420  Chlorine  as  a  Sterilising  Agent.  {Am'j]?™'wirm' 
CHLORINE  AS  A  STERILIZING  AGENT. 
By  Frank  N.  Moerk. 
The  term  chlorination  has  come  into  use  since  the  introduction 
of  liquid  chlorine  as  a  sterilizing  medium.  This  is  not  technically 
correct,  for  it  creates  the  impression  that  sterilization  is  accomplished 
directly  by  the  chlorine.  This,  however,  is  not  the  case,  since  the 
addition  of  chlorine  to  water  results  primarily  in  the  formation  of 
hypochlorous  acid,  according  to  the  equation : 
Cl2  +  H20  =.  HCIO  +  HC1. 
The  hypochlorous  acid,  which  is  unstable  in  the  presence  of  or- 
ganic matter,  liberates  nascent  oxygen,  as  follows : 
HC10  =  HCl  +  0. 
Therefore  it  is  evident  that  chlorination  is  in  reality  an  indirect 
oxidation,  which  depends  upon  the  preliminary  formation  of  hypo- 
chlorous acid  or  its  salts. 
Chlorine  when  dissolved  in,  water  does  not  dissociate  to  any 
great  extent,  since  it  does  not  appreciably  increase  the  conductivity 
of  pure  water.  Yet  we  know  that  its  oxidizing  properties  are  de- 
pendent upon  the  presence  of  moisture.  These  facts  are  explained  by 
the  formation  of  hypochlorous  acid,  the  concentration  of  which  bears 
a  definite  relation  to  that  of  chlorine,  according  to  the  law  of  mass 
action,  and  therefore  the  hypochlorous  acid  is  replenished  as  fast  as 
consumed  at  the  expense  of  the  chlorine. 
This  agrees  with  the  theory  that  chlorine  acts  by  removing  or 
combining  with  hydrogen,  and  thereby  liberates  oxygen,  the  ultimate 
reaction  being 
Cl2  +  H20  =  2HCl  +  0. 
This  equation  does  not  show  the  intermediate  reactions  which  are 
illustrated  by  the  previous  equations. 
Available  Chlorine. — Available  chlorine  is  the  term  used  to 
denote  that  portion  of  the  total  chlorine  content  that  takes  part  in 
the  formation  of  hypochlorites,  whereby  oxygen  is  liberated.  It  will 
be  noted  from  the  first  equation  that  one  molecule,  or  two  atoms  of 
chlorine,  forms  one  molecule  of  hypochlorous  acid,  and  from  the 
second  equation  that  one  molecule  of  hypochlorous  acid  is  capable 
of  liberating  one  atom  of  oxygen.    It  is  this  atomic  oxygen  that  is 
