522 
Theory  of  Indicators,  etc. 
( Am.  Jour.  Pharaa. 
I  November,  1904. 
It  is  further  possible  to  differentiate  between  a  series  of  two  or 
more  acids  or  bases  present  in  the  same  solution  provided  one  can 
find  indicators,  the  combining  affinity  of  which  to  form  neutral 
salts  lies  intermediate  between  those  of  the  bodies  to  be  titrated. 
A  good  illustration  of  this  mode  of  differentiation  is  the  recognition 
of  the  three  acid  groups  of  phosphoric  acid,  the  first  one  of  which 
coincides  with  the  first  alizarin  end  point,  the  second  with  the  sec- 
ond alizarin  end  point,  or  with  the  phenolphthalein  end  point,  and 
the  third  with  the  phenolphthalein  end  point  in  the  presence  of 
barium  chloride ;  or  aspartic  acid,  for  example,  the  first  acid 
affinity  of  which  is  neutralized  at  the  alizarin  or  phenolphthalein 
end  point,  the  second  only  at  the  end  point  of  Porrier's  blue.  Car- 
bonic acid  affords  an  interesting  contrast  to  aspartic  acid,  neither 
of  its  acid  groups  being  strong  enough  to  affect  alizarin,  one  only 
giving  an  indication  with  phenolphthalein  and  both  with  Porrier's 
blue.  It  will  thus  be  seen  that  in  their  behavior  towards  phenol- 
phthalein and  alizarin  we  have  a  sharp  differentiation  between  the 
strongest  acid  affinities  of  aspartic  and  carbonic  acid  respectively. 
At  the  other  end  of  the  scale  phloroglucinvanillin  is  sensitive  to 
the  amido  group  present  in  aspartic  acid,  and  entirely  indifferent 
to  the  acid  groups  referred  to  above.  Whilst  Porrier's  blue  is 
absolutely  indifferent  to  ammonia  and  all  weak  basic  groups, 
alizarin  is  sensitive  to  ammonia,  but  not  to  amido  groups  of  the 
type  present  in  glycocol,  leucin  and  tyrosin.  Phloroglucinvanillin 
as  an  indicator  gives  a  sharp  titration  of  NH2  groups  of  this  class, 
and  one  of  the  NH2  groups  in  asparagin.  It  is  unaffected  by  the 
second  or  amido  group  of  asparagin,  and  those  of  the  same  type 
present  in  acetamid,  formamid,  urea,  etc. 
In  discussing  the  effect  of  albumoses,  peptones,  etc.,  on  indicators, 
it  was  shown  that  perfectly  pure  preparations  of  these  substances 
show  a  wide  range  of  variation  in  titration  end  points,  attributable 
presumably  to  the  influence  of  weak  basic  groups  present  in  the 
molecule ;  further,  that  in  advancing  hydrolysis  the  range  from  the 
lowest  to  the  highest  end  points  was  increased,  and  that  this  effect 
cannot  be  attributable  to  the  influence  of  phosphates,  organic  acids 
and  chlorides  to  the  same  extent  as  has  been  generally  supposed. 
The  comparison  of  volumetric  with  gravimetric  analyses  of  normal 
and  pathological  stomach  contents  showed  that  provided  a  plain 
Ewald,  or,  by  preferences  purely  proteid  test  meal  were  employed, 
