174  So-Called  Tasteless  Iron  Compounds.  {^ApS'iSS*3* 
same  plan,  it  was  also  proposed  to  construct  the  orthophosphate  by 
mixing  ferric  citrate  with  triammonic  phosphate.  Trial  showed  that 
mixtures  of  these  two  kinds  resulted  in  products  having  the  appearance 
of  the  preparations  made  by  the  usual  methods.  It  was,  however, 
ascertained  that  the  precise  imitation  by  inverse  synthesis,  was  impos- 
sible, since,  in  each  of  these  two  cases,  by  the  usual  methods,  an  ex- 
cess of  the  ferric  phosphates  was  absorbed. 
By  an  inspection  of  the  officinal  formula  for  ferric  pyrophosphate,  it 
was  discovered  that  ferric  pyrophosphate  and  triammonic  citrate  react 
upon  each  other  in  the  proportion  of  one  equivalent  of  ferric  pyrophos- 
phate to  three  equivalents  of  triammonic  citrate,  producing  ammonio- 
ferric  pyrophosphate,  ammonio-ferric  citrate  and  free  ferric  citrate, 
thus  :     2(Fe,3P20,)+6(NH4)3C6H507)  =  Fel3P207-3(NH4)4P207)+2 
(FeC6H607.(NH4)3C6H607)+2(FeC6HA.) 
This  shows  that  the  officinal  salt  is  a  rather  complex  compound,  and 
that  the  presence  of  uncombined  ferric  citrate  is  the  cause  of  the  pecu- 
liar tint  described  as  apple  green.  It  also  explains  why  dialysis  can 
not  separate  the  mixture,  as  all  three  of  the  constituents  are  uncrys- 
tallizable,  and,  therefore,  colloidal.  The  nearest  approach  to  this 
compound,  by  means  of  inverse  synthesis,  is  four  equivalents  of  ferric 
citrate  and  three  of  ammonium  pyrophosphate,  which  produces  only 
ammonio-ferric  pyrophosphate  and  ammonio-ferric  citrate,  thus  :  8(Fc 
C6H507)+6((NH4)4P207)  =  Fe43P307.3((NH4)4P207)  +  4(FeC6H507. 
(NH4),C6H5Or) 
This  is  also  equal  to  2(Fe43P207)+8((NH4)3C6H507),  that  is  one 
equivalent  of  ferric  pyrophosphate  and  four  equivalents  of  triammonic 
citrate,  and  differs  from  the  officinal  by  one  equivalent  of  triammonic 
citrate  in  excess. 
Otherwise,  the  difference  in  the  two  compounds  may  also  be  shown 
as  follows  :  4<Fe43PI0,)+"ftNHj,C,H.OI)=F»«3PA+3(4(FeC, 
H507)+3((NH4)4P207.) 
This  indicates  that  the  officinal  salt  is  equal  to  one  equivalent  of 
ferric  pyrophosphate  and  three  of  the  imitation  salt. 
The  salt,  or  rather  mixture  produced  by  retrograde  decomposition, 
has  the  bright  green  color  peculiar  to  ammonio-ferric  citrate,  but  addi- 
tion of  a  certain  proportion  of  ferric  citrate  generates  the  apple  green 
tint.  By  mixing  two  equivalents  of  ferric  citrate  and  one  of  ammo- 
nium pyrophosphate  a  mixture  exactly  analogous  to  the  officinal  salt  is 
