Chlorine (Greek: chloros, meaning "pale green") is a yellow-green poisonous gas with a disagreeable suffocating odor. It is soluble in non-polar liquids; boundedly soluble in water. There is an equilibrium in water chlorine solutions:
Cl2 + H2O --> HClO + H+ + Cl-
In nature chlorine consists of two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%). Its electron configuration is 3s23p5. Chlorine is a powerful oxidant, it exhibits the highest among the elements electron affinity. Its molecule is diatomic. In nature, chlorine is found mainly as the chloride ion.
It combines readily with nearly all metals and non-metals to form chlorides, although it is not as extremely reactive as fluorine. Besides the -1 oxidation state of the chlorides, chlorine also exhibits oxidation state respectively : +1 (hypochlorite, ClO- ), +3 (chlorite, ClO-2), +5 (chlorate, ClO-3) and +7 (perchlorate, ClO-4). Chlorine is a toxic gas that irritates the respiratory system, it became the first killing agent to be employed during World War I.
Almost chlorine gas is produced by electrolysis of sodium or potassium chloride water solution. The co-products of this process are caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H2):
Mercury cell electrolysis. Titanium anodes are located above a liquid mercury cathode in the electrolysis bath with a solution of sodium chloride. When an electrical current is applied, chlorine is released at the titanium anodes and sodium dissolves into the mercury cathode forming an amalgam. This method consumes vast amounts of energy.
Diaphragm cell method. In this route an asbestos or polymer diaphragm is deposited on an iron grid cathode preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing. Sodium hydroxide produced by this method is contaminated with sodium chloride (about 2-3%);
Membrane cell method. The electrolysis cell is divided into two by a ion exchange membrane. This method is nearly as efficient as the diaphragm cell and produces very pure sodium hydroxide but requires very pure sodium chloride solution.
2NaCl + 2H2O --> Cl2 + 2NaOH + H2
There are three commercial methods for the making of chlorine by electrolysis:
in making of bleaching and disinfectant agents such as calcium and sodium hypochlorites, chloramine and others;
as a raw material for metal and non-metal chlorides production (aluminium chloride AlCl3, titanium chloride TiCl4, phosphorus trichloride PCl3 and many others);
as a reagent in manufacturing of chlorinated hydrocarbons, such as vinyl chloride, chloroform, carbon tetrachloride, dichloroethanes and other products;
as an intermediate in pesticides and pharmaceuticals production;
as a bleaching agent in paper production;
as an intermediate in preparation of dyestuffs, food, paits, medicines, textiles, solvents;
for water purification and disinfection;