Iodine was discovered in 1812 by Courtois. Iodides occur in minute quantities in sea water. They are also found as organic compounds in some sea weeds and as deposits of sodium (NaIO3)and calcium trioxoiodates (V), (Ca (IO3)2) in association with sodium trioxonitrate(V).
Laboratory preparation of iodine
In the laboratory, iodine is prepared by the oxidation of sodium iodide, using manganese (IV) oxide in the presence of tetraoxosulphate (VI) acid.
Industrial preparation of iodine
Industrially, iodine is obtained from sodium or calcium trioxoiodate (V) by treatment with sodium hydrogen trioxosulphate (IV).
Physical properties of iodine
- It is a black shiny solid with density of 9gcm cube.
- It sublimes when heated rapidly.
- It is almost insoluble in water.
- It dissolves in potassium iodide solution to give a brown solution of potassium iodate.
- It is also soluble in ethanol, benzene, carbontetrachloride and ethoxyethane, forming brown solutions.
Chemical properties of iodine
- Iodine combines directly with many metals. e.g. silver and aluminium to form their iodides.
- It combines readily with non-metals. e.g. phosphorus.
- Like chlorine and bromine, iodine reacts with hydrogen to form hydrogen iodide which dissolves readily in water to form iodic acid.
- Iodine reacts with alkalis in cold to give iodides.
- Iodine is a mild oxidising agent. It will not perform many of the oxidising actions attributed to chlorine and bromine. It will however oxidise hydrogen sulphide to form hydrogen iodide and yellow sulphur deposit.
Uses of iodine
- Iodine is used in making dyes for colour photography.
- A dilute solution of iodine in ethanol (tincture) is widely used as antiseptic for cuts and wounds.
- Iodine is used in the treatment of simple goitre, a condition caused by a deficiency of iodine in our diet.
Test for iodine and iodides
- Add a drop of the unknown solution to some starch solution; a blue-black colouration due to the formation of an iodine-starch complex indicate the presence of iodine.
- Silver trioxonitrate (V) added to a solution of the iodide acidified with dilute trioxonitrate (V) acid yield yellow precipitate of silver iodide which is insoluble in excess ammonia solution.
- Addition of oxidising agent yield iodide which gives a purple colouration in carbon tetrachloride.