AmjJu°iy!i903arm'}     Quantitative  Estimation  of  Phosphates.  329 
It  may  further  be  noted  for  the  benefit  of  those  who  have  not 
platinum  vessels  at  their  disposal,  and  who  have  to  consider  the 
factor  of  time,  that  a  fairly  accurate  estimate  of  phosphates  may  fre. 
quently  be  made  in  the  following  manner :  After  titrating  the  stom- 
ach contents,  using  phenolphthalein  as  indicator  (in  which  experi- 
ment as  large  a  quantity  of  stomach  contents  as  possible  should  be 
employed),  an  excess  of  three  or  four  cubic  centimeters  of  alkali  is 
added,  the  mixture  boiled  vigorously  and  neutralized  with  sulphuric 
acid  as  in  the  above  experiments.  A  known  excess  of  alkali  is 
then  added,  and  the  five  cubic  centimeters  of  neutral  barium  chloride 
solution ;  the  mixture  once  more  boiled  and  titrated  with  one-tenth 
normal  acid.  The  difference  between  the  amount  of  alkali  employed 
and  the  amount  of  acid  required  for  titrating  back  gives  the  effect 
due  to  the  third  acid  affinity  of  the  phosphoric  acid  between 
Na2HP04  and  Na3P04,  and  is  equal  in  quantity  to  that  exerted  by 
the  phosphates  on  the  alizarin  end  point  for  the  same  quantity  of 
material  from  H3P04  to  Na  H2P04.  It  also  represents  half  the  effect 
on  the  phenolphthalein  end  point  from  H3P04  to  Na2HP04,  and  is 
equal  to  that  between  the  alizarin  and  phenolphthalein  end  point, 
NaH2P04  to  Na2HP04,  and,  therefore,  has  to  be  deducted  from  that 
quantity  in  order  to  obtain  a  more  correct  estimate  of  the  so-called 
combined  hydrochloric. 
In  making  use  of  this  method  without  evaporation  and  ashing,  it 
must  be  remembered  that  the  phenolphthalein  end  point  is  seriously 
interfered  with  and  that  certain  other  disturbing  factors  prevent  as 
much  accuracy  as  is  desirable,  but  still  it  affords  much  better  results 
than  are  obtainable  either  by  means  of  uranium  nitrate  in  the 
stomach  contents  directly,  a  method  which  we  employed  for  a  con- 
siderable period  of  time,  or  any  of  the  methods  previously  suggested, 
such  as  that  obtained  from  the  difference  between  the  dimethyl 
amido-azo-benzol  and  alizarin  end  points,  which,  as  we  have  shown 
above,  is  absolutely  incorrect. 
An  illustration  of  our  method  may  make  it  easier  to  follow.  Fifty 
cubic  centimeters  of  filtered  contents  were  evaporated  to  dryness, 
ignited,  and  extracted  with  boiling  water,  with  addition  of  a  small 
amount  of  acid.  The  whole  was  made  up  to  25  c.c.  Two  lots 
each  of  10  c.c.  were  employed  for  the  titrations,  which  were  carried 
out  as  directed  above. 
(1)  Phenolphthalein  end  point  to  alizarin  end  point  =  0  8  c.c, 
n/10  acid. 
