Strontium

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Strontium

Strontium is a chemical element in the periodic table that has the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestite and strontianite. The ⁹⁰Sr isotope is present in radioactive fallout and has a half-life of 28.90 years.

Notable characteristics

Due to its extreme reactivity to air, this element occurs naturally only in compounds with other elements, as in the minerals strontianite, celestite, etc. It is isolated as a yellowish metal and is somewhat malleable. It is chiefly employed (as in the nitrate) to color pyrotechnic flames red.

Strontium is a bright silvery metal that is softer than calcium and even more reactive in water, which strontium decomposes on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380°C it will only form the oxide spontaneously at room temperature. It should be kept under kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal will ignite spontaneously in air. Volatile strontium salts impart a beautiful crimson color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four stable isotopes.

Applications

At present the primary use for strontium is in glass for color television cathode ray tubes.

Other commercial uses:

Production of ferrite magnets and refining zinc.
Strontium titanate has an extremely high refractive index and an optical dispersion greater than that of diamond, making it useful in a variety of optics applications.
Strontium titanate has been cut into gemstones, in particular for its use as diamond simulant. However, it is very soft and easily scratches so it is rarely used.
Strontium is also used in fireworks for red color.
Strontium aluminate is used as a bright phosphor with long persistence of phosphorescence.
Strontium chloride is sometimes used in toothpastes for sensitive teeth. One popular brand includes 10% strontium chloride hexahydrate by weight.
Strontium oxide is sometimes used to improve the quality of some pottery glazes.
⁸⁹Sr is the active ingredient in Metastron, a radiopharmaceutical used for bone pain secondary to metastatic prostate cancer. The strontium acts like calcium and is preferentially incorporated into bone at sites of increased osteogenesis. This localization focuses the radiation exposure on the cancerous lesion.
⁹⁰Sr has been used as a power source for RTGs. ⁹⁰Sr produces about 0.93 watts of heat per gram (it is lower for the grade of ⁹⁰Sr used in RTGs, which is strontium fluoride). ^* However, ⁹⁰Sr has a lifetime approximately 3 times shorter and has a lower density than ²³⁸Pu, another RTG fuel. The main advantage of ⁹⁰Sr is that it is cheaper than ²³⁸Pu and is found in nuclear waste.
⁹⁰Sr is also used in cancer therapy. Its beta emission and long half-life is ideal for superficial radiotherapy.

History

Adair Crawford recognized the mineral strontianite, named after the Scottish village of Strontian, as differing from other barium minerals in 1790. Strontium itself was discovered in 1798 and metallic strontium was first isolated by Sir Humphry Davy in 1808 using electrolysis.

Strontium was among the radioactive materials released by the 1957 Windscale fire.

Occurrence

Strontium commonly occurs in nature, the 15th most abundant element on earth, averaging 0.034% of all igneous rock and is found chiefly as the form of the sulfate mineral celestite (SrS O₄) and the carbonate strontianite (SrCO₃). Of the two, celestite occurs much more frequently in sedimentary deposits of sufficient size to make development of mining facilities attractive. Strontianite would be the more useful of the two common minerals because strontium is used most often in the carbonate form, but few deposits have been discovered that are suitable for development. The metal can be prepared by electrolysis of melted strontium chloride mixed with potassium chloride:

Sr²⁺ + 2 e^- → Sr
2 Cl^- → Cl_{2 (g)} + 2 e^-

Alternatively it is made by reducing strontium oxide with aluminium in a vacuum at a temperature at which strontium distills off. Three allotropes of the metal exist, with transition points at 235 and 540 °C. Strontium metal (98% pure) in January 1990 cost about $5/oz. The largest commercially exploited deposits are found in England.

See also Strontium minerals.

Isotopes

The alkali earth metal strontium has four stable, naturally occurring isotopes: ⁸⁴Sr (0.56%), ⁸⁶Sr (9.86%), ⁸⁷Sr (7.0%) and ⁸⁸Sr (82.58%). Only ⁸⁷Sr is radiogenic; it is produced by decay from the radioactive alkali metal ⁸⁷Rb, which has a half-life of 4.88 × 10¹⁰ years. Thus, there are two sources of ⁸⁷Sr in any material: that formed during primordial nucleo-synthesis along with ⁸⁴Sr, ⁸⁶Sr and ⁸⁸Sr, as well as that formed by radioactive decay of ⁸⁷Rb. The ratio ⁸⁷Sr/⁸⁶Sr is the parameter typically reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because strontium has an atomic radius similar to that of calcium, it readily substitutes for Ca in minerals.

Sixteen unstable isotopes are known to exist. Of greatest importance is ⁹⁰Sr with a half-life of 28.78 years. It is a by-product of nuclear fallout and presents a health problem since it substitutes for calcium in bone, preventing expulsion from the body. This isotope is one of the best long-lived high-energy beta emitters known, and is used in SNAP (Systems for Nuclear Auxiliary Power) devices. These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, etc, where a lightweight, long-lived, nuclear-electric power source is required. The 1986 Chernobyl nuclear accident contaminated a vast area with ⁹⁰Sr.

Precautions

In its pure form strontium is extremely reactive with air and spontaneously combusts. It is therefore considered to be a fire hazard.

Effect on the human body

The human body absorbs strontium as if it were calcium. Due to the elements being sufficiently similar chemically, the stable forms of strontium do not pose a significant health threat, but the radioactive ⁹⁰Sr can lead to various bone disorders and diseases, including bone cancer. The strontium unit is used in measuring radioactivity from absorbed ⁹⁰Sr.

An experimental drug made by combining strontium with ranelic acid has aided in bone growth, boosted bone density and lessened fractures. (El-Hajj, 2004) Women receiving the drug showed a 6.8% increase in bone density. Women receiving a placebo had a 1.3% decrease. (Meunier et. al, 2004). Half the increase in bone density (measured by x-ray densitometry) is attributed to the higher atomic weight of Sr compared with calcium, whereas the other half is assumed to be a true increase in bone mineral content. Strontium ranelate is registered for treatment of osteoporosis in Europe at a dose of 2 grams daily.

References

External links

WebElements.com – Strontium

Chemical elements | Alkaline earth metals | Pyrotechnic chemicals | Lochaber

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