588  Ferric  Phosphate  and  Pyrophosphate.  {Am-^im&Tm' 
reaction  and  the  reaction  with  acetic  acid  on  boiling.  This  method 
of  proceeding  leaves  the  chemist  very  much  more  satisfied  than  the 
U.  S.  P.  process,  as  the  former  gives  us  a  positive  proof  for  pyro- 
phosphoric  acid,  and  I  can  recommend  it  to  the  attention  of  the 
Committee  for  the  Revision  of  the  U.  S.  P. 
I  found,  however,  that  a  yet  more  convenient  and  satisfactory 
test  could  be  based  on  the  separation  of  the  acids  by  means  of  acetic 
acid,  as  described  in  the  next  article.  The  test  is  not  affected  by 
the  citrates  present.  A  solution  of  alkali  citrate  acidified  with 
acetic  acid  in  any  proportion  gives  no  precipitate  with  magnesium 
sulphate  in  the  cold  or  on  boiling.  At  the  same  time  the  presence 
of  citrates  was  shown  neither  to  prevent  the  precipitation  of  magne- 
sium pyrophosphate  in  acetic  acid  solution  by  boiling  nor  to  cause 
the  precipitation  of  magnesium  phosphate  under  these  conditions. 
The  manner  of  testing  the  scale  salts  in  question  by  this  method 
is  as  follows  : 
5  cc.  of  a  20  per  cent,  aqueous  solution  of  the  salt  are  added  to 
about  9  cc.  of  a  ten  per  cent,  solution  of  potassa,  previously  heated 
to  boiling,  and  the  boiling  continued  for  a  moment ;  after  cooling 
and  filtering,  one  can  use  the  filtrate  which  I  will  call  "  A  "  for  the 
following  tests: 
(i)  If  phosphate  of  iron  is  before  us,  to  a  few  cc.  of  the  alkaline 
filtrate,  some  ammonium  chloride  and  I  to  2  cc.  of  io  per  cent, 
solution  of  magnesium  sulphate  are  added.  The  easily  recognized  crys- 
talline precipitate  of  magnesium  ammonium  phosphate  thus  produced 
is  best  filtered,  washed  with  water  and  converted  into  the  silver  salt. 
It  may  be  dissolved  in  a  few  drops  of  nitric  acid,  ammonia  added 
just  till  the  precipitate  reappears  and  the  precipitate  taken  up  with 
a  drop  of  acetic  acid.  Addition  of  silver  nitrate  solution  now  pro- 
duces, if  the  salt  examined  is  really  iron  phosphate,  a  bright  yellow 
precipitate  of  pure  silver  phosphate,  not  mixed  with  white  silver 
citrate.  A  practised  chemist  will  not  have  to  convert  the  magnesium 
precipitate  into  the  silver  salt,  easily  distinguishing  the  crystalline 
Mg(NHi)P04  from  the  amorphous  pyrophosphate  which  would  be 
thrown  down  if  the  original  salt  were  pyrophosphate  of  iron.  If  the 
quantity  and  nature  of  the  precipitate  of  Mg(NH4)P04  or  a  whitish 
color  of  its  silver  salt  makes  the  chemist  suspect  that  by  an  error 
pyrophosphate  of  iron  has  been  furnished,  he  can  test  another  por- 
tion of  the  alkaline  filtrate  "  A"  for  pyrophosphates  in  the  manner 
just  to  be  described. 
