AmMaynif97arm*}     Valuation  of  Liquor  Iodi  Compositus.  243 
Being  desirous  of  knowing  to  what  degree  the  retail  dispensers 
were  governed  by  the  Pharmacopceial  requirements  for  liquor  iodi 
compositus,  I  obtained  a  few  samples  at  different  pharmacies  and 
estimated  the  per  cent,  of  iodine  by  the  official  method.  The 
results  of  my  titrations  indicated  the  following  percentages  for  the 
samples:  4-96,  4-82,  4-72  and  4-17. 
As  there  is  no  test  given  under  Lugol's  solution  for  estimating  the 
potassium  iodide  present,  I  have  made  some  experiments  for  the  pur- 
pose of  devising  one.  The  following  was  found  to  be  the  best  of  sev- 
eral methods  tried,  and  can  be  recommended  on  the  concordant 
results  which  it  furnished  : 
Take  a  definite  amount  (12-66  grammes)  of  the  solution  and 
titrate  it  according  to  the  official  method  of  estimating  the  iodine. 
The  amount  of  the  latter  is  thus  obtained.  Now  titrate  the  residual 
liquid  with  decinormal  silver  nitrate  volumetric  solution,  using  potas- 
sium chromate  as  an  indicator  if  desired,  until  all  of  the  iodides 
which  the  solution  contains  have  reacted  with  the  silver  nitrate  and 
formed  insoluble  silver  iodide. 
The  iodides  of  the  solution  consist  of  the  potassium  iodide 
originally  present  and  the  sodium  iodide  produced  in  the  reaction 
between  the  sodium  thiosulphate  and  the  free  iodine  of  the  sample. 
The  volume  of  the  solution  of  sodium  hyposulphite  is  the  measure 
of  the  free  iodine  of  the  sample,  and  therefore  the  equivalent  of  the 
volume  of  silver  nitrate  required  to  react  with  the  sodium  iodide 
which  it  forms.  Hence,  if  the  volume  of  sodium  hyposulphite 
required  to  decolorize  the  iodine  of  the  sample  be  deducted  from 
the  volume  of  silver  nitrate  required  to  completely  precipitate  the 
decolorized  liquid,  the  remainder  will  be  the  volume  of  decinormal 
silver  nitrate  volumetric  solution  required  for  the  potassium  iodide 
that  was  present.  Multiply  the  number  of  cubic  centimeters  so 
found  by  0  0 1 65 56,  the  value  of  1  c.c.  of  the  silver  nitrate  solution  in 
potassium  iodide,  to  find  the  amount  of  potassium  iodide.  Then 
by  proportion  calculate  the  percentage  amount  of  it. 
It  was  also  found  that  practical  results  could  be  gotten  by  boiling 
the  Lugol's  solution  after  dilution  with  water  until  all  the  free 
iodine  was  expelled,  and  then  titrating  with  standard  silver  nitrate 
solution,  which  indicated  at  once  the  amount  of  potassium  iodide 
present. 
