Iodine

 

Iodine

Chemical formula	I
Atomic Number	53
OTP appearance	purple crystal
Molar Mass(g/mol)	126.9
Density(g/cc)	4.9
Melting Point(°C)	118
Boiling Point(°C)	183
Solubility in water(g/L)	0.28
Solubility in ethanol(g/L)	131
NFPA 704	0 3 0

Uses

Justification Questioned

Other

• 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.

Hazards

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

Production

Extraction

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.

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

Synthesis

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

Purification

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

Other data

Solubility

Solubility of Iodine

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

• Cooley, Arnold James (1897) "Cooley's Cyclopædia of Practical Receipts and Collateral Information in the Arts, Manufactures, Professions, and Trades, Including Medicine, Pharmacy, and Domestic Economy: Designed as a Comprehensive Supplement to the Pharmacopœia and General Book of Reference for the Manufacturer, Tradesman, Amateur, and Heads of Families"; pp902.
  J. & A. Churchill
• Encyclopedia Britannica, 1911

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.