Am.  Jour.  Pharm.  ' 
October,  1904.  / 
Theory  of  Indicatoi's,  etc. 
457 
their  action  on  the  property  which  they  possess  of  first  reacting  with 
the  strong  acid  present  in  the  standard  solution  employed  for  the 
titration  after  all  the  basic  affinities  present  in  the  solution  have  been 
satisfied. 
In  addition  to  the  group  of  indicators  dealt  with  above  others 
may  be  mentioned,  such  as  dimethyl-amido-azo-benzol  and  methyl- 
violet,  characterized  by  their  basic  instead  of  acid  properties. 
Dimethyl-amido-azo-benzol  being  a  weak  base  falls  into  class  one  of 
indicators  sensitive  to  bases.  When  bases  are  being  titrated  by 
means  of  acid  standard  solutions  the  salt  of  the  basic  indicator  is 
only  formed  after  complete  saturation  of  the  bases  stronger  than  it 
which  are  present  in  the  solution.  Still  another  group  is  charac- 
terized by  the  possession  of  both  acid  and  basic  properties.  To  this 
group  belongs  methyl-orange  and  the  gradual  transition  of  this 
substance  from  a  yellovy  to  a  pink  with  an  intermediate  phase  is 
attributed  to  the  probable  existence  of  a  multivalent  ion  in  the  solu- 
tion, an  ion  possessed  at  the  same  time  of  positive  and  negative 
characteristics.  Wagner,  in  an  exhaustive  study  of  this  question, 
differentiates  still  a  fourth  type  of  indicator  of  which  alizarin 
sulphonic  acid  may  be  taken  as  an  example,  possessed  of  two  nega- 
tive groups  capable  of  reacting  independently  according  to  circum- 
stances. This  indicator  possesses  a  S03H  group  and  an  OH  group. 
The  neutralization  of  the  former  with  alkali  leads  to  the  formation 
of  a  red  color  and  the  saturation  of  both  to  a  purple.  This  differ- 
entiation is  most  marked  in  titrating  phosphoric  acid  which  possesses 
three  acid  groups  differing  considerably  in  their  affinity  for  bases. 
The  first  acid  group  of  the  alizarin  indicator  is  weaker  than  the  first 
acid  group  of  the  phosphate,  but  stronger  than  the  second.  The 
second  acid  group  of  the  indicator  is  weaker  than  the  second,  but 
stronger  than  the  third  acid  affinity  of  the  phosphoric  acid.  As  a 
consequence  we  have  a  sharp  transition  from  green  to  red  on  neu- 
tralization of  one  of  the  acid  affinities  of  the  phosphate  molecule 
and  a  further  transition  from  red  to  purple  after  two  of  the  phos- 
phate affinities  have  been  saturated. 
There  is  nothing  to  be  gained  by  further  multiplying  cases  of  the 
great  variation  in  sensitiveness  of  different  indicators  and  even  of 
one  and  the  same  indicator  under  varying  circumstances.  One  im- 
portant point  should  always  be  borne  in  mind,  viz.,  the  end  point  is 
always  much  sharper  when  the  completion  of  the  reaction  results 
