Manganese

Manganese is a chemical element that has the symbol Mn and atomic number 25. It is found as the free element in nature (often in combination with iron), and in many minerals. The free element is a metal with important industrial metal alloy uses. Manganese ions are variously colored, and are used industrially as pigments and as oxidation chemicals. Manganese (II) ions function as cofactors for a number of enzymes and the element is thus a required trace mineral for all known living organisms.

Compounds

Methylcyclopentadienyl manganese tricarbonyl is used as an additive in unleaded gasoline to boost octane rating and reduce engine knocking.

Manganese dioxide is also used as a reagent in organic chemistry for the oxidation of benzylic alcohols (i.e. adjacent to an aromatic ring). Manganese(IV) oxide (manganese dioxide, MnO₂) is used in dry cells. Manganese can be used to neutralize the greenish tinge in glass caused by trace amounts of iron contamination. Manganese compounds in larger amounts can color glass an amethyst color, and are responsible for the purple color of true amethyst. MnO₂ is also used in the manufacture of oxygen and chlorine, and in drying black paints. Manganese oxide is a brown pigment that can be used to make paint and is a constituent of natural umber. Potassium permanganate, sodium permanganate and barium permanganate are all potent oxidizers. Potassium permanganate, also called Condy’s crystals, is a commonly used laboratory reagent because of its oxidizing properties and finds use as a topical medicine (for example, in the treatment of fish diseases). Solutions of potassium permanganate were among the first stains and fixatives to be used in the preparation of biological cells and tissues for electron microscopy.

Manganese phosphating is used as a treatment for rust and corrosion prevention on steel.

Manganese(IV) oxide (manganese dioxide) was used in the original type of dry cell battery, and is the black material found when opening carbon-zinc type flashlight cells. The same material also functions in newer alkaline batteries (usually battery cells), which use the same basic reaction but a different electrolyte.

The overall level and nature of manganese use in the United States is expected to remain about the same in the near term. No practical technologies exist for replacing manganese with other materials or for using domestic deposits or other accumulations to reduce the complete dependence of the United States on other countries for manganese ore.

Substitutes: Manganese has no satisfactory substitute in its major applications, which are related to metallurgical alloy use. In minor applications, (e.g., manganese phosphating), zinc and sometimes vanadium are viable substitutes. In disposable battery manufacture, standard and alkaline cells using manganese will probably eventually be mostly replaced with lithium battery technology.

The most stable oxidation state for manganese is +2, and many manganese(II) compounds are known, such as manganese(II) sulfate (MnSO₄) and manganese(II) chloride (MnCl₂). This oxidation state is also seen in the mineral rhodochrosite, (manganese(II) carbonate). The +3 oxidation state is also known, in compounds such as manganese(III) acetate, but these are quite powerful oxidizing agents.

Biological role

Manganese is an essential trace nutrient in all forms of life.

The classes of enzymes that have manganese cofactors are very broad and include such classes as oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, lectins, and integrins. The best known manganese-containing polypeptides may be arginase, the diphtheria toxin, and Mn-containing superoxide dismutase (Mn-SOD), which is the enzyme typically present in eukaryotic mitochondria, and also in many bacteria (this fact is in keeping with the bacterial-origin theory of mitochondria). The Mn-SOD enzyme is probably one of the most ancient, for nearly all organisms living in the presence of oxygen use it to deal with the toxic effects of superoxide, formed from the 1-electron reduction of dioxygen. Exceptions include a few kinds of bacteria such as Lactobacillus plantarum and related lactobacilli, which use a different non-enzymatic mechanism, involving manganese (Mn²⁺) ions complexed with polyphosphate directly for this task, indicating how this function possibly evolved in aerobic life. Manganese is also important in photosynthetic oxygen evolution.