Am  Jour.  Pharm. ) 
Sept.,  1882.  )■ 
A  mmonia ca  I  Citro^es. 
455 
AMMONIACAL  CITRATES. 
By  E.  Landrin. 
It  is  well  known  that  many  metallic  oxides  and  citrates,  insoluble 
in  water,  are  soluble  in  the  alkaline  citrates,  even  in  the  presence  of 
reagents,  which  under  ordinary  conditions  precipitate  these  oxides/ 
This  phenomenon  has  been  made  the  basis  of  a  quantitative  method 
for  the  determination  of  phosphoric  acid.    Some  years  ago,  Spiller 
Pharm.  Journ.,''  1858)  carried  on  a  series  of  investigations,  in  order  to 
stndy  the  influence  of  citric  acid  in  preventing  the  ])recipitation  of  the 
metallic  oxides,  and  arrived  at  the  general  result  that  the  neutral 
citrates  possess  the  property  of  combining  with  other  salts  to  form  a 
class  of  compounds  of  the  general  formula  M^gCgHgO^  -f  3M^2^^4? 
which  sulphuric  acid  may  be  replaced  by  carbonic,  chromic,  or  boracic 
acids.  For  instance,  solutions  of  these  compounds  are  not  precipitated 
by  barium  nitrate  until  a  slight  excess  of  sodium  sulphate  is  present. 
Lebaigue  (1864),  however,  considers  that  these  phenomena  are  dne  to  an 
interchange  between  the  acids  and  bases,  which  is  stable  only  so  long  as 
the  citrate  liberated  in  the  nascent  state  is  soluble,  and  thus  the  pecu- 
liar characteristics  of  the  acids  and  bases  present  become  apparent 
when  the  nascent  citrate  has  saturated  the  alkaline  citrate,  viz.,  when 
the  precipitant  is  in  excess  of  the  alkaline  citrate.  Further,  tlie  insolu- 
ble citrates  are  dissolved  in  the  alkaline  citrate  in  definite  proportions, 
and  citric  acid  being  tribasic  can  saturate  not  only  three  equivalents  of 
the  same,  but  also  of  different  bases  to  form  soluble  salts. 
In  order  to  decide  between  these  views,  the  author  has  taken  up  the 
question,  and  has  arrived  at  results  in  accordance  with  those  of 
Lebaigue,  i.  e.,  citrates  insoluble  in  water  dissolve  in  alkaline  citrate.s, 
with  formation  of  double  salts  of  the  composition  MgM^CgHgO^,  in  which 
M  is  an  alkali-metal,  and       a  metal  belonging  to  some  other  class. 
This  result  receives  support  from  the  following  experiments:  If 
barium  carbonate  is  gradually  added  to  citric  acid  saturated  with 
ammonia  until  one  equivalent  of  acid  and  baryta  are  present,  a  clear 
solution  is  obtained,  from  which,  on  cooling,  normal  barium  citrate 
separates  out.  The  supernatant  liquid  contains  in  solution  a  doul)le 
ammonium  barium  citrate.  Analogous  phenomena  were  observed 
with  the  oxides  of  calcium,  strontium,  lead,  and  cadmium.  Again,  if 
aluminium  hydroxide  be  dissolved  in  ammonium  citrate, and  the  solution 
evaporated  over  sulphuric  acid,  white  crystals  of  a  double  ammonium 
