The Complete Medicinal Herbal. New York: DK Publishing, 1993.

PERIODICALS

de Valk, B., and J.J.M. Marx. "Iron, Atherosclerosis, and Is-chemic Heart Disease." Archives of Internal Medicine 159(i14): 1542.

Paula Ford-Martin

Iron

Iron is a metallic chemical element of atomic number 26. Its symbol is Fe, atomic weight is 55.847, specific gravity is 7.874, melting point is 2,795°F (1,535°C), and boiling point is 4,982°F (2,750°C).

Iron is one of the transition metals, occurring in group 8 of the periodic table . Four naturally occurring isotopes exist with atomic weights of 54 (5.8%), 56 (91.7%), 57 (2.2%), and 58 (0.3%). In addition, six radioactive isotopes have been prepared, with atomic weights of 52, 53, 55, 59, 60, and 61. The element was originally known by its Latin name ferrum, from which its chemical symbol is derived.

General properties

Iron is a silver-white or gray metal that is malleable and ductile. In a pure form, it is relatively soft and slightly magnetic. When hardened, it becomes much more magnetic. Iron is the most widely used of all metals. Prior to its use, however, it must be treated in some way to improve its properties or it must be combined with one or more other elements to form an alloy . By far the most common alloy of iron is steel .

One of the most common forms of iron is pig iron, produced by smelting iron ore with coke and limestone in a blast furnace. Pig iron is approximately 90% pure iron and is used primarily in the production of cast iron and steel.

Cast iron is a term used to describe various forms of iron that also contain carbon and silicon ranging in concentrations from 0.5-4.2% of the former and 0.2-3.5% of the latter. Cast iron has a vast array of uses ranging from thin rings to massive turbine bodies. Wrought iron contains small amounts of a number of other elements including carbon, silicon, phosphorus , sulfur , chromium, nickel, cobalt, copper , and molybdenum. Wrought iron can be fabricated into a number of forms and is widely used because of its resistance to corrosion .

Sources of iron

Iron is the fourth most abundant element in the earth's crust and the second most abundant metal, after aluminum . It makes up about 6.2% of the crust by weight. In addition, iron is thought to be the primary constituent of the earth's core as well as of siderite meteorites. Soil samples taken from the Moon indicate that about 0.5% of lunar soil consists of iron.

The primary ores of iron are hematite (Fe2O3), magnetite (Fe3O4), limonite (FeO(OH) • nH2O), and siderite (FeCO3). The element also occurs as the sulfide, iron pyrite (FeS), but this compound is not used commercially as a source of iron because of the difficulty in reducing the sulfide to the pure element. Iron pyrite has a beautiful golden appearance and is sometimes mistaken for elemental gold. This appearance explains its common name of fool's gold. Taconite is a low-grade ore of iron that contains no more than about 30% of the metal.

In nature, oxides, sulfides, and silicates of iron are often converted to other forms by the action of water . Iron(II) sulfate (FeSO4) and iron(II) bicarbonate (Fe(HCO3)2) are the most commonly found of these.

How iron is obtained

Iron is one of the handful of elements that was known to ancient civilizations. Originally it was prepared by heating a naturally occurring ore of iron with charcoal in a very hot flame. The charcoal was obtained by heating wood in the absence of air. There is some evidence that this method of preparation was known as early as 3,000 b.c., but the secret of ore smelting was carefully guarded within the Hittite civilization of the Near East for almost two more millennia.

When the Hittite civilization fell in about 1200 b.c., the process of iron ore smelting spread throughout eastern and southern Europe . Ironsmiths were soon making ornamental objects, simple tools, and weapons from iron. So dramatic was the impact of this new technology on human societies that the period following 1200 b.c. is generally known as the Iron Age.

A major change in the technique for producing iron from its ores occurred in about 1773. As trees (and therefore the charcoal made from them) grew increasingly scarce in Great Britain, the English inventor Abraham Darby (1678?-1717) discovered a method for making coke from soft coal . Since coal was abundant in the British Isles, Darby's technique insured a constant supply of coal for the conversion of iron ores to the pure metal. The modern production of iron involves heating iron ore with coke and limestone in a blast furnace, where temperatures range from 392°F (200°C) at the top of the furnace to 3,632°F (2,000°C) at the bottom. Some blast furnaces are as tall as 15-story buildings and can produce 2,400 tons of iron per day.

Inside a blast furnace, a number of chemical reactions occur. One of these involves the reaction between coke (nearly pure carbon) with oxygen to form carbon monoxide . This carbon monoxide then reacts with iron ore to form pure iron and carbon dioxide . Limestone is added to the reaction mixture to remove impurities in the iron ore. The product of this reaction, known as slag, consists primarily of calcium silicate. The iron formed in a blast furnace exists in a molten form known as pig iron that can be drawn off at the bottom of the furnace. The slag is also molten but less dense than the iron. It is drawn off from taps just above the outlet from which the molten iron is removed.

Efforts to use pig iron for commercial and industrial applications were not very successful. The material was quite brittle and objects of which it was made tended to break easily. Cannons made of pig iron, for example, were likely to blow apart when they fired a shell. By 1760, inventors had begun to find ways of toughening pig iron. These methods involved remelting the pig iron and then burning off the carbon that remained mixed with the product. The most successful early device for accomplishing this step was the Bessemer converter, named after its English inventor Henry Bessemer (1813-1898). In the Bessemer converter, a blast of hot air is blown through molten pig iron. The process results in the formation of stronger forms of iron, cast and wrought iron. More importantly, when additional elements, such as manganese and chromium, are added to the converter, a new product—steel—is formed.

Later inventions improved on the production of steel by the Bessemer converter. In the open hearth process, for example, a charge of molten pig iron, hematite, scrap iron, and limestone is placed into a large brick container. A blast of hot air or oxygen is then blown across the surface of the molten mixture. Chemical reactions within the molten mixture result in the formation of either pure iron or, with the addition of alloying metals such as manganese or chromium, a high grade of steel.

An even more recent variation on the Bessemer converter concept is the basic oxygen process (BOP). In the BOP, a mixture of pig iron, scrap iron, and scrap steel is melted in a large steel container and a blast of pure oxygen is blown through the container. The introduction of alloying metals makes possible the production of various types of steel with many different properties.

How we use iron

Alloyed with other metals, iron is the most widely used of all metallic elements. The way in which it is alloyed determines the uses to which the final product is put. Steel, for example, is a general term used to describe iron alloyed with carbon and, in some cases, with other elements. The American Iron and Steel Institute recognizes 27 standard types of steel. Three of these are designated as carbon steels that may contain, in addition to carbon, small amounts of phosphorus and/or sulfur. Another 20 types of steel are made of iron alloyed with one or more of the following elements: chromium, manganese, molybdenum, nickel, silicon, and vanadium. Finally, four types of stainless and heat-resisting steels contain some combination of chromium, nickel, and manganese alloyed with iron.

Steel is widely used in many types of construction. It has at least six times the strength of concrete , another traditional building material, and about three times the strength of special forms of high-strength concrete. A combination of these two materials, reinforced concrete, is one of the strongest of all building materials available to architects. The strength of steel has made possible some remarkable feats of construction, including very tall buildings (skyscrapers) and bridges with very wide spans. It has also been used in the manufacture of automobile bodies, ship hulls, and heavy machinery and machine parts.

Metallurgists have also invented special iron alloys to meet very specific needs. Alloys of cobalt and iron (both magnetic materials themselves) can be used in the manufacture of very powerful permanent magnets. Steels that contain the element niobium (originally called columbium) have unusually great strength and have been used, among other places, in the construction of nuclear reactors. Tungsten steels are also very strong and have been used in the production of high-speed metal cutting tools and drills. The alloying of aluminum with iron produces a material that can be used in AC magnetic circuits since it can gain and lose magnetism very quickly.

Metallic iron also has other applications. Its natural magnetic properties make it suitable for both permanent magnets and electromagnets. It is also used in the production of various types of dyes, including blueprint paper and a variety of inks, and in the manufacture of abrasives .

Biochemical applications

Iron is essential to the survival of all vertebrates . Hemoglobin, the molecule in blood that transports oxygen from the lungs to an organism's cells, contains a single iron atom buried deep within its complex structure. When humans do not take in sufficient amounts of iron in their daily diets, they may develop a disorder known as anemia . Anemia is characterized by a loss of skin color , a weakness and tendency to faint, palpitation of the heart , and a general sense of exhaustion.

Iron is also important to the good health of plants. It is found in a group of compounds known as porphyrins that play an important role in the growth and development of plant cells. Plants that lack iron have a tendency to loose their color, become weak, and die.

Chemistry and compounds

Iron typically displays one of two valences in forming compounds, 2+ and 3+. According to the older system of chemical nomenclature, these classes of compounds are known as the ferrous and ferric salts, of iron respectively. Because of the abundance of oxygen in the atmosphere, most naturally occurring iron compounds tend to be in the higher (3+) oxidation state .

One of the most widely used of iron compounds is iron(III) (or ferric) chloride, FeCl3. When added to water, it reacts with water molecules forming a thick, gelatinous precipitate of iron(III) hydroxide. The compound is used in the early steps of water purification since, as the precipitate settles out of solution , it traps and carries with it organic and inorganic particles suspended in the water. Iron(III) chloride is also used as a mordant, a substance used in dyeing that binds a dye to a textile. In gaseous form the compound has still another use. It attacks and dissolves metal and can be used, therefore, for etching. Printed circuits, for example, are often first etched with iron(III) chloride.

Iron(II) (ferrous) compounds tend to oxidize rather easily and are, therefore, less widely used than their 3+ cousins. Iron(II) (ferrous) sulfate is an important exception. In solid form, the compound tends not to oxidize as readily as other Fe2+ compounds and is used as an additive for animal feeds, in water purification, in the manufacture of inks and pigments, and in water and sewage treatment operations.

From a commercial standpoint, probably the most important chemical reaction of iron is its tendency to oxidize. When alloys of iron (such as the steels) are used in construction, a major concern is that they tend to react with oxygen in the air, forming a coating or iron oxide, or rust. The rusting process is actually a somewhat complex process in which both oxygen and water are involved. If one or the other of these materials can be prevented from coming into contact with iron, oxidation will not occur. But if both are present, an electrochemical reaction is initiated, and iron is converted to iron oxide.

Each year, billions of dollars are lost when iron-containing structural elements degrade or disintegrate as a result of oxidation (rusting). It is hardly surprising, therefore, that a number of techniques have been developed for reducing or preventing rusting. These techniques include painting, varnishing, galvanizing, tinning, and enameling.

Resources

books

Greenwood, N. N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heinneman Press, 1997.

Hawley, Gessner G., ed. The Condensed Chemical Dictionary. 9th ed. New York: Van Nostrand Reinhold, 1977.

Joesten, Melvin D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

Knepper, W. A. "Iron." Kirk-Othmer Encyclopedia of ChemicalTechnology. 4th ed. Suppl. New York: John Wiley & Sons, 1998.

Seely, Bruce Edsall, ed. Iron and Steel in the Twentieth Century. New York: Facts on File, 1994.

David E. Newton

KEY TERMS

Blast furnace

—A structure in which a metallic ore (often, iron ore) is reduced to the elemental state.

Ductile

—Capable of being drawn or stretched into a thin wire.

Isotopes

—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Malleable

—Capable of being rolled or hammered into thin sheets.

Transition metal

—An element found between groups IIA and IIIA in the periodic table.