460 
Theory  of  Indicators,  etc. 
j  Am.  Jour.  Pharm. 
t      October,  1904. 
is  not  to  be  recommended  when  phenolphthalein  is  equally  service- 
able. 
(4)  The  differentiation  of  organic  acids,  generally  speaking,  from 
mineral  acids,  may  in  the  absence  of  phosphates  be  most  readily 
effected  by  means  of  phloroglucinvanillin  drops  on  a  porcelain  plate, 
the  temperature  of  which  is  kept  slightly  below  the  boiling  point 
of  water,  the  end  point  being  that  point  at  which  the  faint  pink 
finally  disappears,  and  does  not  reappear  on  cooling.  Dimethyl- 
amido.azo-benzol,  commonly  employed  in  volumetric  work,  is  prac- 
tically useless  for  this  purpose,  unless  the  observer  be  especially 
trained  to  note  the  first  change  from  pink  to  orange  red.  This 
indicator  is  not  to  be  recommended.  Tropseolin  oo  may  be  util- 
ized to  distinguish  between  mineral  and  organic  acids  when  used  in 
the  form  of  drops  on  a  hot  plate,  if  the  precaution  is  taken  of  ob- 
serving only  a  permanent  purple  end  point  and  not  a  temporary 
brown  coloration.  In  the  presence  of  phosphates  phloroglucin- 
vanillin drops  give  a  slight  reaction,  but  with  experience  in  using 
the  method  the  effect  produced  by  phosphates,  especially  when 
present  in  small  quantities,  may  be  disregarded.  With  tropoeolin, 
whilst  hydrochloric  and  sulphuric  produce  a  permanent  purple  col- 
oration, phosphoric  acid  first  produces  merely  a  permanent  faint 
slate  color,  and  that  stops  short  of  the  complete  saturation  of  the 
first  acid  affinity  of  this  substance. 
(5)  Carbonic  acid  exhibits  most  interesting  characteristics.  Sodium 
carbonate,  Na2C03,  when  dissolved  in  water,  is  neutral  to  Porrier's 
blue.  The  phenolphthalein  end  point  is  obtained  after  saturation  of 
one  of  the  sodium  affinities  with  acid,  and  the  end  point  of  alizarin, 
hematoxylin,  methyl-orange,  and  especially  of  the  phloroglucinva- 
nillin drops,  is  first  obtained  after  complete  saturation  of  all  the 
sodium  present  with  strong  mineral  acid.  In  other  words,  Porrier's 
blue  is  the  only  indicator  which  recognizes  both  acid  groups  of  the 
hypothetical  carbonic  acid.  Phenolphthalein  is  sensitive  to  one  of 
them,  and  the  other  indicators  enumerated  are  entirely  indifferent 
to  their  presence. 
(6)  The  great  bulk  of  indicators  give  more  or  less  reaction  with 
organic  acids.  Very  few  of  them,  however,  can  be  said  to  afford  a 
satisfactory  final  end  point.  It  is  possible,  for  example,  to  obtain 
an  end  point  including  all  mineral  acids  and  such  organic  acids  as 
lactic,  butyric,  etc.,  provided  the  first  color  changes  are  disregarded 
