176  So-Called  Tasteless  Iron  Compounds.  {AmAJp°rir;x876.rm' 
and  ferric  phosphate  unite  to  produce  a  soluble  compound,  the  solution 
of  which,  when  evaporated  and  scaled,  yields  a  beautiful  secondary  salt, 
having  the  formula  :  Fe2(C6H507)(P04). 
This  ferric  citrophosphate  is  undoubtedly  superior  in  various  respects 
to  the  complex  mixtures  obtained  from  the  ferric  phosphates  by  the 
use  of  monad  citrates.  This  salt  is  entirely  ferruginous,  and  must 
therefore  rank  medicinally  with  the  normal  or  primary  ferric  salts  in 
which  only  one  acid  radical  is  combined.  Its  formation  is  also  remark- 
able, in  that  it  cannot  be  produced  by  treating  ferric  phosphate  with 
citric  acid,  no  combination  taking  place.  If  however  ferric  citrate  is 
treated  with  a  certain  proportion  of  orthophosphoric  acid  a  green  liquid 
results,  but  if  an  excess  of  phosphoric  acid  is  added  all  the  iron  is  pre- 
cipitated as  ferric  phosphate,  which  the  liberated  citric  acid  fails  to  re- 
tain in  solution. 
A  convenient  method  of  preparing  ferric  citrophosphate  will  be  to 
treat  a  concentrated  solution  of  ferric  sulphate  with  an  amount  of  di- 
sodic  orthophosphate  sufficient  to  convert  half  the  iron  into  phosphate 
and  then  adding  disodic  carbonate  in  excess,  washing  the  mixed  phos- 
phate and  oxy-carbonate  by  decantation  and  dissolving  it  in  an  amount 
of  citric  acid  sufficient  to  convert  the  ferric  oxy-carbonate  into  ferric 
citrate. 
Ferric  chloride,  sulphate,  iodide,  etc.,  remain  unchanged  in  presence 
of  ferric  citrate,  for  the  reason,  that  radicals  of  the  same  basicity  can 
only  form  compound  salts  of  this  order ;  hence,  only  ferric  salts  of 
tribasic  acids  can  directly  unite  with  ferric  citrate. 
As  a  general  rule  all  the  amorphous  ferric  salts,  or  mixtures  of  them 
with  other  amorphous  salts,  can  be  obtained  in  transparent  scales,  but 
a  mixture  of  an  amorphous  ferric  salt  with  some  other  crystallizable 
salt,  will  invariably  produce  an  opaque  granular  mass  iususceptible  of 
crystallizing  or  scaling.  Therefore,  if  the  result  of  a  reaction  between 
a  ferric  and  some  other  salt,  yields  on  drying,  a  granular  mass ;  a  crys- 
talline constituent  can  positively  be  inferred,  but  if  such  a  decomposi- 
tion results  in  a  transparent,  scaly  compound,  the  absence  of  crystalline 
bodies  is  unfailingly  pointed  out. 
All  the  green  compound  ferric  citrates  when  treated  with  alkalies  in 
excess,  become  brown  or  red,  and  all  such  compounds  which  have  been 
either  reddened  by  alkalies  or  obtain  this  color  as  the  result  of  the  reac- 
tion which  produced  them,  will  become  green  after  the  addition  of 
