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manganese(măng`gənēs, măn`–) [Lat.,=magnet], metallic chemical element; symbol Mn; at. no. 25; at. wt. 54.93805; m.p. about 1,244°C;; b.p. about 1,962°C;; sp. gr. 7.2 to 7.45, depending on form; valence principally +2, +4, or +7.
Manganese is a pinkish-gray, chemically active metal. It is the first element in Group 7 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . It resembles iron but is harder and more brittle. The metal exhibits allotropyallotropy
[Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes.
..... Click the link for more information. ; it has four different forms with varying physical properties. It can be highly polished. Manganese tarnishes in moist air and oxidizes when heated to form an oxide, Mn3O4. It slowly displaces hydrogen from water. It reacts readily with hydrochloric and sulfuric acids and with the halogens.
In compounds, manganese assumes a number of different oxidation states. It is easily raised to the +2 state, for example, by reaction with hydrochloric acid to form manganous chloride, MnCl2. Manganese is also found in the +3 (manganic) state, but this state is unstable and usually reverts to the +2 state. Both manganous and manganic ions form acidic solutions. Manganese is found in the +4 state largely in manganese dioxide, MnO2 ; the +4 oxidation state is amphoteric, i.e., in the +4 state manganese can either donate or accept electrons in chemical reactions. Manganese also exists in +6 and +7 states; the +6 state is found in the manganate ion (MnO4−−) and the +7 state in the permanganate ion (MnO4−). These ions are stable in basic solutions. There is also evidence for a +1 state (in a complex cyanide) and for an unstable +5 state (in basic solutions). Manganese is found in abundance in nature.
Pyrolusite (MnO2) is the major ore. Manganese ores are produced principally in the countries of the former Soviet Union, India, the Union of South Africa, Ghana, and Morocco, and to a lesser extent in the United States. The metal is prepared commercially by reduction of its ores with aluminum or, with high purity, by electrolysis of a manganese sulfate solution. Manganese is very important in the steel industry, where it is used as a deoxidizing and desulfurizing agent; no substitute has been found. It is also used in large amounts to toughen and harden steel without making it brittle; it is usually added as ferromanganese. Any steel having between 10% and 15% manganese is known as manganese steel, although almost all steel contains some manganese. Manganese is widely used in making alloys. Manganese bronze and manganese brass are alloys containing manganese, copper, tin, zinc, and small amounts of other metals in varying proportions. Certain alloys containing manganese, aluminum, antimony, and small amounts of copper are highly magnetic.
Compounds of manganese are widely used in industry. Manganese dioxide is used as a drying agent; it catalyzes the oxidation of oils in paints and varnishes. It is also used in the dry cell and to remove the green color caused by iron impurities in glass. Potassium permanganate (KMnO4) is a powerful oxidizing agent used industrially for bleaching and in chemistry as an analytical reagent. Other compounds find use in glassmaking, as pigments, and as fertilizers. Manganese is needed as a nutrient in small amounts by many plants and animals and by humans. The purple color of amethyst is due to manganese. The element was first isolated in 1774 by J. G. Gahn, although its existence was previously recognized by T. O. Bergman and by K. W. ScheeleScheele, Karl Wilhelm
, 1742–86, Swedish chemist, b. Stralsund. He is known as the discoverer of many chemical substances. He was a pharmacist in Stockholm, in Uppsala (1770–75), and then in Köping. He prepared and studied oxygen c.
..... Click the link for more information. .
(Latin manganum), Mn, a chemical element in Group VII of the Mendeleev periodic system. Atomic number, 25; atomic weight, 54.9380. A heavy silver-white metal. There is one naturally occurring stable isotope, 55Mn.
Historical information. Manganese minerals have long been known. The ancient Roman naturalist Pliny referred to a black rock that was used to decolorize a fluid vitreous mass; the substance was the mineral pyrolusite, MnO2. From ancient times the Georgians used pyrolusite as an additive in the manufacture of iron. Pyrolusite was long known as black magnesia and was believed to be a variety of lodestone (magnetite). In 1774, K. Scheele showed that it was a compound of an unknown metal. Another Swedish scientist, J. Gahn, obtained manganese contaminated with carbon by strongly heating a mixture of pyrolusite and carbon. The term “manganese” traditionally comes from the German Manganerz, “manganese ore.”
Occurrence in nature. The average manganese content of the earth’s crust is 0.1 percent; its content in most igneous rocks, where it occurs in a diffused state as Mn2+ (an analogue of Fe2+), is 0.06-0.20 percent by weight. On the earth’s surface, where the minerals Mn3+ and Mn4+ also occur, Mn2+ is readily oxidized. In the biosphere manganese migrates energetically under conditions of reduction but is poorly mobile in an oxidizing medium. Manganese exhibits greatest mobility in the acidic waters of tundra and forest regions, where it occurs as Mn2+. Here the manganese content is often increased, and crops suffer from manganese excess in certain areas; ferromanganese concretions and bog ores are formed in soils, lakes, and swamps. In an alkaline oxidizing medium, in dry steppe and desert regions, manganese is poorly mobile; organisms suffer from manganese deficiency, and crops often require manganese trace fertilizers. River waters have a low manganese content (10-6-10-5g/l), although the total discharge of manganese from rivers is extremely large (most of it is deposited in the littoral zone). The manganese content in lakes, seas, and oceans is even smaller. Ferromanganese concretions formed during past geological ages are widely distributed in many areas along the ocean bottom.
Physical and chemical properties. Manganese has a density of 7.2-7.4 g/cm3; melting point, 1245°C; boiling point, 2150°C. Four polymorphic modifications of manganese are known: α-Mn (body-centered cubic lattice with 58 atoms in the unit cell), β-Mn (body-centered cubic lattice with 20 atoms in the cell), γ-Mn (tetragonal lattice with four atoms in the cell), and δ-Mn (body-centered cubic). The conversion temperatures are as follows: α ⇆ β 705°C; β ⇆ γ, 1090°C; γ ⇆ ´ δ,1133°C. The α modification is brittle; they (and sometimes) modification is plastic, which is of considerable importance in the formation of alloys.
Manganese has an atomic radius of 1.30 angstroms (Å). The ionic radii (in Å) are as follows: Mn2+, 0.91; Mn4+, 0.52; Mn7+, 0.46. Other physical properties of ±-Mn include specific heat (at 25°C), 0.478 kilojoules per (kg.°K), or 0.114 calories per (g.°C); temperature coefficient of linear expansion (at 20°C), 22.3 × 10-6 deg-1; thermal conductivity (at 25°C), 66.57 watts per (m-°K), or 0.159 calories per (cm.sec.°C); specific electric resistivity, 1.5-2.6 microhm·m, or 150-260 microhm-cm; temperature coefficient of electric resistivity, (2-3) × 10-4 deg-1. Manganese is paramagnetic.
Manganese exhibits considerable chemical activity, and it reacts vigorously upon heating with nonmetals, such as oxygen (forming a manganese oxide mixture of varying valence), nitrogen (Mn4N, Mn2N, and Mn3N2), sulfur (MnS and MnS2), carbon (MnaC, Mn23C6, Mn7C3, and Mn5C6), and phosphorus (Mn2P and MnP). Manganese does not change when exposed to air at room temperature, and it reacts slowly with water. It dissolves readily in acids (hydrochloric and dilute sulfuric) to form divalent manganese salts. Manganese evaporates readily upon heating in a vacuum, even from alloys.
Manganese forms alloys with chemical elements; most metals dissolve in individual modifications and stabilize them. For example, copper, iron, cobalt, and nickel stabilize the γ modification, and aluminum and silver expand the β- and δ-Mn regions in double alloys. This is important for the preparation of manganese-based alloys that are subjected to plastic deformation (forging, rolling, and stamping).
Manganese compounds usually have a valence of 2 to 7 (the most stable oxidation numbers are +2, + 4, and +7). The oxidative and acidic properties of manganese compounds grow with an increase in the oxidation number. Compounds formed from Mn(+2) are reducing agents. Manganese oxide, MnO, is a grayish green powder that exhibits basic properties and is insoluble in water and alkalies but readily soluble in acids. Manganese hydroxide, Mn(OH)2, is a white substance that is insoluble in water. Compounds formed from Mn(-4) can act as oxidizing agents (a) and reducing agents (b):
(a) MnO2 + 4HCL = MnCl2 + Cl2 + 2H2O
(this reaction is used in the laboratory for the preparation of chlorine);
(b) MnO2 + KCLO3 + 6KOH = 3k2MnO4 + KCL + 3H2O
(this reaction proceeds during melting).
Manganese dioxide, MnO2, is reddish brown in color, and the corresponding hydroxide, Mn(OH)4, is dark brown. Both compounds are insoluble in water and are amphoteric, with a slight predominance of the acidic function. Salts of the type K4MnO4 are called manganites.
Manganic acid and its salts (manganates) are the most typical examples of Mn(+6) compounds. Very important Mn(+7) compounds include permanganic acid, manganese anhydride, and permanganates.
Preparation. The purest manganese is produced industrially according to the method proposed by the Soviet electrochemist R. I. Agladze (1939), which involves the electrolysis of aqueous MnSO4 solutions with an addition of (NH4)2SO4 at pH = 8.0-8.5. This process is conducted using lead anodes and cathodes made from AT-3 titanium alloy or stainless steel. Manganese plates are removed from the cathodes and remelted, if necessary. Manganese with a total impurity content of approximately 0.1 percent is obtained by the halogen process (for example, chlorination of manganese ore) and halide reduction. Less pure manganese is prepared using aluminothermy according to the reaction 3Mn3O4 + 8A1 = 9Mn + 4A12O3, as well as by electrothermy.
Use. The primary consumer of manganese is ferrous metallurgy, which uses an average of 8-9 kg per ton of steel smelted. Ferromanganese, an alloy with iron (70-80 percent Mn, 0.5-7.0 percent C; remainder iron and impurities), is most often used to introduce manganese into steel. Compounds of iron and manganese are smelted in open-hearth and electric furnaces. High-carbon ferromanganese is used for deoxidation and desulfuration of steel; medium- and low-carbon ferromanganese is used in alloying steel. Low-alloy structural and rail steels contain 0.9-1.6 percent manganese. High-alloy, extremely durable steel containing 15 percent Mn and 1.25 percent C (invented by the British metallurgist R. Hadfield in 1883) was one of the first alloy steels to be produced. Nickel-free stainless steel containing 14 percent Cr and 15 percent Mn is produced in the USSR.
Manganese is also used in nonferrous alloys. Alloys of copper and manganese are used in the manufacture of turbine blades, and manganese bronzes, in the production of propellers and other components that require both strength and corrosion resistance. Nearly all industrial aluminum alloys and magnesium alloys contain manganese. Wrought manganese-based alloys with copper, nickel, and other elements are being developed. Galvanic manganese coatings are used to protect metal articles against corrosion.
Manganese compounds are also used in the manufacture of galvanic cells and in glassmaking, ceramics, printing, and the dye industry, as well as in agriculture.
F. N. TAVADZE
Manganese in the organism. Manganese is widespread in nature as a fixed constituent in plant and animal organisms. The manganese content of plants varies from ten-thousandths to hundredths of a percent and from hundred-thousandths to thousandths of a percent in animals. Invertebrates have a higher manganese content than vertebrates. Plants that accumulate a substantial quantity of manganese (up to several percent in ash) include certain species of rust fungi, water chestnuts, and duck-weed, bacteria of the genera Leptothrix and Crenothrix, and certain diatomic algae (Cocconeis). Red ants and certain mollusks and crustaceans are examples of animals that accumulate manganese (up to hundredths of a percent).
Manganese activates a number of enzymes; takes part in respiratory processes, photosynthesis, and the biosynthesis of nucleic acids; intensifies the activity of insulin and other hormones; and affects hematopoiesis and mineral metabolism. Manganese deficiency in plants induces necrosis, chlorosis in apple and citrus trees, and cereal, potato, and barley blight. Manganese is present in all human organs and tissue (the largest quantities are found in the liver, skeleton, and thyroid gland). The daily manganese requirement for animals and humans is a few milli-grams (humans ingest 3-8 mg in the diet). The manganese requirement increases under physical stress and in the case of insufficient exposure to sunlight; children require a larger quantity of manganese than adults. A manganese deficiency in animal feed has been shown to have an adverse effect on growth and development, to induce anemia and grass tetany, and to disrupt mineral metabolism in bone tissue. Manganese salts are added to the feed to prevent such disorders.
G. IA. ZHIZNEVSKAIA
In medicine, certain magnesium salts (for example, KMnO4) are used as disinfectants. Manganese compounds, which are used in many sectors of industry, may have a toxic effect on the organism. Manganese enters the organism primarily through the respiratory tract, is accumulated in the parenchymal organs (liver and spleen), bones, and muscles, and is removed gradually, over a period of several years. The maximum permissible concentration of manganese compounds in the air is 0.3 mg/m3. Damage to the nervous system, accompanied by the characteristic manganese parkinsonism syndrome, is observed in cases of pronounced poisoning.
The course of treatment includes vitamin therapy and cholinolytics. The preventive measures require observance of the rules of practical hygiene.
REFERENCESSully, A. H. Marganets. Moscow, 1959. (Translated from English.)
Proizvodstvo ferrosplavov, 2nd ed. Moscow, 1957.
Pearson, A. “Marganets i ego rol’ v fotosinteze.” In the collection Mikroelementy. Moscow, 1962. (Translated from English.)