318  Acetic  Acid  in  Extracting  Drugs.  {Kmi™l'S£xm' 
few  seconds.  On  close  inspection  by  reflected  light  no  difference 
in  tint  between  the  lower  and  upper  parts  of  the  wetted  portion  will 
be  discoverable.  If  this  be  the  case,  one  or  two  drops  more  of  the 
decinormal  alkali  is  added  and  the  testing  repeated  with  a  new  strip 
of  wetted  neutral  paper.  Now,  a  small  patch  of  faint  bluish  tint  will 
be  discoverable  about  the  middle  of  the  wetted  portion,  and  this  in- 
dicates as  nearly  as  need  be  the  point  when  all  the  acid  salts  have 
been  reduced  to  neutral  salts. 
The  number  of  cubic  centimetres  of  decinormal  alkali  used  to 
reach  this  point  subtracted  from  the  number  of  cubic  centimetres 
of  decinormal  acid  taken  for  the  solution  gives  the  number  of 
cubic  centimetres  of  the  acid  saturated  by  the  alkaloids  to  form 
the  neutral  salts,  and  this  number  divided  by  10  gives  the 
amount  of  normal  acid  equivalent  to  the  decinormal  acid  used. 
This  multiplied  by  the  normal  molecular  weight  of  the  alka- 
loids would  give  the  weight  of  alkaloids  obtained  from  the  I  a 
grammes  of  cinchona  taken.  But  there  are  many  alkaloids  of 
different  molecular  weights  in  cinchona,  so  that  it  is  impracticable 
to  get  a  molecular  weight  that  would  accurately  represent  any  sam- 
ple of  total  alkaloids.  Perhaps  the  best  that  can  be  done,  as  has 
often  been  done  before,  is  to  make  an  arbitrary  composite  combin- 
ing number.  All  that  can  be  said  of  this  proceeding  is  that  it  is 
very  convenient — that  it  admits  of  titration — that  the  results  can- 
not be  more  than  about  0-3  per  cent,  from  the  truth  in  rare  cases, 
and  that  it  is  always  closer  than  is  the  weighing  of  a  chloroform  or 
ether  extract  as  total  alkaloids. 
The  alkaloids  of  cinchona  may  be  usefully  divided  into  three 
groups: 
(1)  The  quinine  group  with  a  molecular  weight  of  about  0-324. 
(2)  The  cinchonine  group  with  a  molecular  weight  of  about 
0-294. 
(3)  The  remaining  alkaloids  with  a  molecular  weight  of  about 
0-312. 
Practically  no  cinchona  for  pharmaceutical  uses  should  contain 
less  than  5  per  cent,  of  total  alkaloids,  and  at  least  2-5  per  cent,  of 
these  should  be  of  the  quinine  group,  125  per  cent,  of  the  cincho- 
nine group,  and  1-25  per  cent,  of  the  remaining  alkaloids.  This 
proportion  being  arbitrarily  assumed  gives  a  combining  weight  of 
0  314  as  follows : 
