168     Ferric  Citro- Phosphate  and  Citro-Fyrophosphate. 
and  223  grains  of  sodic  pyrophosphate  yield  405  grains  of  the  salt, 
whose  formula  is  therefore  Fe2Na4Ci2(P207).  3  Aq.,  and  molecular  weight 
810.  Its  constitutional  formula  is  susceptible  of  quite  a  number  of 
possible  variations,  but  perhaps  its  most  probable  form  is 
Fe  P,0, 
I  II 
Ci  —  Na^.    8  Aq. 
Fe  ==  c'i 
It  may  hence  be  viewed  as  a  derivative  of  the  sodio-ferric  pyrophos- 
phate by  replacing  two  pyrophosphoric  by  two  citric  radicles,  which  of 
course  decreases  the  sodium  by  two  ms. 
Fe  P,0, 
I 
Fe 
The  old  official  salt  can  also  be  considered  as  a  mixture  of  two  salts 
composed  of  one  m.  of  the  derivative  just  treated,  and  one  m.  of 
Fe  = 
I  II 
Ci  —  Na. 
I  II 
Fe  =  P2OV 
Now  instead  of  accepting  the  proposed  salt  it  was  deemed  preferable 
to  adopt  the  more  complex  salt  into  the  Pharmacopoeia,  which  is 
rendered  still  more  indefinite  by  reason  of  the  error  regarding  the 
formula  of  ferric  citrate.  The  new  salt  is  stated  to  contain  11*5  per 
cent,  of  metallic  iron,  which  would  make  its  molecular  weight  1948, 
with  somewhat  less  than  9J  ms.  of  hydrous  water. 
In  the  same  paper  ("American  Journal  of  Pharmacy,'^  April,  1876), 
on  the  inverse  synthesis  of  these  salts,  the  w^riter  also  proposed  a  sodio- 
ferric  citrophosphate  to  be  prepared  by  this  general  method.  In  this 
instance,  however,  no  special  recommendation  was  made.  But  the 
writer  stated  that  the  nearest  approximation  to  the  salt  made  by  dis- 
solving two  ms.  of  ferric  phosphate,  FePO^,  in  one  m.  of  trisodic 
citrate,  NagCi,  is  obtained  by  mixing  one  m.  of  ferric  citrate  with  one 
m.  of  trisodic  phosphate.  It  was  incidentally  mentioned  that  ferric 
citrate,  in  considerable  excess,  formed  green  solutions  with  disodic 
phosphate.  Upon  this  vague  suggestion  a  compound  was  made  official, 
represented  by  one  m.  of  ferric  citrate  and  one  m.  of  disodic  phos- 
phate. It  is  to  contain  13*5  per  cent,  of  metallic  iron,  indicating  a 
molecular  weight  of  415  with  somewhat  more  than  1^  m.  of  hydrous 
water. 
