326  Quantitative  Estimation  of  Phosphates.    { AmjJu^>1S§arm' 
of  one-tenth  normal  soda,  different  indicators  being  employed  in 
order  to  determine  the  point  of  saturation  of  the  individual  acid 
groups. 
(1)  Phosphoric  acid  is  completely  converted  into  mono-sodium 
phosphate,  XaH2P04,  at  the  alizarin  end  point. 
(2)  The  acid  is  completely  converted  into  di-sodium  phosphate, 
Na.2HP04,  at  the  phenolphthalein  end  point. 
(3)  The  acid  is  completely  saturated  with  the  formation  of 
Na3P04,  or  its  equivalent,  when  an  excess  of  neutral  barium  chlor- 
ide1 and  a  definite  excess  of  the  one-tenth  normal  alkali  employed 
are  added  to  the  solution,  which  is  boiled,  and  the  residual  excess 
of  alkali  titrated  by  means  of  one-tenth  normal  acid,  phenolphtha- 
lein still  being  employed  as  indicator. 
If  a  solution  of  phosphoric  acid  is  prepared  of  such  a  strength 
that,  using  alizarin  as  indicator,  10  c.c.  are  exactly  equivalent  to  10 
of  one-tenth  normal  alkali,  then  10  c.c.  of  the  same  solution  would 
require  20  of  the  alkali,  using  phenolphthalein  as  indicator,  and  30 
using  phenolphthalein  in  the  presence  of  an  excess  of  neutral  barium 
chloride.  As  regards  other  indicators  which  have  been  carefully 
tested,  tropaeolin  and  phloroglucin  vanallin  give  practically  no  reac- 
tion. Dimethyl-amido-azo-benzol  gives  an  indefinite  end  point, 
starting  shortly  after  the  commencement  of  the  titration  and  reach- 
ing the  green  stage  shortly  before  the  alizarin  end  point.  Methyl- 
orange,  benzo-purpurin  and  congo  give  fair  end  points  in  the 
neighborhood  of  the  alizarin  end  point.  Litmus,  lakmoid  and 
rosolic  acid  give  indefinite  end  points,  somewhere  in  the  neighbor- 
hood of  the  phenolphthalein  end  point.  In  the  presence  of  barium 
chloride,  rosolic  acid  is  affected  in  just  the  same  manner  as  phenol 
phthalein,  giving  an  indication  at  the  end  point,  Na3P04.  We 
should  not,  however,  recommend  any  of  these  indicators  for  use  in 
estimation  of  phosphates,  as  alizarin  and  phenolphthalein  have  been 
found  to  give  excellent  results  when  employed  in  the  manner  which 
will  be  described  later. 
xThe  explanation  of  this  reaction  is  as  follows:  Na2HP04  is  neutral  to 
phenolphthalein,  but  Na3P04  is  alkaline.  The  addition  of  neutral  barium 
chloride  to  the  solution  leads  to  the  immediate  precipitation  of  neutral  barium 
phosphate,  Ba3(P04l2,  so  soon  as  formed,  in  such  a  manner  that  final  end  point 
with  phenolphthalein  is  obtained  on  complete  saturation  of  the  phosphoric 
acid.    See  " Sutton's  Volumetric  Analysis." 
