Iron

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.