Preparation of Bromine in Lab

Bromine is the third lightest halogen element with characteristic fuming red-brown fumes at room temperature. Bromine can easily evaporate readily to form a similarly coloured gas. Elemental form of bromine is very reactive and thus does not occur free in nature. Bromine was discovered independently by two chemists, Carl Jacob and Antoine Balard,in 1825 and 1826, respectively.

Bromine Properties

Bromine being a nonmetal, positioned in group 17 of the periodic table. Its properties are thus similar to those of neighbouring elements like fluorine, chlorine, and iodine, and tend to be intermediate between those of the two neighbouring halogens, chlorine and iodine. Bromine has the electron configuration [Ar]3d104s24p5, with the seven electrons in the fourth and outermost shell acting as its valence electrons.

Bromine has two stable isotopes, 79Br and 81Br. These are the only two natural isotopes, with 79Br making up 51% of natural bromine and 81Br making up the remaining 49%.

The simplest compound of bromine is hydrogen bromide (HBr). It is mainly used in the production of inorganic bromides and alkyl bromides and as a catalyst for many reactions in organic chemistry.

Bromine Occurrence

Bromine is significantly less abundant in the crust than fluorine or chlorine. It is the forty-sixth most abundant element in Earth's crust.It is significantly more abundant in the oceans, resulting from long-term leaching process.

Bromine Synthesis

To make bromine in the laboratory we need to find a chlorine source and react it with a solution of bromide ions. After formation of the bromine, it must be separated from the solution in a suitable way.

Bromine is obtained by oxidation of bromide ions:
2Br-(aq) -> Br2(l)+2e-

The process occurs in two stages:
a) oxidation of bromide ions to bromine
b) purification of bromine

(a) Oxidation of bromide ions to bromine

Chlorine and water are pumped up a tower, down which hot brine, rich in bromide ions, is flowing.

Bromine is liberated from the solution by oxidation of the bromide ions by the chlorine gas:
Cl2(g) + 2Br-(aq) -> 2Cl-(aq) +Br2(g)

(b) Purification of bromine

The crude bromine, containing water, chlorine and organic matter, is purified by distillation.The bromine in the solution is 'stripped out' with steam.It is necessary to heat the system to prevent the bromine from remaining in solution. Heating is provided by steam. In one process, steam is pumped through the liquid, under reduced pressure, until it is boiling. The bromine is collected together with the condensed steam and separated into two layers as bromine is only slightly soluble in water.

The gas mixture containing the bromine vapours, residual chlorine and steam rises to the top of the tower, while the liquid brine accumulates at the bottom of the tower. The tower is packed with suitable filling materials to increase the contact area and the reaction time between the gases and the solution.

Bromine Separation Steps
A mixture of hot gases containing bromine, chlorine and water vapour leaves the top of the tower. This mixture undergoes a number of work-up steps.

1. Condensation
The first step is to cool the gas mixture. The hot gas mixture arrives in the condenser, which has a temperature at which bromine, but not chlorine, condenses. At the temperature conditions in the condenser, the chlorine gas is separated from the liquid and after leaving the bromine and water-rich condenser it is returned to the reaction tower. The liquid phase containing chlorine and water-containing bromine is transferred t0 a separator.

2. Separation
Two layers are formed in the separator. The heavy, lower layer is the bromine. The lighter, upper layer is the aqueous layer. The aqueous layer contains bromine and chlorine, which are slightly soluble in water. After separation, this layer is recycled to the reaction tower. The bromine layer, which contains chlorine and water as impurities, is further purified as necessary.

3. Purification and Drying
Bromine obtained after the separation step is not completely pure and contains chlorine and water. The chlorine and most of the water are separated by distillation and recycled to the reaction tower. Residual water is removed by a drying process, such as by treatment of wet bromine with concentrated sulfuric acid.

There is also an industrial process, by which bromine can be synthesised.

Blowing-Out Process

The blowing-out process uses air rather than steam. (The cost of steam to heat ocean water, with its very low bromine content of 65 mg/L, is prohibitively expensive.) In this process, bromide-containing water is pumped to the top of blowing-out towers. Sulfuric acid and chlorine are added above the pumps to ensure that mixing occurs in the brine during its ascent. An excess of about 15% chlorine is used over the theoretical amount required. Air is drawn through the towers, thus sweeping out a mixture of bromine and chlorine (or bromine chloride) from the descending water.

The bromine-laden air is drawn next through the absorber towers in which it is scrubbed counter currently with a sodium carbonate solution. To remove the spray from the air, small packed chambers are interposed between the absorber towers and the fans. When the alkalinity of the scrubber solution is nearly depleted, the solution is transferred to a storage tank and then to a reactor where it is treated with sulfuric acid and steamed to release bromine. Subsequently, the bromine is condensed.

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