586  Ferric  Phosphate  and  Pyrophosphate.  {^£*iJ£Min- 
consideration.  The  frequent  use  of  pyrophosphate  as  even  a  better 
preparation  than  the  phosphate  and  the  desirability  of  having  the 
pharmacopceial  requirements  perfectly  reliable  in  all  cases  caused 
me  to  examine  the  subject  somewhat  extensively. 
The  present  pharmacopoeia  distinguishes  the  two  preparations  by 
the  silver  salts  of  phosphoric  and  of  pyrophosphoric  acid,  which  is 
satisfactory;  but  by  neglecting  first  to  effect  the  separation  of  tjie 
respective  acid  from  the  citric  acid,  also  present,  before  making  the 
silver  test,  its  directions  prove  inadequate  to  any  one  examining 
the  salts  frequently  and  carefully.  Citric  acid  gives,  under  the  con- 
ditions of  the  test,  also  a  white  precipitate  with  silver  nitrate  :  At 
the  same  time  I  have  never  had  a  sample  of  pyrophosphate  of  iron 
U.  S.  P.  which  was  entirely  free  from  the  corresponding  phosphate, 
and  yet  the  contamination  is  sometimes  so  slight  that  the  prepara- 
tions could  not  be  condemned.  Now  the  consequence  of  the  precipita- 
tion, with  silver  nitrate,  of  phosphoric  acid  in  the  presence  of 
citrates,  and  on  the  other  hand  of  pyrophosphoric  acid  in  the  pres- 
ence both  of  citric  acid  and  varying  amounts  of  phosphoric  acid  is, 
that  the  behavior  of  both  salts  is  nearly  identical.  By  total  precipi- 
tation a  very  pale  yellow  salt  is  obtained,  which  one  is  loth  to 
regard  as  either  phosphate  or  pyrophosphate.  By  fractional  pre- 
cipitation both  preparations  give  white,  yellowish  and  yellow  tints, 
the  phosphate  often  giving  first  a  white  precipitate  of  silver  citrate, 
and  only  with  a  larger  amount  of  silver  nitrate  a  yellow  precipitate. 
The  presence  of  acetic  acid  makes  the  reaction  yet  worse.  Phosphate 
of  silver  appears  to  be  more  soluble  in  this  acid  than  silver  citrate  or 
silver  pyrophosphate  :  If  after  complete  precipitation  a  little  ammonia 
is  added,  bright  yellow  silver  phosphate  is  thrown  down  both  in  the 
case  of  phosphate  and  generally  of  commercial  pyrophosphate. 
My  time  was  too  limited  to  investigate  whether  the  appearance  of 
i  phosphate  in  the  pyrophosphate  of  iron  is  due  to  any  converting 
action  during  the  course  of  preparation;  it  probably  is  due  to  the 
use  of  an  impure  sodium  pyrophosphate.  But  certainly  it  would 
hardly  be  advisable  to  condemn  every  pyrophosphate  of  iron  con- 
taining a  slight  amount  of  phosphate. 
In  order  to  do  away  with  this  uncertainty,  and  especially  to  have 
a  positive  reaction  for  pyrophosphate  without  the  possibility  of  mis- 
taking the  citric  acid  present  for  it,  I  tried  several  different  methods 
of  proceeding. 
