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Chemical formula I
Atomic Number 53 
OTP appearance purple crystal 
Molar Mass(g/mol) 126.9 
Melting Point(°C) 118 
Boiling Point(°C) 183 
Density(g/cc) 4.9 
Solubility in water(g/L) 0.28 
Solubility in ethanol(g/L) 131
NFPA 704


Justification Questioned


  • Feedstock for WHO LEM lugol's solution
  • Prophylaxis for iodine-131 (radiation) poisoning

Natural Sources

  • Elemental iodine does not occur naturally.
  • Iodine occurs in kelp and seawater.


Like most halides, iodine is poisonous, but it is the least poisonous of the bunch.



The general source of iodides (and thus iodine) is from burnt kelp. The references at kelp show 0.1-5 grams (.02 to 1 ml) of iodine per kilo of wet kelp.

with sulfuric acid

  1. Gather dry kelp
  2. Place in alembic
  3. Add concentrated sulfuric acid in excess
  4. Organics are reduced to C,CO2,N2,H2O,O2
  5. Heat sublimes iodine which condenses as crystals or is absorbed by the water
  6. Wash condenser thoroughly with distilled water
  7. Combine wash water with distillate
  8. Evaporate
  9. Crystals contain iodine.

with hydrogen peroxide

high yield

by burning

To maximize retention of vaporous iodine, direct gases from charring through a water bubbler

  1. Gather dry kelp
  2. Char at the lowest possible temperature
  3. Combine gas entrainment water with residue
  4. Add more water until all soluble material is dissolved
  5. Add calcium hydroxide in excess
  6. Filter
  7. Discard the residue
  8. Evaporate
  9. Crystals contain iodine


from alkaline iodides

via sulfuric acid

Combine with sulfuric acid and an oxygen donor such as manganese dioxide.

2 KI + MnO2 + 3 H2SO4 I2 + 2 KHSO4 + MnSO4 + 2 H2O
via copper sulfate

Combine with copper sulfate, which undergoes cation exchange and spontaneous decomposition. (Yield: 50%)

2 CuSO4(aq) + 4 KI(aq) 2 CuI2(s) + 2 K2SO4(aq)
2 CuI2 2 CuI + I2


  • Sublime and condense between 113.7°C and 184.3°C
  • Iodine dissolves in diethyl ether but not in water.

Other data


Solubility of Iodine[1]
Temp 0°C 25°C 35°C
Solvent Wt% Mol% Wt% Mol% Wt% Mol%
carbon disulfide 7.7 16.49 21.63
Cyclohexane 2.723 3.907
Ethanol 16.67 3.51 21.38 4.71 24.70 5.59
Diethyl ether 19.34 6.54 25.22 8.96 28.50 10.43
Para-xylene 16.55 7.66 20.11 9.54
Trimethyl benzene 13.51 6.89 20.20 10.72




See Also

Iodine is obtained either from kelp (the ashes of burnt seaweed) or from the mother-liquors obtained in the purification of Chile saltpetre. In the former case the seaweed is burnt in large heaps, care being taken that too high a temperature is not reached, for if the ash be allowed to fuse much iodine is lost by volatilization. The product obtained after burning is known either as kelp or varec. Another method of obtaining kelp is to heat the seaweed in large retorts, whereby tarry and ammoniacal liquors pass over and a very porous residue of kelp remains. A later method consists in boiling the weed with sodium carbonate; the liquid is filtered and hydrochloric acid added to the filtrate, when alginic acid is precipitated; this is also filtered off, the filtrate neutralized by caustic soda, and the whole evaporated to dryness and carbonized, the residue obtained being known as kelp substitute.

The kelp obtained by any of these methods is then lixiviated with water, which extracts the soluble salts, and the liquid is concentrated, when the less soluble salts, which are chiefly alkaline chlorides, sulphates and carbonates, crystallize out and are removed. Sulphuric acid is now added to the liquid, and any alkaline sulphides and sulphites present are decomposed, while iodides and bromides are converted into sulphates, and hydriodic and hydrobromic acids are liberated and remain dissolved in the solution. The liquid is run into the iodine still and gently warmed, manganese dioxide in small quantities being added from time to time, when the iodine distils over and is collected. In the second method it is found that the mother-liquors obtained from Chile saltpetre contain small quantities of sodium iodate NaIO3; this liquor is mixed with the calculated quantity of sodium bisulphite in large vats, and iodine is precipitated:

2 NaIO3 + 5 NaHSO3 H2O + 2 Na2SO4 + 3 NaHSO4 + I2

The precipitate is washed and then distilled from iron retorts. Iodine may also be prepared by the decomposition of an iodide with chlorine, or by heating a mixture of an iodide and manganese dioxide with concentrated sulphuric acid. Commercial iodine may be purified by mixing it with a little potassium iodide and then subliming the mixture; in this way any traces of bromine or chlorine are removed. J. S. Stas recommends solution of the iodine in potassium iodide and subsequent precipitation by the addition of a large excess of water, the precipitate being washed, distilled in steam, and dried in vacuo over solid calcium nitrate, and then over solid caustic baryta.


  1. Hildebrand, J. H.; Benesi first3=L. M, H. A; Mower (1950) "Solubility of Iodine in Ethyl Alcohol, Ethyl Ether, Mesitylene, p-Xylene, 2,2-Dimethylbutane, Cyclohexane and Perfluoro-n-heptane"
    J. Am. Chem. Soc. 72(2); pp1017–1020. 
    link courtesy ACS.