A.m.  Jour.  Pharm.  1 
October,  1904.  J 
Theory  of  Indicators,  etc. 
459 
drops  ot  phloroglucinvanillin,  tropaeolin  and  sugar  resorcin  on  a 
warm  porcelain  plate,  were  employed.  In  the  second  group,  alizarin, 
hematoxylin,  lakmus,  rosolic  acid  and  guaiac  tincture,  and  in  the 
third  group  tropaeolin  ooo,  phenolphthalein,  alpha-naphthol-benzine 
and  Porrier's  blue  were  made  use  of. 
It  would,  of  course,  be  impossible,  with  the  limited  space  at  our 
disposal,  to  tabulate  all  the  results  obtained.  Such  a  work  would 
in  any  case  be  superfluous.  The  great  majority  of  indicators  were 
found,  for  some  reason  or  other,  to  have  little  value  in  dealing  with 
physiological  solutions  owing  to  their  complete  failure  to  give  defi- 
nite end  points.  These  experiments  were  necessarily  carried  on  at 
the  same  time  as  those  enumerated  in  Sections  4  and  5  on  bases, 
proteids,  albumoses,  etc.;  as  a  result,  many  indicators  possessed 
of  considerable  value  for  differentiating  acids  had  to  be  ruled  out  ot 
consideration  on  account  of  their  failure  to  give  a  sharp  end  point 
in  the  presence  of  weak  bases. 
In  order  to  effect  an  economy  of  space  the  general  tables  of  this 
section  have  been  omitted,  the  most  important  points  being  sum- 
marized in  the  following  paragraph; 
Notes  and  Conclusions. — Phosphoric  acid  may  most  readily  be 
titrated,  making  use  of  alizarin  as  indicator  for  the  determination  of 
the  first  acid  group  of  phenolphthalein  to  indicate  the  termination  of 
the  second  acid  group,  and  of  phenolphthalein  in  the  presence  of 
barium  chloride  and  an  excess  of  alkali  in  a  boiling  solution  for  indi- 
cation of  complete  saturation  of  all  the  acid  affinities.1 
(2)  Phenolphthalein  gives  a  good  end  point  for  determining  the 
total  acidity  of  any  of  the  acids  enumerated  above,  with  the  exception 
of  the  third  acid  group  of  phosphoric  acid,  and  failing  beyond  the  first 
acid  affinity  of  malic  and  aspartic  ;  but  on  account  of  its  sensi- 
tiveness to  ammonia  cannot  be  employed  in  the  presence  of  large 
quantities  of  that  substance,  as  will  be  seen  later. 
(3)  Porrier's  blue  may  be  employed  in  the  place  of  plenolphthalein 
where  large  quantities  of  ammonia  are  present,  as  it  is  entirely  indif- 
ferent to  that  substance.  It  is  also  slightly  more  sensitive  than 
phenolphthalein  to  some  of  the  weaker  organic  acids,  but,  generally 
speaking,  gives  an  inferior  end  point,  consequently  its  employment 
1  See  this  Journal,  July,  1903.  A  note  on  the  quantitative  estimation  of 
phosphates  in  stomach  contents. 
