Iodine, I, is a dark violet (Greek, ioeides, violet) non-metallic halogen
element belonging to
Group VIIb (i.e. the
Halogen Group of elements) of the periodic table.
There is one stable isotope, iodine-127 and fourteen radioactive
- Atomic Number : 53
- Atomic Mass : 126.904
- Melting Point : 113.5 degC
- Boiling Point : 1834 degC
- Density : 4.94
Iodine was discovered by B Courtois in 1811AD. He treated the
liquor obtained from the extraction of kelp (i.e. seaweed), which
is now known to contain iodide salts, with sulphuric acid to produce
a vapour with a violet colour.
2 NaI + H2SO4 ==> l2 + Na2SO4
In 1812AD, Guy Lussac demonstrated
that iodine was an element and its chemical relationship to chlorine. About the same time,
Davy confirmed many of these properties.
Because of its reactivity, iodine does not exist in the free
elemental state in nature, but small quantities are combined
in many silver ores found in Mexico.
Compounds of iodine occurs naturally in seaweed.
Iodide salts are found in liquor from the extraction of saltpeter from deposits in Chile.
Iodine is extracted from kelp, which is obtained by burning seaweed.
Salts such as sodium chloride, potassium chloride, and potassium sulphate
are removed from the kelp by washing with water. The residue is then
heated with manganese dioxide and concentrated sulphuric acid, and the
iodine is liberated.
2 I(-) + MnO2 + H(+) ==> Mn(2-) + 2 H2O + I2
Iodine can be prepared in the laboratory by heating potassium
Iodide or sodium iodide with dilute sulphuric acid and manganese dioxide.
2 KI + MnO2 + 3 H2SO4 ==> I2 + 2 KHSO4 + MnSO4 +2 H2O
Iodine is manufactured industrially
- from the liquor remaining after the extraction of potassium
nitrate from saltpeter found in deposits in Chile,
- by extraction from the ash from the combustion of seaweed. In the
process, the sea-weeds are burnt in shallow pits and the resulting kelp
(i.e. the ash from the process) contains potassium salts and from
0.4 per cent to 1.3 per cent of iodine in the form of iodide salts
(e.g. potassium iodide),
- in a wet extraction of the seaweed with sodium carbonate solution
is prepared and an organic adhesive is precipitated on acidifying
of this solution, and
- by extraction from salt brine.
Iodine is a violet dense solid, which is insoluble in water, but it
is soluble in non-polar solvents. When heated it gives a violet
vapour that sublimes.
Iodine is soluble in a solution of potassium iodide due to the
formation of the soluble potassium tri-iodide.
KI + I2 ==> KI3
Iodine resembles chlorine and bromine in many of its properties, but
it is much less reactive.
Iodine combines directly with many elements, although it is usually necessary to provide
heat and/or a catalyst.
Iodine reacts quantitatively with sodium thiosulphate
I2 + 2 Na2S2O3 ==> 2 NaI + NaS4O6
This reaction is of importance in
volumetric analysis for the
estimation of oxidising agents.
Iodine is used
- in medicine, where its solution in alcohol, called "tincture of iodine",
is used as a disinfectant,
- in the manufacture of compounds used in photography (e.g.
silver iodide which is a light sensitive material used in film),
- in the manufacture of dyestuffs and drugs, and
- as a reagent in analytical chemistry.
Detection and Analysis
Iodine also produces a deep blue colour when it forms a complex with
starch solution. Thus, a starch solution is used as an indicator
for free elemental iodine in solution during titration's.
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