S6 
Ionic  Dissociation. 
(Am.  Jour.  Pharm. 
(     February,  1920. 
is  believed  by  some  that  at  absolute  zero  conductivity  would  be 
infinite. 
The  neutralizing  value  (strength)  of  acid  and  base  is  usually 
determined  by  titrating  a  solution  of  one  with  a  standard  (so-called 
"Volumetric")  solution  of  another  of  opposite  chemical  character, 
the  point  of  neutrality  being  determined,  more  or  less  accurately, 
by  having  in  the  solution  which  is  being  titrated  a  small  quantity  of 
a  so-called  "indicator,"  usually  an  organic  compound,  which  shows 
by  change  of  color  the  presence  of  a  slight  excess  of  the  reagent  being 
used  for  neutralization. 
In  all  cases  of  neutralization,  whether  of  an  acid  by  a  base  or  a 
base  by  an  acid,  the  reaction  takes  place  by  reason  of  the  H  ions  from 
the  acid  combining  with  the  OH  ions  from  the  base  to  form  undisso- 
ciated  water,  while  the  anions  from  the  acid  and  the  cations  from 
the  base  remain,  at  least  in  part,  as  ions  in  solution.  When  the  sol- 
vent is  removed,  generally  by  evaporation,  these  ions  combine  to 
form  molecules  of  a  salt.    As  one  writer  has  expressed  it: 
"In  every  case  of  neutralization  the  products  are:  i.  Undissoci- 
ated  water.  2.  A  solution  containing  cations  from  the  base  and 
anions  from  the  acid.    3.  Energy  in  the  form  of  heat." 
With  strong  acids  and  strong  bases  in  dilute  solution  the  heat 
from  gram-equivalents  of  each  is  13,700  calories — the  heat  of  com- 
bination of  one  gram  of  H  ion  with  seventeen  grams  of  OH  ion. 
With  the  acid  or  base,  or  both,  weak,  the  heat  liberated  may  be 
greater  or  less  than  13,700  calories,  heat  being  taken  up  in  some  cases, 
and  given  off  in  other  cases,  in  the  ionization  of  molecules  which  were 
undissociated  when  the  process  of  neutralization  was  begun. 
It  has  been  estimated  that  three-fourths  of  all  known  chemical 
reactions  involve  the  formation  of  water,  and  if  these  were  eliminated 
from  our  chemistry  we  would  have  but  little  on  which  to  build  a 
science. 
There  are  two  theories  with  reference  to  what  takes  place  when 
an  organic  indicator  changes  color  in  acidimetry  and  alkalimetry. 
The  one  held  by  Ostwald  is  that,  under  one  set  of  conditions, 
only  molecules  of  the  indicator  (with  its  characteristic  color)  exist 
in  the  solution,  while,  under  opposite  conditions  as  to  acidity  or 
alkalinity,  a  salt  of  the  indicator  (formed  when  the  excess  of  reagent 
is  added)  undergoes  ionization,  the  new  color  being  that  of  a  com- 
plex ion  from  this  salt.    Most  indicators  are  weak  acids,  and  accord- 
