302         Atropine  Per  iodides  and  I o  do  mer  curates.  {Amju0nUe^is98arm' 
amount  of  iodine,  free  as  well  as  bound  to  potassium,  by  shaking 
them  in  a  flask  with  an  excess  of  powdered  metallic  zinc  till  the 
color  of  iodine  disappeared,  filtering  the  solution  of  zinc  iodide  and 
potassium  iodide  thus  obtained,  washing  the  flask  and  the  excess  of 
zinc  with  hot  water  and  precipitating  the  iodides  in  the  filtrate  with 
silver  nitrate  and  nitric  acid.  From  the  weight  of  silver  iodide, 
washed  and  dried  in  the  usual  way,  we  calculated  the  amount  of 
total  iodine  present  in  our  iodine  solution.  By  subtracting  the 
amount  of  free  iodine  from  that  of  the  total  iodine,  we  obtained  the 
amount  of  iodine  present  in  our  solution  as  potassium  iodide.  We 
thus  found  that  the  iodine  solution  contained  I  per  cent,  free 
iodine  and  1-2312  per  cent,  combined  iodine. 
We  then  put  40  c.c.  of  this  iodine  solution  in  a  100  c.c.  measuring 
flask,  added  20  c.c.  of  a  three-tenths  per  cent,  solution  of  atropine 
alkaloid,  and,  after  shaking  well,  diluted  the  liquid  in  the  flask  to 
100  c.c.  We  now  took  two  vials  and  in  each  filtered  off  25  c.c. 
from  the  flask.  In  the  first  vial  the  amount  of  free  iodine  titrated 
back  with  sodium  thiosulphate  was  found  to  be  0-0465.  From  this 
it  is  easy  to  deduce  that  the  20  c.c.  of  the  three-tenths  per  cent, 
atropine  solution  consumed  0-214  gramme  of  the  free  iodine.  In 
the  second  vial  the  total  iodine  was  determined  in  the  same  way  as 
described  above  and  the  quantity  of  silver  iodide  was  found  to  be 
0-3023  gramme,  which  shows  that  after  the  treatment  with  atropine 
the  liquid  contained  1-633  Per  cent,  total  iodine.  From  these  data 
we  deduce  that  the  20  c.c.  of  the  atropine  solution  have  taken  up 
0-02528  gramme  iodine  from  the  bound  iodine.  As  the  20  c.c.  of 
atropine  solution  contained  0  06  gramme  atropine  we  find  for  100 
parts  of  precipitated  enneaiodide 
Found.  Calculated. 
Per  cent.  Per  cent. 
Atropine                                                     20*05  20-26 
Iodine  taken  from  the  free  iodine  of  the 
iodine  solution  7i'5Q  70*88 
Iodine  from  the  iodine  bound  as  KI    .  .  .   8-45  8*86 
We  see  that  in  aqueous  acidulous  solutions  the  potassium  iodide 
takes  an  active  part  in  the  reaction  by  joining  in  a  yield  of  hydriodic 
acid  for  the  normal  hydriodide  of  the  alkaloid,  necessary  to  hold  the 
additive  iodine  of  the  periodide.  When  carbon  disulphide  or  carbon 
tetrachloride  is  used  as  solvent  for  the  atropine  and  the  iodine,  no 
periodide  seems  to  be  formed,  possibly  for  the  reason  of  there  being 
