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Iodine

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.

Atomic Number : 53
Atomic Mass : 126.904
Melting Point : 113.5 degC
Boiling Point : 1834 degC
Density : 4.94

There is one stable isotope, iodine-127 and fourteen radioactive isotopes.

Discovery

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.

Occurrence

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.

Extraction

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

Preparation

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

Manufacture

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.

Properties

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

Reactions

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.

Uses

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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