Bromine is the third lightest halogen element that has characteristic red-brown fumes at room temperature. Bromine can be evaporated easily to form a similarly coloured gas. The elemental form of bromine is extremely reactive and therefore does not occur freely in nature. Bromine was discovered by two chemists independently i.e., Carl Jacob in 1825 and Antoine Ballard in 1826. The occurrence of Bromine is significantly less abundant in the crust as compared to fluorine or chlorine. Bromine is the forty-sixth most abundant element found in the Earth's crust. Bromine is significantly more abundant in the oceans because of a long-term leaching process.
The synthesis of bromine can be done in the laboratory if we have chlorine source and react with a solution of bromide ions. Once bromine is formed, it must be separated from the solution appropriately.
Bromine can be obtained by oxidizing bromide ions:
2Br-(aq) -> Br2(l)+2e-
The process is divided into two stages:
a) Oxidation of bromide ions to bromine
b) Purification of bromine
Oxidation of Bromide Ions to Bromine
Chlorine and water are pumped up the blowing out tower, down the hot brine, in which rich bromide ions are flowing.
We can liberate Bromine from the solution by oxidizing the bromide ions by the chlorine gas:
Cl2(g) + 2Br-(aq) -> 2Cl-(aq) + Br2(g)
Purification of Bromine
The crude bromine, which is a mixture of water, chlorine and organic matter, is purified by the process of distillation. The bromine present in the solution is 'stripped out' with steam. It is important to heat the system to prevent the bromine from being left out in the solution. In the process, steam is passed through the liquid, under reduced pressure, until it boils. The bromine is therefore collected along with the condensed steam and separated into two layers as bromine is slightly soluble in water only.
The gas mixture that has the bromine vapours, residual chlorine and steam rise to the peak of the tower, while the liquid brine gets accumulated at the bottom of the tower. The tower is packed with suitable filling materials to maximize the contact area and the reaction time between the gases and the solution.
Bromine, chlorine and water vapour form a mixture of hot gases that are left on the top of the blowing out tower. This mixture goes through several steps that are discussed below.
As the name suggests, the gas mixture is left to cool down in this step. The hot gas mixture reaches the condenser, having a temperature at which only bromine can condense. The temperature condition is not suitable for chlorine to condense in the condenser, therefore, chlorine gas is separated from the liquid. As it gets separated, Chlorine leaves bromine and the water-rich condenser to return to the reaction tower. The liquid phase with chlorine and the water-containing bromine is shifted to a separator.
In the separator, two layers are formed i.e., the heavy, lower layer is the bromine. The lighter and the upper layer is the aqueous layer. The aqueous layer has bromine and chlorine, slightly soluble in water. After the separation is done, the layers are recycled to the reaction tower. The bromine layer containing chlorine and water as its impurities is further purified as necessary.
3. Purification and Drying
Even after the separation step, Bromine is not completely pure and has chlorine and water in it. The chlorine and most of the water are separated from Bromine by distillation and are recycled back to the reaction tower. The Residual water is then removed by a drying process, such as by treatment of wet bromide with concentrated sulfuric acid.
Bromine can also be synthesized by an industrial process.
Q1. What is a Blown-out Process?
Ans: The blown-out process makes use of air instead of steam because heating the ocean water using steam with its very low bromine content of 65 mg/L will be expensive. In the blown-out process, water containing bromide is pumped to the top of the blowing-out towers. To make sure that the mixing occurs in the brine during its ascent, Sulfuric acid and chlorine are added above the pumps. An additional 15% of chlorine is utilized over the theoretical amount required.
The air from the tower is drawn, sweeping out a mixture of bromine and chlorine (or bromine chloride) from the descending water. Next, through the absorber towers, the bromine-laden air is drawn in which it is scrubbed counter-currently with a sodium carbonate solution. Small packed chambers are interposed between the absorber towers and the fans to remove the spray from the air. Once 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.
Q2. What are the properties of Bromine?
Ans: Bromine is a non-metal that is located in group 17 of the periodic table. The properties of Bromine are therefore alike to those of its neighbouring elements such as fluorine, chlorine and iodine.
It also tends to be intermediate between the two neighbouring halogens i.e., chlorine and iodine.
The electronic configuration of Bromine is [Ar]3d104s24p5.
It has seven electrons in the fourth shell, which is the outermost shell, therefore, acting as its valence electrons.
79Br with 51% of natural bromine and 81Br with 49% of natural bromine are the two stable and natural isotopes of Bromine.
The simplest compound of bromine is hydrogen bromide (HBr) which is mostly used in the production of inorganic bromides and alkyl bromides.
It is also used as a catalyst for many reactions in organic chemistry.