Iodine: a purple nonmetal

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

Bi­o­log­i­cal role

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.