Am.  Jour.  Pharm.  \ 
November,  1904.  / 
Theory  of  Indicators,  etc. 
521 
and  it  may  be  said  that  some  serious  disturbance  in  the  meta- 
bolic equilibrium  has  taken  place  when  urine  is  either  acid  or 
alkaline  to  both  alizarin  and  phenolphthalein.  In  other  words, 
since  the  passage  from  alizarin  to  phenolphthalein  represents  the 
neutralization  of  the  second  acid  group  of  phosphoric  acid,  the 
excreta  of  the  kidneys  cannot  be  said  to  be  satisfactory  unless  the 
first  acid  group  of  the  phosphates  is  entirely  neutralized  by  bases 
and  the  third  entirely  free,  the  end  point  lying  somewhere  between 
the  neutralization  points  of  the  first  and  the  second. 
The  presence  of  small  quantities  of  amido  bodies  (lying  between 
ammonia  and  urea  in  basic  properties)  could  not  produce  a  very 
appreciable  effect  upon  indicators  in  comparison  with  the  large 
amount  of  phosphoric  acid  present  in  the  urine,  and  it  is  conse- 
quently not  a  matter  of  such  importance  to  determine  their  influence 
as  was  the  case  in  dealing  with  stomach  contents,  serum,  etc. 
In  the  presence  of  any  considerable  quantity  of  ammonia,  which, 
in  contradistinction  to  urea  exerts  a  powerful  effect  upon  alizarin 
and  phenolphthalein,  the  urine  would  be  less  acid,  or  even  alkaline 
to  phenolphthalein  and  more  strongly  alkaline  to  alizarin  than  is 
normally  the  case. 
In  conclusion  the  writer  wishes  to  express  his  indebtedness  to 
Dr.  David  E.  Wheeler,  by  whom  a  considerable  number  of  the 
analyses  referred  to  in  this  paper  were  carried  out. 
XI.  SUMMARY. 
In  this  paper  the  physico-chemical  theory  of  indicators  has 
been  discussed  in  so  far  as  its  principles  are  found  to  be  applicable 
to  the  solution  of  problems  involved  in  the  volumetric  analysis  of 
physiological  solutions. 
Practically  all  indicators  are  sensitive  to  strong  acids  and  strong 
bases.  They  may,  however,  be  divided  into  three  distinct  classes 
when  tested  with  a  series  of  weak  acids  and  weak  bases  of  the  type 
present  in  physiological  solutions.  At  one  end  of  the  scale  are 
those  which  in  virtue  of  an  extremely  weak  acid  affinity  are  particu- 
larly affected  by  weak  acids  and  are,  comparatively  speaking,  indif- 
ferent to  weak  bases ;  whilst  at  the  other  end  of  the  scale  are  to  be 
found  those  which  are  possessed  of  strong  acid  or  weak  basic  com- 
bining groups  which  render  them  very  susceptible  to  the  influence 
of  weak  bases  but  more  or  less  indifferent  to  that  of  weak  acids. 
