Magnesium

Magnesium is the chemical element in the periodic table that has the symbol Mg and atomic number 12. Magnesium is the eighth most abundant element and constitutes about 2% of the Earth's crust by weight (Gardner,2005), and it is the third most plentiful element dissolved in seawater. This alkaline earth metal is primarily used as an alloying agent to make aluminium-magnesium alloys.

Contents

1 Notable characteristics 2 Uses 3 History 4 Sources 5 Compounds in living organisms 6 Food sources 7 Isotopes 8 Precautions

Notable characteristics

Magnesium is a fairly strong, silvery-white, light-weight metal (one third lighter than aluminium) that slightly tarnishes when exposed to air. In a powder, this metal heats and ignites when exposed to moisture and burns with a white flame. It is difficult to ignite in bulk, though it is easy to light if it is shaved into thin strips. Once ignited, it is difficult to extinguish, being able to burn in both nitrogen (forming magnesium nitride), and carbon dioxide.

Uses

Magnesium compounds, primarily magnesium oxide, are used mainly as refractory material in furnace linings for producing iron and steel, nonferrous metals, glass, and cement. Magnesium oxide and other compounds also are used in agricultural, chemical, and construction industries. This element's principal use is as an alloying additive to aluminium with these aluminium-magnesium alloys being used mainly for beverage cans. Magnesium alloys also are used as structural components of automobiles and machinery. Another use of this metal is to aid the removal of sulfur from iron and steel.

Other uses include:

• Magnesium, like aluminium, is strong and light, so it is used in several high volume automotive and truck components. Specialty, high grade car wheels of magnesium alloy are called "mag wheels."
• Photoengraved plates in the printing industry.
• Combined in alloys, this metal is essential for airplane and missile construction.
• When used as an alloying agent, this metal improves the mechanical, fabrication and welding characteristics of aluminium.
• Additive agent for conventional propellants and used in producing nodular graphite in cast iron.
• Reducing agent for the production of pure uranium and other metals from their salts.
• Its hydroxide is used in milk of magnesia, its chloride and sulfate in Epsom salts and its citrates in medicine.
• Dead-burned magnesite is used for refractory purposes such as brick and liners in furnaces and converters.
• Magnesium is also flammable, burning at a temperature of approximately 4000°F.
• The extremely high temperature at which magnesium burns makes it a handy tool for starting emergency fires during outdoor recreation.
• Magnesium carbonate (MgCO₃) powder is also used by athletes, such as gymnasts and weightlifters, to improve the grip on objects – the apparatus or lifting bar.
• Magnesium stearate is a slightly flammable white powder with lubricative properties. In pharmaceutical technology it's used in the manufacturing of tablets, to prevent the tablets from sticking to the equipment during the tablet compression process (i.e., when the tablet's substance is pressed into a tablet form).
• Other uses include flashlight photography, flares, and pyrotechnics, including incendiary bombs.

History

The name originates from the Greek word for a district in Thessaly called Magnesia. Joseph Black in England recognized magnesium as being an element in 1755, Sir Humphry Davy electrolytically isolated pure magnesium metal in 1808 from a mix of magnesia and HgO and A. A. B. Bussy prepared it in coherent form in 1831. Magnesium is the eighth most abundant element in the earth's crust. It is an alkaline earth metal and therefore does not occur uncombined with other elements. It is found in large deposits of magnesite, dolomite, and other minerals.

Sources

In the United States this metal is principally obtained by electrolysis of fused magnesium chloride from brines, wells, and sea water. Although magnesium is found in over 60 minerals, only dolomite, magnesite, brucite, carnallite, talc, and olivine are of commercial importance.
Isolation (* follow):
cathode: Mg²⁺* + 2e^- → Mg
anode: 2Cl^-* → Cl₂ (gas) + 2e^-

Compounds in living organisms

Organic magnesium is important in both plant and animal life. Chlorophylls are magnesium-centered porphyrins. The adult daily nutritional requirement, which is affected by various factors including gender, weight and size, is 300-400 mg/day. Many enzymes require the presence of magnesium ions for their catalytic action, especially enzymes utilizing ATP. Inadequate magnesium intake frequently causes muscle spasms, and has been associated with cardiovascular disease, diabetes, high blood pressure and osteoporosis. Acute deficiency is rare. It is never due to an insufficiency of food contribution. It is the fact of various specific diseases whose majority are rare.

Food sources

Green vegetables such as spinach provide magnesium because the center of the chlorophyll molecule contains magnesium. Nuts, seeds, and some whole grains are also good sources of magnesium.

Although magnesium is present in many foods, it usually occurs in small amounts. As with most nutrients, daily needs for magnesium cannot be met from a single food. Eating a wide variety of foods, including five servings of fruits and vegetables daily and plenty of whole grains, helps to ensure an adequate intake of magnesium.

The magnesium content of refined foods is usually low. Whole-wheat bread, for example, has twice as much magnesium as white bread because the magnesium-rich germ and bran are removed when white flour is processed. The table of food sources of magnesium suggests many dietary sources of magnesium.

Water can provide magnesium, but the amount varies according to the water supply. "Hard" water contains more magnesium than "soft" water. Dietary surveys do not estimate magnesium intake from water, which may lead to underestimating total magnesium intake and its variability.

Following are some foods and the amount of magnesium in them:

• spinach (1/2 cup) = 80 milligrams (mg)
• peanut butter (2 tablespoons) = 50 mg
• black-eyed peas (1/2 cup) = 45 mg
• milk, low fat (1 cup) = 40 mg

Isotopes

Magnesium-26 is a stable isotope that has found application in isotopic geology, similar to that of aluminium. Mg-26 is a radiogenic daughter product of Al-26, which has a half-life of 717000 years. Large enrichments of stable Mg-26 have been observed in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites. The anomalous abundance of Mg-26 is attributed to the decay of its parent Al-26 in the inclusions. Therefore, the meteorite must have formed in the solar nebula before the Al-26 had decayed. Hence, these fragments are among the oldest objects in the solar system and have preserved information about its earliest history.

It is conventional to plot Mg-26/Mg-24 against an Al/Mg ratio. In an isochrone plot, the Al/Mg ratio plotted is Al-27/Mg-24. The slope of the isochron has no age significance, but indicates the initial Al-26/Al-27 ratio in the sample at the time when the systems were separated from a common reservoir.

Precautions

• Magnesium metal and alloys are highly flammable in their pure form and melts when it is a powder. Magnesium metal quickly reacts exothermically upon contact with air or water and should be handled with care. One should wear safety glasses while working with magnesium. The bright white light (including ultraviolet) produced by burning magnesium can damage the eyes. Water should not be used to extinguish magnesium fires, because it can actually feed the fire, according to the reaction:

Mg_(s) + 2 H₂O_(l) → Mg(OH)_2(aq) + H_2(g)

Carbon dioxide fire extinguishers should not be used either, because magnesium can burn in carbon dioxide. A Class D dry chemical fire extinguisher should be used if available, or else the fire should be covered with sand.

• The DRI upper tolerated limit for supplemental magnesium is 350 mg/day. The most common symptom of excess magnesium is diarrhea. Infants should not be given magnesium supplements.