422 
ALKALOIDS  IN  THE  CINCHONA  BARKS. 
who  have  first  to  prepare  the  perfectly  neutral  reagent.  Sul- 
phate of  quinine,  containing  traces  of  free  sulphuric  acid,  (in 
the  case  of  its  having  been  recrystallized  from  strongly  acidu- 
lated water)  is  thereby  rendered  more  soluble,  and  since  all  so- 
lutions of  quinine  salts,  which  are  more  soluble,  are  precipitated 
by  iodide  of  potassium,  the  result  of  the  test  would  in  that  case 
be  faulty.  In  the  presence  of  very  slight  traces  of  foreign  al- 
kaloids a  large  excess  of  the  reagent  frequently  dissolves  the 
precipitate  as  rapidly.again. 
Several  years  ago  Howard  proposed  to  employ  the  slight  sol- 
ubility of  the  sulphate  of  quinine  in  cold  water  as  a  criterion  of 
its  purity  ;  but  the  solubility  of  even  the  purest  sample  varies, 
and  is  not  at  all  constant,  and  this  mode  of  testing  might,  there- 
fore, in  some  cases,  cause  the  analyst  to  overlook  from  2  to  5 
per  cent,  of  quinidine.  Accurate  determinations  of  the  solubili- 
ty of  organic  substances,  containing  water  of  crystallization, 
are  not  among  the  easiest  of  tasks  of  analytical  chemistry,  and 
the  possession  of  a  characteristic  qualitative  test  becomes  there- 
fore a  thing  all  the  more  to  be  desired.  A  method  for  the  rec- 
ognition of  foreign  cinchona  bases  in  sulphate  of  quinine  may, 
however,  be  based  upon  the  difference  in  the  solubility  of  the 
sulphates  of  our  alkaloids,  provided  another  factor  of  solubility 
is  taken  into  account.  Dilute  solution  of  caustic  ammonia  ex- 
hibits very  different  solvent  powers  with  the  different  cinchona 
alkaloids.  Quinine  is  dissolved  with  comparative  facility,  «  qui- 
nidine with  much  more  difficulty,  and*  quinidine  and  cinchoni- 
dine  only  slightly,  whilst  cinchonine  is  all  but  perfectly  insolu- 
ble in  ammonia.  The  solubility  of  the  cinchona  bases,  in  ammo- 
nia, stands  therefore  in  an  inversed  ratio  to  that  of  their  sul- 
phates in  water.  Upon  this  fact  is  based  the  following  quinine 
test : — 
A.  Solubility  of  the  cinchona  bases  in  dilute  ammonia. 
The  dried  alkaloids,  whether  amorphous  or  crystalline,  dis- 
solve very  difficultly  and  slowly,  and  rarely  equally  in  ammo- 
nia ;  they  do  not  even  dissolve  in  constant  proportions  when 
freshly  precipitated,  and  when  in  a  pasty  state.  I  found,  how- 
ever, that  they  are  rendered  considerably  more  soluble  by  em- 
ploying ammonia  for  the  precipitation  of  the  alkaloids  from  their 
saline  solutions,  and  by  adding  ammonia  in  excess  till  the  bases 
