Bleaching Lime

Bleaching lime (chlorinated lime; Calcaria chlorata, Calcaria hypochlorosa; German. Chlorkalk, Bleichkalk; French. Chlorure de chaux sec; English. hypochlorite of lime), a product of the action of free chlorine on calcium hydroxide: 2Ca(OH)₂+4Cl= =Ca(OCl)₂+CaCl₂+2H₂O. Its chemical composition is usually represented as Ca(OCl)₂·CaCl₂. Technical bleaching lime can be considered a mixture of calcium hypochlorite, calcium chloride, and calcium hydroxide in undefined proportions. Bleaching lime is a white, lumpy, dry powder with the odor of hypochlorous acid and chlorine and an alkaline reaction. When treated with 10 parts water, calcium hypochlorite and calcium chloride go into solution, while calcium hydroxide remains mostly undissolved. When treated with a small amount of water, calcium chloride primarily goes into solution. Under the action of light and heat, bleaching lime gradually gives off Cl and O, forming calcium chlorate [Ca(ClO₃)₂] and calcium chloride. When treated with excess strong acids, bleaching lime releases free chlorine: Ca(OCl)₂·CaCl₂+4HCl=2H₂O+2CaCl₂+4Cl; Ca(OCl)₂·CaCl₂+2H₂SO₄=2H₂O+2CaSO₄+4Cl. When treated with solutions of weak acids (acetic, boric, carbonic) and weak solutions of strong acids, no chlorine is released, and free hypochlorous acid appears in the solution, partially decomposing with the formation of chlorine oxide and water: Ca(OCl)₂+CO₂+H₂O=CaCO₃+2HOCl; 2HOCl=Cl₂O+H₂O. Hypochlorous acid is a very unstable compound that easily gives up its oxygen, which acts especially energetically in statu nascendi: 2HOCl=2HCl+O₂. Technically, bleaching lime is obtained by saturating slaked lime with free chlorine. Commercial evaluation of bleaching lime is based on the amount of free, so-called "active" chlorine (as opposed to "inactive" bound chlorine in chlorides) that can be released from bleaching lime under the action of strong acids. The content of active chlorine in France and some other countries is expressed in Gay-Lussac degrees, representing the number of liters of gaseous chlorine, reduced to 0° and 760 mm pressure, released by 1 kg of bleaching lime. In the USSR, England, America, and Germany, the content of active chlorine is expressed as weight percentages of gaseous chlorine relative to the weight of bleaching lime. Good bleaching lime contains at least 30% active chlorine. With an active chlorine content below 25%, bleaching lime is already of low quality. Many methods have been proposed for the exact determination of active chlorine. The best is the modified Bunsen method—the iodometric determination of active chlorine: from an average sample of bleaching lime, take a 3.456 g portion and thoroughly grind in a mortar with a small amount of distilled water to completely break up the lumps; then pour the liquid together with the precipitate into a liter measuring flask, rinsing the mortar several times, and add water to the mark. After mixing the solution in the flask, take 10 cubic cm of the solution into an Erlenmeyer flask, add 50 cubic cm of distilled water, 10 cubic cm of 10% potassium iodide, 10 drops of strong HCl, carefully mix with a glass rod to avoid loss of free chlorine, and quickly add from a burette n/₁₀₀ solution of Na₂S₂O₃ (hyposulfite) until a pale yellow color appears, after which add 1 cubic cm of 1% solution of soluble starch and titrate with n/₁₀₀ hyposulfite solution until the blue color disappears. The number of cubic cm of exactly n/₁₀₀ Na₂S₂O₃ solution used for titrating 10 cubic cm of the solution gives the content of active chlorine in percent. Bleaching lime finds extensive application in technology as a bleaching agent, in a number of technological processes as a source for the rapid and convenient production of chlorine, and in sanitary technology as a disinfectant for water chlorination and for disinfecting impurities. Recently, bleaching lime has been replaced to a considerable extent: in bleaching processes by electrolytically obtained salts of hypochlorous acid and peroxides, and in sanitary technology for water chlorination by liquid chlorine.