MEDICAL  ANALYSIS. 
267 
It  will  be  observed  that  only  one-third  of  an  equivalent  in 
grains  is  neutralized  by  the  100  measures  of  volumetrical  solu- 
tion of  soda,  although  that  number  of  measures  contain  a  full 
equivalent  in  grains  of  soda,  which  would  neutralize  or  —  one 
equivalent  of  any  monobasic  acid,  but  citric  acid  is  what  is  termed 
a  tribasic  acid,  and  the  neutral  salt  is  a  trimetallic  salt.    Thus — 
Citric  Acid  C12H8  Ou 
Acid  Monometallic  Salt  Ci2H7  MQu 
Acid  Dimetallic  Salt  Ci2II6  M2  Oi4 
Neutral  Trimetallic  Salt  C12H5  M3  Ou 
Therefore  we  have  the  following  decomposition  occurring  in  the 
neutralization  of  citric  acid  by  the  volumetric  solution  of  soda : — 
,  3HO,C12H5  Ou  +  3Na  0  =  C12H5  Na.3  614  +  3HO 
[2C6  H8  O7         +  3Na2  0  ==  2C6  H5  Na3  O10  +  3IP  0] 
Citric  Acid.  Soda.  Tricitrate  of  Soda. 
.-.    C    :    NaO    :  :    67    :  31 
Citric  acid  is  frequently  slightly  tinged ;  this  is  due  to  the 
charring  of  the  organic  matter  by  the  sulphuric  acid  employed  in 
its  manufacture,  and  from  other  causes. 
Aeidum  Grallicum. 
3  HO,CuH3  07  +  2  HO  =  [C7  IP  O5  +  HO2 .] 
This  is  a  tribasic  acid.  When  pure  it  occurs  in  white  acicular 
prisms,  but  is  generally  found  in  commerce  more  or  less  colored, 
(described  in  the  Pharmacopoeia  as  a  pale  fawn  color).  It  dis- 
solves in  100  parts  of  cold  and  3  parts  of  boiling  water.  It  is 
very  soluble  in  alcohol,  but  less  soluble  in  ether. 
If  we  except  the  coloring  matter,  this  acid  is  generally  pure. 
The  Pharmacopoeia  tests  will  meet  every  case— viz., 
"  It  leaves  no  residue  when  burned  with  free  access  of  air.  Its 
solution  gives  no  precipitate  with  gelatine." — Medical  Press, 
Dublin,  May  31,  1865. 
crop  of  crystals  the  most  impure  when'recrystallizing  ;  he  recommends 
dissolving  the  ordinary  acid  in  sufficient  water  to  give  from  10  or  15  per 
cent,  of  the  original  weight  employed,  when  crystallization  has  taken 
place  these  are  to  be  rejected  ;  mother  liquors  yield  a  purer  crystal.  This 
treatment  may  be  repeated  until  oxalic  acid  is  procured  practically  pure. 
The  writer  would  recommend  acidulating  the  liquor  with  nitric  acid  before 
the  last  recrystallization. 
