Strontium

Strontium (pronounced /ˈstrɒntiəm/) is a chemical element with 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 celestine and strontianite. The ⁹⁰Sr isotope is present in radioactive fallout and has a half-life of 28.90 years.

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Notable characteristics

Due to its extreme reactivity to air, this element occurs naturally only in compounds with other elements, as in the minerals strontianite and celestite.

Strontium is a bright silvery metal that is softer than calcium and even more reactive in water, which strontium decomposes on contact with 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 powered strontium metal will ignite spontaneously in air. Volatile strontium salts impart a 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

As a pure metal strontium is being used in strontium 90%-aluminium 10% alloys of an eutectic composition for the modification of aluminium-silicon casting alloys. The primary use for strontium compounds is in glass for colour television cathode ray tubes to prevent X-ray emission.

Other uses:

• 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 radioisotope thermoelectric generators (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).^[2] 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.
• Strontium is one of the constituents of AJ62 alloy, a durable magnesium alloy used in car and motorcycle engines by BMW.

⁸⁷Sr/⁸⁶Sr ratios are commonly used to determine the likely provenance areas of sediment in natural systems, especially in marine and fluvial environments. Dasch (1969) showed that surface sediments of the deep Atlantic displayed ⁸⁷Sr/⁸⁶Sr ratios that could be regarded as bulk averages of the ⁸⁷Sr/⁸⁶Sr ratios of geological terranes from adjacent landmasses. One good example of a fluvial system to which Sr isotope ratio studies have been frequently employed is the River Nile (Krom et al, 1999; Krom et al, 2002; Talbot et al. 2000). Due to the vastly differing ages of the rocks that constitute the majority of the Blue and White Nile catchment areas the changing provenance of sediment reaching the River Nile delta, and East Mediterranean Sea beyond, can be discerned through Sr isotopic studies. This information is useful as it can elicit information regarding climate change over geologic time scales.

More recently, ⁸⁷Sr/⁸⁶Sr ratios have also been used to determine the source of ancient archaeological materials such as timbers and corn in Chaco Canyon, New Mexico (English et al, 2001, Benson et al, 2003). ⁸⁷Sr/⁸⁶Sr ratios in teeth may also be used to track animal migrations (Porder et al., 2003) or in criminal forensics.

Compounds

• 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. This quality has also led to it being cut into gemstones, in particular as a diamond simulant. However, it is very soft and easily scratches so it is rarely used.
• Strontium carbonate, Strontium nitrate, and Strontium sulfate are commonly 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% total strontium chloride hexahydrate by weight.
• Strontium oxide is sometimes used to improve the quality of some pottery glazes.
• Strontium is also commonly used in aerosol paint, such as the Spanish Montana (Montana Hardcore). This is one of the most likely sources of exposure to the public.
• Strontium ranelate is used in the treatment of osteoporosis

History

The mineral strontianite is named after the Scottish village of Strontian, having been discovered in the lead mines there in 1787.^[3] Adair Crawford recognized it as differing from other barium minerals in 1790. Strontium itself was discovered in 1798 by Thomas Charles Hope, 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

  In 2005, China was the top producer of strontium with almost two-thirds world share followed by Spain and Mexico, reports the British Geological Survey.

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 (SrSO₄) 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. The largest commercially exploited deposits are found in England.

See also strontium minerals.

Isotopes

Main article: Isotopes of strontium

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 fission which is found in 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 innovative drug made by combining strontium with ranelic acid has aided in bone growth, boosted bone density and lessened vertebral, peripheral and hip fractures.^{[4] [5]} Women receiving the drug showed a 12.7% increase in bone density. Women receiving a placebo had a 1.6% decrease. 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 a true increase in bone mass. It means that strontium ranelate creates new and strong bone. Strontium ranelate (marketed under the trade names Protelos, Osseor, Protos, Bivalos, Protaxos, Ossum) is registered for treatment of osteoporosis in many countries all over the world. Strontium ranelate has been shown to strengthen bones, according presentations given the IOF World Congress on Osteoporosis, in June of 2006. It also reduced bone resorbtion.

Strontium ranelate is registered as a prescription drug in Europe and many countries worldwide. It needs to be prescribed by a doctor, delivered by a pharmacist and requires a strict medical supervision. Currently, (early 2007) it is not available in Canada or the United States.

Several other salts of strontium such as strontium citrate or strontium carbonate are often presented as natural therapies and sold at a dose that is several hundred times higher than the usual strontium intake. Despite the lack of strontium deficit referenced in the medical literature and the lack of information about possible toxicity of strontium supplementation, such compounds can still be sold in the United States under the Dietary Supplements Health and Education Act of 1994.

However, their long-term safety and efficacy have never been evaluated on humans using large-scale medical trials. Such compounds should not be administered to humans before further studies are conducted.

Allegedly, an attempt was made in 1968 to poison Alexander Dubček with Sr-90, but it failed.

See also

• Strontium compounds

References

1. ^ Strontium: strontium(I) fluoride compound data. Bernath.UWaterloo.ca. Retrieved on 2007-12-10.
2. ^ http://www.qrg.northwestern.edu/projects/vss/docs/Power/3-what-are-the-fuels-for-rtgs.html
3. ^ Murray, W.H. (1977) The Companion Guide to the West Highlands of Scotland. London. Collins
4. ^ Meunier PJ, Roux C, Seeman E et al. (2004). "effects of strontium ranelate on the risk of vertebral fracture in women with postmenopausal osteoporosis.". New England Journal of Medicine 350: 459-468. PMID 14749454.
5. ^ Reginster JY, Seeman E, De Vernejoul MC et al. (2005). "Strontium ranelate reduces the risk of nonvertebral fractures in postmenopausal women with osteoporosis: treatment of peripheral osteoporosis (TROPOS) study". J Clin Metab. 90: 2816-2822. PMID 15728210.

• Los Alamos National Laboratory – Strontium. Retrieved on August 5, 2005.
• Dasch, J. (1969). Strontium isotopes in weathering profiles, deep-sea sediments, and sedimentary rocks. Geochimica et Cosmochimica Acta, Vol. 33, pp. 1521-1552.
• Krom et al. (1999). The characterisation of Saharan Dusts and Nile particulate matter in surface sediments from the Levantine basin using Sr isotopes. Marine Geology, Vol. 155, pp. 319-330.
• Krom et al. (2002). Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development. Geology, Vol. 30, pp. 71-74.
• Talbot et al., (2000). Strontium isotope evidence for late Pleistocene reestablishment of an integrated Nile drainage network. Geology, Vol. 28, pp. 343-346.
• Benson, L., Cordell, L., Vincent, K., Taylor, H., Stein, J., Farmer, G., and Kiyoto, F. (2003) Ancient maize from Chacoan great houses: where was it grown?: Proceedings of the National Academy of Sciences , v. 22, p. 13111-13115.
• English, N.B., Betancourt, J.L., Dean, J.S. and J. Quade (2001) Strontium Isotopes Reveal Distant Sources of Architectural Timber in Chaco Canyon, New Mexico. Proceedings of the National Academy of Sciences of the United States of America, Vol. 98, pp. 11891-11896
• Porder, S., Paytan, A., and E.A. Hadly (2003) Mapping the origin of faunal assemblages using strontium isotopes. Paleobiology, 29: 197 - 204.