i8o SEWAGE AND ITS PURIFICATION 



besides killing all life. In localized situations, therefore, 

 chlorine and its compounds are effectively used for dealing 

 with special nuisances. The offensive gases of putrefaction 

 are decomposed, sulphuretted hydrogen being resolved into 

 sulphur and hydrochloric acid — 



H2S + Cl2 = 2HCl + S 



phosphuretted hydrogen being also decomposed, while ammonia 

 and compound ammonias give a corresponding chloride and 



nitrogen — 



8NH3 + 3Cl2= 6NH4CI+ N2, 



hence the copious white fumes frequently noticed when a 



chlorine mixture is thrown into a dung - pit. With more 



chlorine intensely acrid vapours, which attack the eyes and 



lungs, due to chlorides of nitrogen and chloropicrin, C(N02)Cl3, 



are produced. In dealing with cesspools, ashbins, or privies 



this becomes strongly prominent in chlorine disinfection. 



Chlorine acts as an oxidizing agent by decomposition of 



water — 



H20 + Cl2 = 2HCl + 0, 



the nascent oxygen so liberated being far more energetic than 

 atmospheric oxygen, and acting directly on organic substances. 

 A cheap source of chlorine is chloride of lime or bleaching 

 powder, CaCl20, which, on dissolving in water, breaks up into 

 calcium chloride, CaClo, and calcium hypochlorite, Ca(C10)o ; 

 the latter only is available for chlorinating or oxidizing. The 

 commercial dry powder contains as a rule about one-third of 

 its weight of active or ** available " chlorine.^ When mixed 



^ "Available chlorine" means that portion of the whole chlorine which 

 liberates oxygen on reaction with water. Hydrochloric acid and most chlorides 

 liberate none. Free chlorine, for every molecule Clg, or 71 parts by weight, sets 

 free i atom, weighing 16 parts, of oxygen : 



Cl2+H20 = 2HCl + ; 



that is, the weight of chlorine used is about 4^ times the oxygen obtained. 



Hypochlorous acid and hypochlorites can break up directly into hydrochloric 

 acid or chlorides and oxygen : 



HC10 = HCl + J 

 Ca(C10)o = CaCU + 20*^ 

 NaC10 = NaCr+0. 



Hence f>ure hypochlorous acid, or a fiirc hypochlorite, would give i atom of 

 oxygen for i of chlorine, or double the amount yielded by free chlorine. 

 Commercially, however, the hypochlorite is always obtained mixed with an 

 equivalent amount of the inert chloride, as in the formation of solutions of chloride 

 of Hme and chlorinated soda : 



2NaOH + CU=NaCl + NaC104-HoO. y 

 2Ca(0H)o + 2CU = CaClo + Ca(C10)2 +'2HoO. 



Therefore, apart from the question of difference of activity, the total amount of 

 chlorine present in these chlorinated products bears the same relation to the 



