Iodine: a purple nonmetal
Experiments with iodine and its areas of application
Iodine (I₂) is a hard, lustrous, purplish-grey nonmetal that sublimes easily to form a purple gas. It dissolves easily in organic solvents such as ethanol. Iodine and its compounds are relatively rare – iodine is only the 61st most common substance in the earth’s crust.
Iodine is one of the few simple substances capable of sublimating (passing straight from solid to gaseous form) under normal circumstances. This characteristic is widely applied both to purify iodine and to reveal fingerprints in criminal analysis. In addition to subliming, iodine dissolves easily in the fats in sweat traces and, as it gets “stuck” in these fats, reveals any fingerprints.
History of discovery
In 1811, Bernard Courtois accidentally used a higher concentration of sulfuric acid than usual while obtaining potassium nitrate from seaweed ash. A purple gas appeared, which condensed as purple crystals – iodine! However, Courtois didn’t pursue any study of this unfamiliar gas. A few years later, independently of one another, Humphry Davy and Joseph Louis Gay-Lussac proved that the gas Courtois discovered was in fact a new chemical element. They gave the new substance the now-familiar name of “iodine,” which means “purple” in Greek.
Creation and chemical properties
Iodine can be created in a laboratory by mixing solutions of potassium iodide (KI), hydrogen peroxide (H₂O₂), and citric acid (C₆H₈O₇):
6KI + 3H₂O₂ + 2C₆H₈O₇ → 3I₂↓ + 2K₃C₆H₅O₇ + 6H₂O
Meanwhile, the resulting molecular iodine reacts with the remainder of potassium iodide to form potassium triiodide, which tints the solution a reddish-brown color:
KI + I₂ → KI₃
Iodine is a relatively active nonmetal. It reacts vigorously with aluminum (Al) to form aluminum iodide (AlI₃). The reaction is initiated by adding a few drops of water, in which iodine partially dissolves to form hydroiodic acid (HI) and hypoiodous acid (HIO). The resulting acids dissolve the oxide film on the surface of the aluminum, which then begins to react with iodine. The reaction is accompanied by an intense release of heat, which causes some of the iodine to sublime, forming a purple gas.
I₂ + H₂O → HI + HIO
Al₂O₃ + 3HI + 3HIO → AlI₂ + Al(IO)₃ + 3H₂O
2Al + 3I₂ → 2AlI₃
As iodide ions are powerful reductive agents, potassium iodide (KI) reacts with copper sulfate solutions (CuSO₄) to form copper(I) iodide (CuI) and molecular iodine.
4KI + 2CuSO₄ → 2CuI↓ + I₂↓ + 2K₂SO₄
Again, the resulting molecular iodine reacts with the remaining potassium iodide to create potassium triiodide, which tints the solution a reddish-brown color:
KI + I₂ → KI₃
A solution can be tested for the presence of iodide ions using a qualitative reaction. Iodide ions react with a solution of silver nitrate (AgNO₃), to produce a yellow precipitate of silver iodide (AgI).
AgNO₃ + KI → AgI↓ + KNO₃
Another qualitative reaction used to test for the presence of iodide ions is the reaction known as “Golden Rain.” Mixing heated solutions of potassium iodide and lead(II) nitrate (Pb(NO₃)₂) and acidifying the solution slightly will lead to the formation of beautiful golden crystals of lead(II) iodide (PbI₂) as the solution cools.
2KI + Pb(NO₃)₂ → PbI₂↓ + 2KNO₃
Iodine forms a characteristic dark-blue starch-iodine complex with starch. This characteristic is used in chemistry to test for the presence of molecular iodine, and in the stunning “Egyptian Night” experiment.
Another popular experiment, “Elephant's toothpaste,” can be performed by combining liquid soap with solutions of hydrogen peroxide and potassium iodide in a flask. When the solutions of hydrogen peroxide and potassium iodide are mixed, a redox reaction begins, and the reaction mixture heats up. Over the course of the reaction, oxygen and water vapor are released, which whip the liquid soap into a foam.
3H₂O₂ + 2KI= I₂ + O₂ + 2H₂O + 2KOH
Iodine is present in an important thyroid hormone known as triiodothyronine, which participates in the growth and development of the body. Iodine deficiencies can lead to stunted growth, thyroid disorders, and impaired cognitive function.
People living in iodine-poor areas, such as those far from the sea or high in the mountains, must consume more marine products such as seaweed, fish, and mollusks to compensate. There is, however, a cheaper alternative – iodized salt, ordinary kitchen salt with added potassium iodate. People need approximately 150 mcg of iodine daily, about half a teaspoon of iodized salt, for normal development and function.
Applications of iodine and its compounds
Iodine is used in chemical manufacturing as a catalyst in the production of acetic acid.
In pharmaceutical manufacturing, it is used in aqua-alcoholic solutions of iodine such as sodium iodide or potassium iodide, which are used as antiseptics. Iodine is also contained in many contrast media, which are used to improve the visibility of various bodily structures in X-rays visualization methods such as computer tomography.