786 



CHLORIDE OF LIME 



Mr. Graham found that hydrate of lime, dried at 212, absorbed afterwards little 

 or no chlorine ; but that, when dried over sulphuric acid, it -was in the most favour- 

 able condition for becoming chloride of lime. A dry, white, pulverulent compound 

 is obtained by exposing the last hydrate to chlorine, which contains 41-2 to 41-4 

 chlorine in 100 parts, of which 39 parts are available for bleaching, the remainder 

 going to form chloride of calcium and chlorate of lime. This appears to be the 

 maximum absorption of chlorine by dry hydrate of lime ; but the bleaching powder 

 of commerce rarely, even -when fresh prepared, contains more than 30 per cent, of 

 chlorine, and after being kept for several months, the proportion often falls as low as 

 20 per cent. A compound containing one equivalent of chlorine and one equivalent 

 of hydrate of lime, should contain 48 - 57 chlorine and 51-43 hydrate of lime ; a com- 

 pound of one equivalent of chlorine and two of hydrate of lime, should contain 32-42 

 chlorine and 67"58 hydrate of lime ; and these are about the proportions in good com- 

 mercial specimens. It would not be advisable to attempt to manufacture a more 

 highly chlorinated product, as the stability of the compound is increased by an excess 

 of lime. Where a stream of chlorine is transmitted through water holding hydrate 

 of lime in suspension, the lime is entirely dissolved, and the full equivalent of chlorine 

 is absorbed. Different views have been held by chemists respecting the constitution 

 of bleaching powder, but it is now generally admitted that the substance consists 

 essentially of chloride of calcium and hypochlorite of lime, with excess of slaked lime. 

 Water poured upon bleaching powder dissolves out the bleaching combination, leaving 

 a large residue of lime. Ten parts of water are required for one part of dry chloride. 

 The solution emits the peculiar odour of hypochlorous acid. The reaction which occurs 

 in the formation of ' chloride of lime ' may be thus represented : 

 2(CaO.HO) + 201= CaCl + CaO.CIO + 2HO. 

 2CaH 2 O' + Cl ' = CaCl 2 + CaCl O - + 2H -O . 



Slaked lime and chlorine thus yield chloride of calcium, hypochlorite of lime and 

 water. Whether the chloride and hypochlorite are united as a combination or 

 merely mixed together, may be open to question. But good bleaching powder is not 

 deliquescent, neither does alcohol dissolve anything from it, both which should occur 

 if the compound contained free chloride of calcium. It is possible, however, that the 

 two salts may exist in bleaching powder in the form of a double salt, or that the 

 chlorine is in direct combination with the oxide. If the compoxind be supposed to 

 be pure chloride of lime, the reaction is simply an absorption of chlorine ; and the 

 same should be the case with the other bleaching compounds chloride of soda, 

 for instance. But when carbonate of soda, saturated with chlorine (Labarraque's 

 Liquor), is evaporated, no chlorine is evolved, and the residue still possesses 

 bleaching properties. The true nature of bleaching powder is open, therefore, to 

 speculation. 



The bleaching action of solution of chloride of lime is very slow tmless an acid be 

 added to it. When dilute sulphuric acid in insufficient quantity is employed, no 

 chlorine is evolved, but hypochlorous acid, which may be distilled off and condensed 

 in a suitable receiver ; but with excess of acid, chlorine only is liberated. When 

 calicoes and other woven goods are to be bleached, they are first thoroughly cleansed 

 by boiling successively with lime-water and a weak solution of caustic soda ; they are 

 then digested in a solution of bleaching powder, specific gravity T02, containing about 

 2$ per cent, of chloride of lime ; after which they are immersed in very dilute sulphuric 

 acid, which, by liberating the chlorine within the fibres of the cloth, rapidly removes 

 the colour. The goods are then washed, a second time steeped in alkali, and agj'in 

 passed through a weaker solution of chloride, and then through dilute acid ; after which 

 they are thoroughly washed in water. The quantity of liquor necessary for 700 Ibs. of 

 cloth is 971 gallons, containing 388 Ibs. of chloride. When white figures are required 

 on a coloured ground, the pattern is printed on the cloth with tartaric acid, thickened 



