1 06  Bismuth  and  Iron.  {Am  ]&!7^ 
nium  citrate,  with  the  equivalent  of  citric  acid  contained  in  the  iron 
citrate,  and  hence  the  true  or  normal  ammonio-feroxycitrate  is  also 
produced  as  follows  : 
Fe///C6H5074-3(NH4OH)=(NH4)3C6H5OrFe(OH)3,  that  is  tri- 
ammonium  citrate  combined  with  normal  ferric  hydrate.  It  is,  there- 
fore, the  iron  analogue  of  ammonio-bismuthous  citrate. 
The  writer  has  also  ascertained  that  besides  the  normal  green  ammo- 
nio-ferric  citrate,  two  other  ammonio-ferric  citrates  exist,  correspond- 
ing to  the  di  and  mon-ammonium  citrates.  The  formulae  for  the  3 
green  double  salts  are,  for  triammonio-ferric  citrate  Fe'"(NH4)32(C6H5 
07),  for  diammonio-ferric  citrate  Fe"'(NHJ2C6H607.C6H607or Fe"'(N 
H4)2H2(C6H507),  and  for  monammonio-ferric  citrate  Fe//r(NH4)2(C6H& 
07)  or  F///(NH4)H22(C6H507).  The  first  one  only  is  neutral,  the  other 
two  are  acid  salts.  If  either  of  these  salts  is  treated  with  ammonia  in 
excess,  the  solution  becomes  brown,  and  in  some  cases,  as  for  instance 
with  the  potassium  double  citrates,  which  do  not  hold  the  iron  as  firmly 
as  the  corresponding  ammonium  salts,  ferric  hydrate  is  precipitated. 
The  reaction,  therefore,  indicates  that  a  monado-fer-oxy-citrate  was  pro- 
duced. It  is,  however,  remarkable  that  the  addition  of  citric  acid  or 
an  acid  monad  citrate  does  not  immediately  restore  the  green  color> 
but  in  some  instances  proceeds  very  slowly,  the  transition  from  brown 
to  green  is  not  simultaneous  throughout  the  solution,  but  progresses 
from  the  bottom  upwards,  even  if  the  solution  is  occasionally  shaken,  it 
resumes  this  order  of  progressive  action. 
The  most  surprising  relationship,  is  however,  evinced  when  ammonio- 
bismuthous  citrate  and  triammonio-ferric  citrate  are  brought  together, 
and  the  change  of  color  is  the  same  as  when  triammonium  citrate  is 
added  to  the  iron  salt.  The  light  green  solution  is  further  remarkable 
for  the  very  important  fact  that  moderate  or  strong  acidulation  with 
citric  or  nitric  acid  fails  to  separate  the  bismuthous  citrate.  This  prop- 
erty, therefore,  renders  it  possible  of  holding  bismuthous  citrate  in  acid- 
ulated solution,  providing  ammonio-ferric  citrate  is  also  present. 
The  same  amount  of  citric  acid  in  combination  as  ammonio-bismuth- 
ous citrate,  is  required  to  change  an  equal  equivalent  of  the  acid  com- 
bined as  ferric  citrate  into  the  green  ammonio-ferric  citrate,  as  would 
be  required  in  the  condition  of  triammonium  citrate.  Therefore  the 
result  is  expressed  as  follows  : 
Fe^^C6H507+(NH4)3C6H5OrBi(OH)3=Fe^(NH4)32(C6H507).Bi(OH)3. 
That   is,  one   equivalent   of  triammonio-ferric   citrate  combined 
