We continue to review some of the most important materials in heat treatment and metallurgy.
Iodine (chemical symbol: I)
Iodine comes from the Greek word ioeides (violet colored), and it is one of the most important elements to mankind. The human body requires a daily intake of about 0.1 milligrams of iodine to help produce thyroxin, a hormone excreted by the thyroid gland that regulates the body's growth and temperature.
Our bodies typically contain up to 20 milligrams of iodine, mainly in the thyroid gland. Iodine is added to common table salt (iodized salt) to prevent diseases such as goiter, a swelling of the thyroid gland. Iodine, however, is poisonous in large amounts.
Iodine was discovered by French chemist Barnard Courtois in 1811 during the extraction of other elements (sodium, potassium) from seaweed ash. Iodine is found in seawater, although it is only present in trace amounts, in the order of 0.05 parts per million.
During the addition of sulfuric acid (H2SO4) to further process the ash, he accidentally added too much and a violet-colored cloud (Fig. 1) erupted from the mass. The gas condensed on metal objects in the room, creating solid iodine. Iodine exists as a lustrous, purple-black nonmetallic solid (Fig. 2) at standard temperature and pressure that sublimes readily to form a violet gas. Today, iodine is chiefly obtained from deposits of sodium iodate (NaIO3) and sodium periodate (NaIO4) in Chile, Bolivia and Japan.
Other chemical uses for iodine are as an antiseptic for external wounds when mixed with alcohol and as a radioisotope (Iodine-131) for the treatment of thyroid cancer. Iodine is used as a catalyst in the production of acetic acid and some polymers. Potassium iodide (KI) is found in photographic film.
Iodine is a member of group 17 (halogens) in the periodic table and is the fourth such element, below fluorine, chlorine and bromine. It is the heaviest member of its group that remains stable over time. Like the other halogens, it is one electron short of a full octet and is hence a strong oxidizing agent, but iodine is the weakest of the halogen family.
Here are a few important facts about iodine.[2,5]
- Atomic number: 49
- Atomic weight: 126.90447
- Melting point: 386.85 K (113.7°C or 236.7°F)
- Boiling point: 457.51 K (184.4°C or 363.8°F)
- Density: 4.93 grams per cubic centimeter @ 300 K
- Phase at room temperature: Solid
- Element classification: Nonmetal
- Period number: 5
- Group number: 17
- Group name: Halogen
- Electron configuration: [Kr] 4d105s25p5
- Periodic Table of the Elements, Sargent-Welch Catalog No. WLS-18806-10, 2004
- Chemicool (www.chemicool.com)
- Royal Society of Chemistry (rsc.org)