Strontium

Strontium was discovered in the late 18th century, when Scottish physician and chemist Adair Crawford examined a mineral from the Strontian region in Scotland in 1790. The element takes its name from this geographic location. In 1793, the German chemist Martin Heinrich Klaproth confirmed that it was indeed a new mineral. In 1808, British chemist Sir Humphry Davy succeeded in isolating metallic strontium for the first time.

In the 19th century, strontium hydroxide was initially used in sugar refining. Later, the discovery of the intense red flame coloration of strontium salts made the element an important component of fireworks.

Industrial use began in the 20th century, when strontium oxide was used in television tubes to absorb X-rays.
Strontium has also been used as an alloying additive in aluminum and cast iron to improve their mechanical properties.

Strontium carbonate is the most common strontium compound and serves as the primary raw material for the production of other strontium compounds.

The main application of strontium is in pyrotechnics. Strontium salts, such as strontium nitrate, are responsible for the intense red coloration in fireworks, as well as in military and marine signal flares.

In metallurgy, strontium is used in aluminum–silicon alloys to improve their mechanical properties. In iron and steel production, it acts as a deoxidizing agent, helping to remove sulfur and phosphorus impurities.

Beyond pyrotechnics, strontium plays an important role in the production of ferrite magnets. The use of strontium in magnetic materials enables the development of more reliable and efficient components for motors, sensors, and audio devices.

Strontium compounds are also used in the glass and ceramics industry. When added to glass formulations, strontium can enhance optical properties and improve durability of the final product.

The radioisotope strontium-89 is used in medicine to relieve pain caused by bone metastases, acting as a beta emitterthat selectively destroys cancer cells in bone tissue.

New applications of strontium are currently being researched in medical, technological, and metrological fields, including its use in ultra-precise optical atomic clocks.
In 2024, a strontium atomic clock was recognized as the most accurate clock ever built.

Die wichtigsten Strontium-haltigen Minerale sind Coelestin und Strontianit. Coelestin ist wirtschaftlich bedeutender, da es häufiger vorkommt

Die größten Strontiumhersteller sind Iran und Spanien sowie Chin. In Spanien betreibt das deutsche Unternehmen Kandelium Barium Strontium GmbH zwei Minen bei Granada. Die Erze werden nach Bad Hönningen zur Weiterverarbeitung verschifft. Kandelium hat einen Weltmarktanteil von etwa 35 Prozent an der Stroniumproduktion und deckt 90 Prozent des EU-Bedarfs an Strontium ab.

Die Weltproduktion von Strontium beläuft sich auf etwa 500.000 Tonnen im Jahr.

Barium can replace strontium in ceramic ferrite magnets. However, the resulting barium compound has a lower maximum operating temperature compared to strontium-based materials.

The replacement of strontium in pyrotechnics is challenging, as it is difficult to achieve the same brilliance and visibility that strontium and its compounds provide.

Physical Properties

In its highest purity, strontium is a bright, pale golden metal, while in less pure form it appears silvery-white.
With a melting point of 777 °C and a boiling point of 1380 °C, strontium lies between the lighter calcium and the heavier barium, with calcium having a higher and barium a lower melting point. Strontium has the lowest boiling point of all alkaline earth metals after magnesium and radium.

With a density of 2.6 g/cm³, strontium is classified as a light metal. It is very soft, with a Mohs hardness of 1.5, and can be easily bent or rolled.

At room temperature, strontium crystallizes in a face-centered cubic structure (space group Fm3m, No. 225; copper type) with a lattice parameter a = 608.5 pm and four formula units per unit cell.
Two additional high-temperature modifications are known: Above 215 °C, it transforms into a hexagonal close-packed structure (magnesium type) with lattice parameters a = 432 pm and c = 706 pm. Above 605 °C, it adopts a body-centered cubic structure (tungsten type), which is the most stable phase at high temperatures.

Chemical Properties

Strontium is the third most reactive alkaline earth metal, following barium and radium. It reacts readily with halogens, oxygen, nitrogen, and sulfur, forming compounds in which it always appears as a divalent cation (Sr²⁺). When heated in air, strontium burns with a crimson-red flame, producing strontium oxide (SrO) and strontium nitride (Sr₃N₂).

As a very electropositive metal, strontium reacts vigorously with water, releasing hydrogen gas and forming strontium hydroxide [Sr(OH)₂]. The same hydroxide also forms upon exposure to moist air. Strontium is soluble in liquid ammonia, producing blue-black ammoniates.

In groundwater, strontium behaves similarly to calcium. Strontium compounds are generally insoluble under slightly acidic to basic conditions, but become soluble at lower pH values. During weathering processes, the removal of carbon dioxide (CO₂) promotes the precipitation of strontium together with calcium as strontium or calcium carbonates.
Additionally, a high cation exchange capacity (CEC) in soils enhances the binding of strontium.

A total of 34 isotopes and nine additional nuclear isomers are known. Of these, four—84Sr, 86Sr, 87Sr, and 88Sr—occur naturally. In the natural isotopic composition, the isotope 88Sr predominates with a proportion of 82.58%. 86Sr with 9.86%, 87Sr with 7.0%, and 84Sr with a proportion of 0.56% are less common.

90Sr is a beta emitter with a decay energy of 0.546 MeV and decays with a half-life of 28.78 years to 90Y, which in turn rapidly (t1/2 = 64.1 h) with the emission of high-energy beta radiation (ZE = 2.282 MeV) and gamma radiation to form stable 90Zr. It usually occurs as a secondary fission product. It is produced within a few minutes by multiple beta decay from primary fission products with a mass number of 90, which occur in 5.7% of all nuclear fissions of 235U in nuclear power plants and atomic bomb explosions. This makes 90Sr one of the most common fission products of all.

Large quantities of 90Sr are released into the environment during all nuclear disasters. Accidents in which 90Sr was released into the environment include the Windscale fire, in which 0.07 TBq of 90Sr was released, and the Chernobyl disaster, in which the released activity of 90Sr amounted to 800 TBq. After above-ground nuclear weapons tests, particularly in 1955–58 and 1961–63, the level of 90Sr in the atmosphere rose sharply. This, together with the contamination with 137Cs in 1963, led to the adoption of the Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space, and Under Water, which prohibited such tests in the signatory states. As a result, the contamination of the atmosphere decreased significantly again in the following years. The total activity of 90Sr released by nuclear weapons was approximately 6 × 1017 Bq (600 PBq).

The absorption of 90Sr, which can enter the body through contaminated milk, for example, is dangerous. The high-energy beta radiation of the isotope can alter cells in bones or bone marrow, thereby triggering bone tumors or leukemia. It is impossible to remove the strontium absorbed into the bones with chelating agents, as these prefer to complex calcium and the strontium remains in the bones. Decorporation with barium sulfate is only possible if it is carried out quickly after incorporation, before it can be incorporated into the bones. Degradation through biological processes is also very slow, with a biological half-life of 49 years in bones and an effective half-life of 90Sr of 18.1 years. It is possible that 90Sr binds to parathyroid cells. This would explain the high incidence of hyperparathyroidism among liquidators at the Chernobyl reactor.

87Sr is the decay product of the rubidium isotope 87Rb, which has a very long half-life of 48 billion years. The ratio of the different strontium isotopes can therefore be used in strontium isotope analysis to determine the age of rubidium- and strontium-containing rocks such as granite.

Strontium is stored in bones and teeth in varying amounts under different conditions. At the same time, the isotope ratio of 86Sr and 87Sr depends on the surrounding rocks. Therefore, the isotope ratios of strontium can sometimes be used to draw conclusions about the migratory movements of prehistoric humans.

Biological Significance

Only a few organisms use strontium in biological processes. These include Acantharia, single-celled eukaryotic organisms that belong to the radiolarians and are a common component of zooplankton in the sea. They are the only protists that use strontium sulfate as a building material for their skeletons. In doing so, they also cause changes in the strontium content in individual layers of the sea by first absorbing strontium and then sinking to deeper layers after they die, where they dissolve.

Physiological and Therapeutic Significance

Strontium is not essential, and only a few biological effects of the element are known. It is possible that strontium has an inhibitory effect on tooth decay.

In animal experiments with pigs, a diet rich in strontium and low in calcium caused symptoms such as coordination disorders, weakness, and paralysis.

Strontium has properties very similar to calcium. However, unlike calcium, it is only absorbed in small amounts through the intestines. This may be due to the larger ion radius of the element. The average strontium content in a 70-kilogram man is only 0.32 g, compared to about 1000 g of calcium in the body. Like calcium, the strontium absorbed is mainly stored in the bones, which is a treatment option for osteoporosis. A correspondingly high bioavailability is achieved through salt formation with organic acids such as ranelic acid or malonic acid.

89Sr is used as chloride (under the trade name “Metastron”) for radionuclide therapy of bone metastases.

Like other alkaline earth metals, strontium is combustible. It reacts with water or carbon dioxide, so these cannot be used as extinguishing agents. Metal fire extinguishers (class D) should be used for extinguishing, and dry sand, salt, and extinguishing powder can also be used. Furthermore, contact with water causes the formation of hydrogen, which is explosive. Small quantities of strontium can be disposed of by reacting it with isopropanol, tert-butanol, or octanol.

Like all alkaline earth metals, strontium occurs in stable compounds exclusively in the oxidation state +2. These are usually colorless salts that are often highly soluble in water.

Halides

Strontium forms halides with the halogens fluorine, chlorine, bromine, and iodine, each with the general formula SrX2. These are typical, colorless salts that are highly soluble in water, with the exception of strontium fluoride. They can be produced by reacting strontium carbonate with hydrohalic acids such as hydrofluoric acid or hydrochloric acid. Among other things, strontium chloride is used as an intermediate product for the production of other strontium compounds and in toothpaste, where it is intended to combat sensitive teeth.

Salts of Oxygen Acids

The strontium salts of oxygen acids such as strontium carbonate, strontium nitrate, strontium sulfate, and strontium chromate are particularly important in industry. Strontium carbonate is the most important commercial form of strontium compounds, with the majority of mined celestite being converted into strontium carbonate. It is mainly used in the manufacture of X-ray absorbing glass for cathode ray tubes, but also in the manufacture of strontium ferrite for permanent magnets or electroceramics. Strontium nitrate is mainly used in pyrotechnics for the red flame color typical of strontium, while yellow strontium chromate serves as a primer against corrosion of aluminum in aircraft and shipbuilding.

Other Strontium Compounds

Strontium(I) compounds have been detected as unstable intermediates in hot flames. Strontium(I) hydroxide, SrOH, similar to strontium(I) chloride, SrCl, is a strong emitter in the red spectral range and acts as the sole colorant in bright and deeply saturated red pyrotechnic flares.

Organic Strontium Compounds

Organic strontium compounds are only known and studied to a limited extent because they are very reactive and can also react with many solvents such as ethers. However, they are insoluble in nonpolar solvents. Among other things, a metallocene with pentamethylcyclopentadienyl anions (Cp*) was presented, which, unlike other metallocenes such as ferrocene, is angled in the gas phase.