carbonate, a mixture of the chloride and hypochlorite of the alkali metal is formed. But if any excess of chlorine is introduced, the hypochlorite is decomposed into chloride and free hypochlorous acid (HOCl) :-
1)K2C03 +H20 +2 Cl = RCl +ROCI + H20 +CO2,
2) K2C03 +H"O +4 Cl = 2 KCl + CO2 +2 HOCl.
This solution of hypochlorous acid is a powerful bleaching and oxidizing agent. It was first made about 17S0, and brought into trade in France as a "bleach liquor" under the name of eau de Javelle, or eau de LabarNUjUe. In 170S or 1799 Charles Tennant took out a patent in England for a "bleach liquor" made by passing chlorine into" milk of lime," by which a solution of calcium chloride (Lndhypochlorite was formed:-
2 Ca(OH)2 + 4 Cl = CaC12+ Ca(OCl)2 + 2 H20.
This bleach liquor is cheaper, stronger, and more convenient to use than bleaching powder (see below), but since it is unstable, evolving oxygen even when kept in a closed vessel in the dark, it is usually made only for immediate use.
The tanks in which the milk of lime is treated with chlorine are provided with stirring apparatus; the temperature must not rise much above 30° C., or chlorates are formed. A dilute chlorine may be used. The density of the solution obtained is about 8° Tw. Calcium carbonate suspended in water may also be employed for preparing bleach liquor:-
CaCOa +H20 +4 Cl = CaC12+CO2+2 lWCI.
These liquors are chiefly used for bleaching vegetable fibres and for disinfectants.
The absorption of chlorine in milk of lime soon led <to trials of dry, slaked lime or calcium hydroxide for the same purpose. A dry bleaching powder, fairly stable and constant in strength, resulted; but its composition is not the same as that of the bleach liquor made from milk of lime. It was at first supposed that a direct combination took place between the lime and chlorine, and that the powder was simply calcium hypochlorite [Ca (OCl)2J, so the name" chloride of lime" was given to it. Other investigations Jed to the view that it contained a mixture of calcium chloride and hypochlorite. But this was disproved by Lunge * and his students; Lunge assigns to Cl
it the formula Ca< . Hence it is an oxychloride of calcium. O-Cl
When dissolved in water, this forms hypochlorite and chloride of calcium.
desirable. It is slaked carefully, so that the resulting hydroxide contains about 24.5 to 25.5 per cent of water. That is, there should be a' slight excess of water over that necessary to form calcium hydroxide.
The absorption chambers are brick; cast iron, or lead, and are usually 6.5 feet high, and have about 200 square feet of floor area per ton of bleach made per week. Brick chambers are tarrell inside to make them gas tight and to protect them from the chlorine; large ones are usually made from lead, much like the vitriol chambers, and may have a floor area of 30 by 100 feet. The slaked lime is sifted through screens with from 20 to 25 meshes per lineal' inch, as only the fine powder is suitable. This is spread three or foul' inches deep on the floor, and is furrowed with a special rake in order to assist the absorption by increasing the surface. The chlorine is introduced at the top of the chamber, and settling to the bottom because of its density, is at first rapidly absorbed by the lime. After a time the process goes on more slowly, and i1nally the gas enters under some pressure. In modern works there are three or more chambers in a series, the strongest chlorine entering that containing the most nearly finished bleach, and passing out through that containing the fresh lime. The degree of absorption of chlorine is judged by the color of the gases seen through the glass "sights" in the chamber walls. The powder is turned over once or twice, and the treatment (" gassing") continued until tests show that it contains from 36 to 37 per cent of "avai]ab]e chlorine." If under strength ( "weak "), after the third" gassing," it should be packed and sold for what it will bring, for further exposure will cause the formation of chlorate and chloride with loss of strength.
During the absorption considerable heat is generated; for strong powder the temperature should not exceed 40° - 46° C. '*' The chlorine should be admitted in a very slow stream, and should be concentrated, dry, and free from hydrochloric or carbonic acids. 'When dilute (as from Deacon's apparatus), a large, special chamber provided with numerous shelves, on which the slaked lime is spread to secure a greater absorbing surface, is employed. 'rhe yield from 100 pounds of good lime is about 150 pounds.
Bleaching powder is a yellowish white substance, which should be perfectly dry and free from lumps. On exposure to the air, it absorbs moisture and carbon dioxide, giving off hypochlorous acid, the evolution of which gives bleach its peculiar odor. Good samples contain about 36 per cent "available chlorine." Its chief 1ise is for bleaching vegetable fibres for the textile and paper industries. In order to liberate the chlorine for bleaching purposes, the powder is usually decomposed by a mineral acid, thus: the fibre having been saturated with the bleaching powder solution, is passed into a dilute acid bath, where the hypochlorite is decomposed and the chlorine set free. The nascent chlorine combines with the hydrogen of the water, liberating nascent oxygen, which, in turn, destroys the organic coloring matter in the fibre.
Organic Chemistry for the industry
Inorganic Chemistry for the industry
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