Halogen element - Production and use

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Production and use

Rock salt deposits are usually mined; occasionally water is pumped down and , containing about 25 percent sodium chloride, is brought to the surface. When the brine is evaporated, impurities separate first and can be removed. In warm climates salt is obtained by evaporation of shallow seawater by the sun, to give bay salt.

Chlorine is produced on a large scale by any of a number of different methods:

1. By electrolysis of a concentrated solution of sodium chloride in water. Hydrogen is evolved at the cathode, and chlorine at the anode. At the same time, an alkali metal hydroxide is produced in the electrolyte, and hence this process is often referred to as chlorine-alkali-electrolysis.

The chemical reactions that take place at each electrode and the overall cell process are given in the following equations:

in which the symbol e- represents a single electron. In the reaction vessel, free chlorine and hydroxide ions must not come in contact with each other, because chlorine would be consumed according to the reaction

To accomplish the separation of chlorine gas and the hydroxide ion, a porous wall is inserted between the electrodes (diaphragm process), or the iron cathode is replaced by a cathode consisting of liquid mercury (mercury cathode process) which avoids the production of hydroxide ions at the electrode. Instead, free sodium is discharged at the cathode, and this metal is readily dissolved in the mercury forming an amalgam, as follows:

The amalgam is allowed to react with water outside the cell:

The overall process is equivalent to the cell process given above.

2. By electrolysis of fused sodium chloride, which also produces metallic sodium; chlorine is again evolved at the anode.

3. By electrolysis of fused magnesium chloride, in which chlorine is formed as a by-product in the manufacture of metallic magnesium.

4. By oxidation of hydrogen chloride. In this process gaseous hydrogen chloride mixed with air or oxygen is passed over pumice in contact with cupric chloride as a catalyst, as shown in the following equation:

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The amalgam is allowed to react with water outside the cell:

The overall process is equivalent to the cell process given above.

2. By electrolysis of fused sodium chloride, which also produces metallic sodium; chlorine is again evolved at the anode.

3. By electrolysis of fused magnesium chloride, in which chlorine is formed as a by-product in the manufacture of metallic magnesium.

The equilibrium constant for this reaction decreases with increase of temperature; i.e., the reaction proceeds less extensively at higher temperatures. In practice, however, a temperature of 400 C is required to achieve a reasonable rate of conversion.

5. Of historical interest is the process in which a mixture of almost any solid chloride and manganese dioxide (MnO2) yields chlorine when heated with concentrated sulfuric acid (H2SO4). The reaction occurs, as follows:

In the laboratory chlorine is frequently prepared by the oxidation of concentrated hydrochloric acid with permanganate or dichromate salts:

Similarly, chlorine may be prepared on a small scale by addition of any dilute acid to bleaching powder, a mixture of dry calcium salts, including the hydroxide, chloride, and hypochlorite.

Most of the chlorine produced is used for chemical processes involving the introduction of chlorine into organic compounds, yielding (used as a solvent, a fire extinguisher, and a dry-cleaning agent) and glycols (used as antifreeze), and other organic compounds for the manufacture of plastics (polyvinyl chloride) and synthetic rubber. , made by the action of chlorine on carbon disulfide or by combining sulfur and chlorine, is used in the vulcanization of rubber and as a chlorinating agent in organic synthesis. Chlorine and carbon monoxide form carbonyl chloride, or , which has been employed as a war gas and is used in metallurgy to transform certain oxides into chlorides. Much chlorine is used to sterilize water and wastes; and the substance is employed either directly or indirectly as a bleaching agent for paper or textiles. Chlorine is applied in the manufacture of hydrochloric acid, the extraction of titanium with formation of titanium(IV) chloride, the removal of tin from old tinplate, and in the production of antiknock materials for gasolines, such as tetraethyl lead. Anhydrous aluminum chloride is made by the reaction of chlorine with scrap aluminum or with aluminum oxide and carbon. Chlorine is also used to prepare silicon(IV) chloride and methyl chloride, which are employed in the synthesis of silicon materials. Chlorine enters directly, or indirectly as an intermediate, into many organic syntheses of industrial importance.

Production and use

Iodine is produced commercially either from Chile saltpetre or from iodine-containing brines. In the former process, the salt is dissolved in hot water and the saltpetre allowed to crystallize on cooling. The mother liquor is used for further extractions until the extracts contain up to nine grams of iodine per litre. Sodium hydrogen sulfite is then added in order to reduce all iodate to iodide, and the solution is nearly neutralized with sodium carbonate. Fresh mother liquor is then added until all iodide is oxidized by the iodate to free iodine, according to the equation:

The solid, containing up to 80 percent iodine, is collected, washed with water, and pressed into cheeselike blocks. These are heated to distill off both iodine and water.

Natural brines, or brines extracted from oil wells containing up to 150 milligrams of iodine per litre, are found in Java, California, and northern Italy. Impurities, such as clay, sand, and oil, are removed by filtration, and the solution is passed through a stream of sulfur dioxide and then through a number of containers containing bundles of copper wire. The copper(I) iodide that forms is removed by filtration, washed with water, dried, and finely ground. The product is heated with potassium carbonate to give potassium iodide, which is then oxidized to the free element with dichromate and sulfuric acid:

In an alternate process, chlorine is used as the oxidizing agent:

It is characteristic of the atoms of the halogen elements that each carries seven electrons in its outermost . The seven outermost electons of an atom of a halogen element are in two different kinds of orbitals, designated s (with two electrons) and p (with five). Potentially, a halogen atom could hold one more electron (in a p orbital), which would give the electrons the same arrangement (configuration) as that of the noble gas next to it in the periodic table, the noble gases all having exceptionally stable configurations of electrons in their atoms. It is in acquiring such an electron that an atom of a halogen element acts as an oxidizing agent (or is itself reduced). In the process, the atom acquires an electrical charge (in this case a negative one) and becomes a charged particle or ion. Halogen elements exist in their salts as halide ions, which are relatively large ions, colourless and extremely stable.

Iodine enters the chemical industry on a smaller scale. The largest producer is Japan, where iodine is obtained from . Seawater contains only about 0.05 part per million iodine, but some species of seaweed are able to concentrate this iodine manyfold, so that commercial extraction of the iodine is possible.

The most important industrial use of iodine compounds is the small amount of silver iodide used with silver bromide in photography. Iodine is important also in (although this is not a large-scale use) in the treatment of certain thyroid conditions, and it is added to common table salt to prevent such conditions. It is also used directly as a disinfectant. Iodine is a component of a few useful dyes. The laboratory chemist frequently makes use of iodine or iodine compounds in synthesis and also in analysis. Crystalline silver iodide is useful in cloud seeding.