Am.  Jour.  Pharm.) 
October  1904.  j 
Theory  of  Indicators,  etc. 
461 
and  only  the  final  end  points  observed,  by  employing  such  indi- 
cators as  alizarin,  hematoxylin  (?),  litmus,  benzo-purpurin,  rosolic 
acid,  etc.,  but  for  some  cause  or  other  the  majority  of  these  indi- 
cators are  ruled  out  by  later  experiments.  There  is,  however,  a 
noticeable  difference  in  the  behavior  of  the  organic  acids  towards 
the  same  indicator  as  regards  the  first  appearance  of  the  color 
change  and  final  end  point.  The  strength  of  these  acids  diminishes 
as  we  ascend  the  series,  as  would  be  expected  from  our  knowledge 
of  their  constitution. 
IV.   BEHAVIOR  OF  INDICATORS  TOWARDS  BASES,  SODA,  AMMONIA,  ETC., 
AND  THE  AMIDO  BASES. 
In  this  work,  which  was  carried  out  simultaneously  with 
that  recorded  in  a  previous  section,  the  same  series  of  indi- 
cators was  employed.  All  indicators  give  sharp  end  points  with 
NaOH  and  KOH.  Towards  ammonia  Porrier's  blue  is  prac- 
tically indifferent  when  properly  employed.  Phenolphthalein  is 
materially  affected,  giving  an  end  point  extending  over  -4  to  -5  c.c. 
when  10  c.c.  of  n/10  solutions  are  employed.  This  indicator  can- 
not, therefore,  be  used  in  the  presence  of  any  considerable  quantities 
of  ammonia.  Alizarin,  litmus,  dimethyl-amido-azo-benzol,  rosolic 
acid  and  methyl  orange  give  a  better  end  point  than  that  of  phe- 
nolphthalein, but  still  far  from  satisfactory.  Haematoxylin  and  congo- 
red  give  sharp  end  points  so  long  as  strong  acids  are  employed  for 
titration.  Drops  of  phloroglucinvanillin,  tropaeolin,  etc.,  give  the 
sharpest  end  points. 
Having  observed  the  behavior  of  indicators  towards  ammonia,  a 
fresh  series  of  experiments  was  carried  out  with  a  series  of  amido 
acids,  asparagin,  glycocol,  leucin,  tyrosin  and  aspartic  acid  with  a 
view  to  observing  the  effects  produced  by  the  weak  NH.,  groups 
present  in  each  of  these  compounds  upon  the  indicator  in  question. 
Exact  n / 10  solutions  of  these  substances  were  prepared,  and  their 
behavior  towards  certain  typical  indicators  compared  with  that  of 
the  ammonium  salts  of  formic,  acetic,  lactic  acid  on  the  one  hand, 
and  bodies  like  acetamid,  formamid  and  urea  on  the  other,  and  the 
results  tabulated.  See  Table  I.  In  order  to  insure  as  great  accu- 
racy as  possible,  an  effort  was  made  to  employ  an  amount  of  sub- 
stance which  should  require  25  c.c  of  a  n/  10  solution  for  the  recog- 
nition of  each  acid  or  alkali  group,  especially  where  such  indicators 
