66 
The  Purity  Rubric  and  US.P.  Tests. 
i  Am.  Jour.  Pharm. 
\  February,  1910. 
would,  of  course,  obviate  this  difficulty.  Although  no  determination 
of  the  potassium  iodide  in  tincture  of  iodine  is  prescribed  by  the 
Pharmacopoeia,  a  definite  amount  of  this  ingredient  is  specified  by 
the  formula.  It  therefore  appears  that  a  quantitative  test  for 
potassium  iodide  might  well  be  included.  A  satisfactory  method 
for  this  determination  is  as  follows :  25  c.c.  of  the  sample  are 
evaporated  to  dryness  in  a  dish  and  the  free  iodine  removed  by 
volatilization.  The  residue  after  being  heated  to  dull  redness  is 
dissolved  in  water,  the  solution  filtered  and  diluted  to  100  c.c. ; 
25  c.c.  of  the  latter  are  then  placed  in  a  glass  stoppered  bottle  with 
50  c.c.  of  cone.  HC1  and  10  c.c.  of  chloroform  and  titrated  to  dis- 
appearance of  pink  color  of  the  chloroform  with  0.1  N  KI03  solu- 
tion. Several  determinations  upon  two  samples  of  tincture  of  iodine 
gave  the  following  results : 
No.       used       empty      and. residue      .Residue      residue    Gm.  KI  tinctures 
234  25  31.8809  33.I  l60  I.2351  18.6  =  0.3089  4.94 
229  25  31.8915  32.9640  I.0725  16.0  =  0.2656  4.25 
229        25        3I-8855        32.9747        I.0892        16.3=0.2706  4.32 
The  preceding  results  indicate  that  this  iodate  titration  method 
can  probably  be  applied  with  success  to  many  of  the  iodine  com- 
pounds of  the  Pharmacopoeia  for  which  quantitative  methods  are 
at  present  not  given.  It  may  also  be  expected  that  the  determination 
of  mercury  in  many  of  its  salts  can  be  satisfactorily  accomplished 
by  a  modification  for  the  method  described  above  for  red  mercuric 
iodide. 
Lead  Acetate. — The  purity  rubric  of  the  U.S. P.  requires  that 
this  product  should  contain  not  less  than  99.5  per  cent,  of 
Pb(CH3COO)2  +  3H20,  since,  however,  no  quantitative  method  is 
prescribed  by  which  this  limit  of  purity  may  be  determined,  the  fol- 
lowing plan  was  applied  to  two  samples  of  the  salt  with  satisfactory 
results :  A  weighed  sample  was  dissolved  in  about  50  c.c.  of  water 
and  a  slight  excess  of  dilute  H2S04  added.  The  precipitated  lead 
sulphate  was  filtered  on  a  weighed  Gooch  crucible,  heated  gently 
over  a  Bunsen  flame,  cooled,  and  weighed.  The  results  were  as 
follows : 
Wt.  of  Platinum  Dish 
C.C.0.1KIO3 
for  y± 
Gm.  KI 
per  100  c.c. 
Sample  C.c. 
Wt.  of  Gooch  crucible 
Sample  Gms. 
No.  used 
207  0.5 
226  0.5 
7-1533 
8.3308 
Alone 
+PbS04 
7.5706 
8.7368 
Wt.  of       Calc.  % 
PbS04  Pb(CH3COO)o+3HoO 
O.4173  IO4.3 
0.4060      10 1. 6 
