We continue to review some of the most important materials in heat treatment and metallurgy.
Strontium (chemical symbol: Sr)
Strontium is a soft metal that has a dull, green, grayish-white appearance (Fig. 1). It is highly chemically reactive. Like its group-2 neighbors magnesium, calcium and barium on the periodic table, it reacts with water to form hydrogen gas. Strontium is obtained from the naturally occurring minerals celestine and strontianite and is mostly mined from these. While elemental strontium is stable and nontoxic, a synthetic strontium isotope generated during detonation of nuclear weapons is radioactive and deadly to humans since it is readily absorbed by the body in a similar manner to calcium.
Strontium was discovered in 1790 by Irish chemist Adair Crawford in the city of Strontian, after which it was named. The element was first isolated and identified in 1808 by renowned English chemist Sir Humphry Davy, through the electrolysis of strontium chloride (SrCl2) and mercuric oxide (HgO). Davy, a pioneer of electrochemistry, was a mentor for famous scientist Michael Faraday, who pioneered the relationship between electricity and magnetism
One of the first major commercial uses for strontium was in cathode ray tubes for early color television sets. It was used on the screen to block dangerous X-rays. Today, strontium is used as a refining agent for zinc and is used with iron to make ferrite magnets. Two compounds of strontium – strontium nitrate (Sr(NO3)2) and strontium carbonate (SrCO3) – emit a bright, red flame when burned. Hence, they are used in signal flares, flare guns and fireworks.
Strontium carbonate is also used to make certain types of glass. Strontium aluminate, when combined with a dopant, absorbs light and releases it in the dark over time, making it useful in glow-in-the-dark paints, plastics and toys (Fig. 2). Strontium chloride is used at concentrations of 10% in toothpaste for people that have sensitive teeth (dentin hypersensitivity). The strontium ions block water and other fluids from flowing through microscopic canals in the tooth dentin called dentinal tubules, thereby relieving pain caused by hot, cold and certain foods.
Here are a few important facts about strontium.
- Atomic number: 38
- Atomic weight: 87.62
- Melting point: 1050 K (777°C or 1431°F)
- Boiling point: 1655 K (1382°C or 2520°F)
- Density: 2.64 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 5
- Group number: 2
- Group name: Alkaline-earth metal
- Minerals Education Coalition