Am*jineri9ih8.rm'  ?    Chlorine  as  a  Sterilising  Agent.  421 
determined  analytically  and  calculated  to  chlorine,  and  this  chlorine 
equivalent  is  what  is  known  as  available  chlorine.  Potential  oxygen 
would  be  a  far  more  suitable  term. 
Chlorinating  or  Oxidizing  Media. — There  are  two  chemicals 
now  in  use  for  sterilization  of  water  and  sewage,  namely  chlorinated 
lime  or  bleaching  powder,  and  liquid  chlorine.  The  former  has  for 
its  chief  ingredients,  calcium  oxy-chloride,1  CaOCl2,  and  calcium 
hydroxide,  Ca(OH)2.  When  chlorinated  lime  is  added  to  water  the 
oxy-chloride  becomes  a  true  hypochlorite,  according  to  the  equation : 
2CaOCl2  +  H20  =  Ca(OCl)2  +  CaCl2  +  H20. 
Thus  we  have  an  alkaline  hypochlorite  solution,  due  to  the  calcium 
hydroxide. 
Liquid  chlorine,  on  the  other  hand,  when  added  to  water  forms 
hypochlorous  and  hydrochloric  acids  in  dilute  solutions,  as  follows : 
Cl2  +  H20  =  HCIO  +  HC1. 
In  this  case  an  acid  hypochlorite  solution  results. 
Ionic  Influence  on  Hypochlorites. — Since  acid  and  alkaline 
solutions  contain  an  excess  of  hydrogen  and  hydroxyl  ions  respec- 
tively, it  may  be  stated  that  these  ions  exert  an  influence  upon  the 
action  of  the  hypochlorites.  The  natural  tendency  of  all  unstable 
substances  is  to  become  stable,  so  there  is  a  spontaneous  reaction 
among  the  hypochlorite  molecules  which  results  in  the  formation  of 
chlorates  and  chlorides,  as  illustrated  by  the  equation : 
3HC10  =  HC103  +  2HC1. 
The  chlorates  are  more  stable  than  the  hypochlorites  in  dilute 
solutions,  and  are  therefore  inferior  oxidizing  agents,  while  the 
chlorides  are  inert.  The  reaction  depicted  above  is  accelerated  by 
hydrogen  and  retarded  by  hydroxyl  ions,2  which  means  that  hypo- 
chlorites are  more  stable  in  alkaline  solutions.  The  hydrogen  ion 
also  increases  the  reaction  velocity  of  the  hypochlorites,  or  in  other 
words  the  velocity  of  oxidation  is  greater  in  acid  solutions. 
Processes  Involved  During  Diffusion. — In  practice,  the  chlori- 
nating media  are  applied  locally  and  have  to  diffuse  throughout  the 
entire  mass  of  the  water  or  sewage  to  be  sterilized,  which  means 
that  the  ultimate  degree  of  sterilization  is  largely  dependent  upon 
the  rate  of  diffusion. 
1  Ellms. 
2  LeBlanc. 
