Bromine, Br, is a red volatile liquid at room temperature, having
a red brown vapour and a member of
Group VIIb (i.e. the
Halogen Group of elements) of the periodic table.
It forms compounds in which its oxidation states is 1, 3, 5, or 7.
- Atomic Number : 35
- Atomic Mass : 79.909
- Melting Point : -7 degC
- Boiling Point : 58 degC
- Density : 3.1
The liquid is corrosive and harmful to human tissue and
bromine vapour irritates the eyes and throat.
Bromine was discovered by A J Balardin 1826AD, by the action of
Chlorine on the residues (i.e. Bromide salts) after the
crystallisation of the salt from the salt-marshes of Montpellier.
NaBr + Cl2 ==> NaCl + Br2
Balard also demonstrated that the liquid was an element and he
suggested the name Bromine (Greek, stench) for the liquid.
- Because of its reactivity, Bromine does not exist in the free
elemental state in nature, but small quantities are combined in
many silver ores.
- Compounds of Bromine also occurs in marine animals and plants.
- Bromide salts are found in seawater and in the salts deposits
Bromine occurs primarily as bromide salts (e.g. Sodium Bromide, NaBr)
in sea-water. On passing a stream of chlorine gas through a solution
containing Bromide Ions, Br(-), free elemental bromine is released.
2 Br(-) + Cl2 ==> Br2 + 2 Cl(-)
Bromine can be prepared in the laboratory by heating
Potassium Bromide, KBr, with dilute Sulphuric Acid, H2SO4,
and Manganese Dioxide, MnO2.
2 KBr + MnO2 + 3 H2SO4 ==> Br2 + 2 KHSO4 + MnSO4 +2 H2O
Bromine is manufactured industrially by allowing a solution of
Potassium Bromide to flow down a tower, against a stream
of Chlorine gas rising up through the tower.
2 KBr + Cl2 ==> Br2 + 2 KCl
Bromine is used in a vide range of industries :
- in the manufacture of 1,2-DibromoEthane, CH2BrCH2Br, which is
used as a petrol additive to prevent lead being deposited in the
cylinders (i.e. from the anti-knock agent, Tetra-Ethyl Lead, Pb(C2H5)4),
- in the manufacture of compounds used in
photography (e.g. Silver Bromide, AgBr, which is
the light sensitive material in film),
- in the manufacture of dyestuffs and drugs,
- in the analytical laboratory in testing for unsaturation
in organic compounds, where bromine adds across the multiple bonds
of unsaturated compounds,
- as a disinfectant, adsorbed in sticks of a diatomic brick,
- in making dyes, and
- in gold extraction
Detection and Analysis
Bromine is detected by the brown colour of its vapour, and
by its action on Iodide salts, (e.g. Sodium Iodide, NaI),
which are oxidises to free elemental Iodine.
2 NaI + Br2 ==> 2 NaBr + I2
The Iodine liberated in this reaction turns starch indicator
solution to a blue colour.
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