Am.  jour.  Pharm.)  jlie  purity  Rubric  and  US.P.  Tests. 
February,  1910.    j  J 
65 
with  about  50  c.c.  of  H20,  5  Gms.  KOH,  and  10  to  20  c.c.  of  3 
per  cent,  solution  of.  hydrogen  dioxide;  the  solution  warmed  gently 
until  the  decomposition  of  the  mercuric  iodide  is  complete.1  The 
gray  mercury  residue  is  filtered  on  a  weighed  porcelain  Gooch 
crucible,  washed  with  cold  H20,  dried  at  about  6o°  and  weighed. 
Hg  X  2.2692  =  Hgl2.  For  the  iodide,  the  filtrate  from  the  mercury 
is  evaporated  to  about  50  c.c,  cooled  and  filtered  into  a  glass 
stoppered  bottle;  about  twice  its  volume  of  concentrated  HC1  and 
10  c.c.  of  chloroform  are  added.  The  mixture  is  then  titrated  to 
disappearance  of  the  pink  color  of  the  chloroform  wth  0.1  N  KIOs 
solution.2  The  potassium  iodate  solution  may  be  conveniently 
standardized  against  pure  potassium  iodide.  Duplicate  determina- 
tions upon  a  sample  of  red  mercuric  iodide  gave  the  following 
results : 
Wt.  of  Gooch  Crucible  Calc.  %   C.c.  o.i  N  Calc.  % 
Sample  ist.  2nd.         Gm.  Hg.     Hgl2.     KIO3  Sol.  Hgl2. 
[a]  i.oGm.       8.1786       8.6152      0.4366       09.1       43.7  99-2 
[b]  I.oGm.       7.1758       7.6143       0.4385       99.5       437  99-2 
The  process  as  here  outlined  is  rapid  enough  for  all  ordinary 
purposes.  Only  one  weighed  portion  of  the  sample  is  required  for 
the  determination  of  both  constituents,  and  the  method  will  no 
doubt  yield  concordant  results  in  the  hands  of  different  analysts. 
Potassium  Iodide. — The  quantitative  method  given  by  the  Phar- 
macopoeia for  this  salt  is  the  usual  argentometric  titration  which,  of 
course,  does  not  differentiate  between  the  several  halogens.  The 
titration  with  standard  potassium  iodate  as  outlined  above  offers 
certain  advantages  over  the  use  of  silver  nitrate,  which  more  detailed 
experiments  will  no  doubt  demonstrate.  It  is  to  be  noted  in  con- 
nection with  the  iodate  titration  that  the  end  point  of  the  reaction 
is  to  be  taken  at  the  disappearance  of  the  pink  color  of  the  chloro- 
form indicator  without  reference  to  the  pale  yellow  color  of  the 
aqueous  layer  which  persists. 
Tincture  of  Iodine. — The  titration  of  a  5  c.c.  portion  of  tincture 
of  iodine  as  prescribed  by  the  Pharmacopoeia  gives  rise  to  somewhat 
variable  results  due  to  inaccuracies  in  correctly  measuring  this 
relatively  small  volume  of  the  sample.  The  quantity  of  iodine 
calculated  from  several  duplicate  titrations  may  vary  by  as  much 
as  0.3  Gm.    The  use  of  aliquot  portions  of  the  sample  after  dilution 
1  M.  Kohn,  Z.  anorg.  chem.,  59,  108-10;  Chem.  Abstracts,  2,  2911. 
2L.  W.  Andrews,  /.  Am.  Chem.  Soc,  25,  756,  1903. 
