Am.  Jour.  Pharm. ) 
October,  1904.  / 
Theory  of  Indicators,  etc. 
455 
(2)  Those  which  are  equally  sensitive  both  to  alkalies  and  acids, 
as  for  example,  alizarin,  haematoxylin,  lakmus  and  rosolic  acid,  indi- 
cators which  would  be  employed  by  preference  when  we  have  to 
deal  with  mixtures  of  moderately  strong  acids  and  bases. 
(3)  Those  which  are  especially  sensitive  to  acids,  as,  for  example, 
phenolphthalein,  Porrier's  blue,  etc.,  indicators  which  would  be 
employed  by  preference  in  estimating  the  quantity  of  any  extremely 
weak  acid. 
Indicators  of  the  type  enumerated  above  belong  to  the  group  of 
organic  dyestuffs,  and  are  endowed  in  addition  with  more  or  less 
marked  acid  properties  ranging  from  those  in  the  first  class  referred 
to  above  which  possess  one  or  more  strongly  negative  or  acid 
groups,  to  those  in  the  third  class,  the  acid  properties  of  which  are 
extremely  weak.  The  color  changes  observed  on  titration  are 
attributable  to  the  formation  of  a  salt  of  the  indicator  in  question 
possessed  of  a  different  color  to  that  of  the  original  acid  from  which 
it  is  derived.  This  result  is  obtained,  just  as  would  be  expected,  at 
that  point  at  which  all  the  acids  in  the  solution  which  are  stronger 
than  the  acid  group  of  the  indicator,  and  consequently  possessed  of 
a  greater  affinity  than  the  latter  for  alkali,  have  been  neutralized. 
It  will  thus  be  seen  that  in  order  to  titrate  the  weakest  possible 
acids  it  is  simply  necessary  to  find  indicators  which  possess  even 
weaker  acid  properties  and  less  affinity  for  bases  than  those  pos- 
sessed by  the  weak  acid  in  question.  It  is  also  obvious  that  if  we 
desire  to  determine  the  proportions  of  two  or  more  acids  present  in 
a  solution,  this  may  be  readily  effected,  provided  the  acids  in  ques- 
tion differ  sufficiently  in  strength  to  enable  us  to  find  indicators 
possessing  acid  affinities  lying  intermediate  between  those  of  the 
acids  involved.  There  are  considerable  limitations  to  the  extension 
of  this  scheme,  one  of  the  principal  difficulties  being  the  impossibil- 
ity of  obtaining  sharp  end  points  with  extremely  weak  acids  or 
bases,  owing  to  the  slow  and  incomplete  formation  of  salts  and  con- 
sequent imperfect  dissociation  of  salts  into  ions,  upon  which  the 
color  reactions  really  depend. 
It  is  a  general  principle  in  volumetric  analysis  that  strong  bases 
should  be  employed  to  titrate  weak  acids,  and  strong  acids  to  titrate 
weak  bases.  This  is  due  to  the  fact  that  weak  acids  and  weak  bases 
only  combine  with  considerable  difficulty  to  form  salts  and  the 
speed  of  reaction  even  at  high  temperatures  is  extremely  slow. 
