Am.  Jour.  Pharm.) 
February,  1920.) 
Ionic  Dissociation. 
87 
ing  to  this  theory,  the  color  of  their  neutral  or  acid  solutions  is  due 
to  molecules  of  the  indicator,  while  in  alkaline  solutions  their  wholly 
or  partly  ionized  salts  exist,  with  a  different  color  because  of  these 
ions. 
The  other  theory,  advanced  by  Stieglitz,  and  quite  generally  ac- 
cepted by  physical  chemists,  is  that  in  passing  from  acidity  to  alka- 
linity, or  vice  versa,  there  is  a  chemical  change  within  the  molecule 
of  the  indicator  to  form  a  new  compound  which,  structurally,  differs 
from  the  original  substance  only  in  the  arrangement  of  atoms  and 
atomic  groups  (a  so-called  "tautomeric  compound").  Generally 
the  "lactoid"  group  — CeH4 —  group  becomes  the  "quinoid"  group 
=  C6H4=,  or  vice  versa,  one  of  these  being  called  a  "chromophoric" 
group.  The  compound  with  the  lactoid  group  has  one  color,  that 
with  the  quinoid  group  another  color. 
Of  the  rather  large  number  of  substances  which  may  be  used  as 
indicators  we  find  that  few  of  them  change  color  at  the  same  stage 
of  acidity  or  alkalinity,  hence  cannot  be  used  interchangeably  in 
many  processes  involving  neutralization,  particularly  those  where 
results  approaching  exactness  are  desired. 
Hildebrand^  gives  in  tabular  form  the  relative  values  of  a  lot  of 
those  in  more  or  less  general  use,  the  values  being  expressed  in  terms 
of  hydrogen-ion  concentration,  explained  more  fully  further  on: 
in  this  paper.    From  this  table  the  following  data  were  taken : 
Cochineal.    Yellow  at  io~^,  yellow-pink  at  lo"^,  lilac  at  lo"^ 
Litmus.  Red  at  lo""^,  red-violet  at  IO~^  violet  at  io~'^,  blue  at 
io~^. 
Methyl  Orange.    Rose  at  i.o,  orange  at  io~^,  yellow  at  lo'"^ 
Phenol phthalein.    Colorless  at  io~^,  red  at  lo"^. 
Methyl  Red.    Red  at  io~^,  pink  at  Io~^  yellow  at  io~^ 
Inasmuch  as  lo"^  is  the  figure  for  neutrality  and  those  with  in- 
dexes of  less  than  ~^  indicate  acidity  (increasing  as  the  index  de- 
creases) and  those  with  indexes  higher  than  ~^  indicate  alkalinity 
(increasing  as  the  index  increases),  it  will  be  seen  at  a  glance  that 
indicators  do  not  generally  change  color  at  a  neutral  point,  but 
rather,  at  a  definite  hydrogen-ion  concentration  peculiar  to  each 
indicator. 
Since  these  differences  exist,  since,  as  has  been  shown,  different 
substances  ionize  in  different  degrees,  and  since  it  is  not  always  pos- 
^  Jour.  Am.  Chem.  Soc,  35:  856,  1913. 
