118  Ferric  Citrate  and  its  Double  Salts.       { ^"^Mar '1883™" 
The  action  of  hydro-sodic  carbonate  on  ferric  citrate  is  peculiar^ 
When  equal  equivalents  are  used,  that  is  272  grains  of  the  citrate  and 
84  grains  of  the  carbonate,  285  grains  of  an  elegantly  scaled  and  very 
soluble  light-red  non-deliquescent  compound  is  produced.  Hence  its 
formula  is 
Fe  m  Ci  (OH)  or  Fe— (OH) 
II  and  molecular  weight  285. 
Ci— Na 
When  two  equivalents  of  the  carbonate  are  employed,  only  a  part  of 
its  carbonic  oxide  (not  carbonous  oxide,  formerly  called  carbonic  oxide) 
is  expelled,  the  remainder  being  retained  in  the  formation  of  a  remark- 
able double  citro  carbonate,  whose  formula  FeNa2CiC03,  or 
Fe  =  CO3 
I  Hence  a  sodioferric  citrocarbonate. 
Ci  =  m,. 
When  three  equivalents  of  the  carbonate^ire  added,  a  still  more  com- 
plicated effect  is  produced,  68  grains  of  ferric  citrate  and  63  grains  of 
the  carbonate  yielding  98  grains  of  a  blistered,  scaly,  very  soluble  mass^ 
The  formula  of  this  compound  is  then  FeNajCi  (OH)  or 
(OH)— Fe— COsNa 
I  and  molecular  weight  391,  it  being  a  sodioferric 
Ci  =  Na, 
hy  d  ro-ci  tr  ocarbonate . 
All  of  the  above-described  double  and  secondary  salts  are  deriva- 
tives by  substitution  within  a  single  molecule  of  citrate.  An  entirely 
different  order  of  double  and  secondary  salts  results  from  the  coales- 
cence of  two  molecules.  The  most  characteristic  of  the  ferric  double 
citrates  are  those  formed  by  the  normal  monad  citrates.  They  are 
sharply  distinguished  from  the  preceding  order  by  their  splendid  green, 
color. 
Ferric  citrate  unites  with  normal  citrates,  as  the  writer  first  pointed 
out,  molecule  for  molecule.  When,  for  instance,  68  grains  of  ferric 
citrate  is  dissolved  in  81  grains  of  potassic  citrate  in  the  presence  of 
water,  the  characteristic  green  solution  of  the  double  salt  results.  This 
on  evaporation  to  a  dense,  syrupy  consistence,  rapidly  becomes  granular 
and  opaque,  and  on  drying  leaves  a  yellowish-green  powdery  residue, 
weighing  138  grains.  This  rather  astonishing  and  unexpected  result 
led  the  writer  to  question  the  condition  of  the  potassic  citrate,  and. 
therefore  repeated  the  experiment  by  generating  this  citrate  directly  in. 
the  process.    Hence  75  grains  of  hydro-potassic  carbonate  was  added. 
