Am.  Jour.  Pharm.  ) 
March,  1910.  J 
Assay  of  Medicinal  Plasters. 
"5 
it  with  20  cc.  alcohol  as  before,  pour  of!  and  mix  the  decanted  fluid 
with  the  other  two  portions  in  the  flask  which  now  contains  all  of  the 
salicylic  acid  in  solution.  Fill  the  flask  up  to  the  250  cc.  mark  with 
alcohol.  Remove  the  cloth,  which  should  now  be  white  and  clean, 
from  beaker  No.  1,  allow  it  to  dry  spontaneously,  and  weigh.  Sub- 
tract its  weight  from  the  total  weight  of  plaster  used,  thus  ascertain- 
ing the  weight  of  plaster  compound  taken  for  assay. 
Standard  Salicylic  Acid  -  Solution. — Weigh  out  exactly  0.5  Gm. 
pure  salicylic  acid  and  dissolve  it  in  50  per  cent,  alcohol.  Transfer 
to  a  500  cc.  glass  stoppered  graduated  flask,  rinse  out  the  vessel, 
in  which  the  acid  was  dissolved,  with  repeated  portions  of  50  per 
cent,  alcohol,  adding  each  portion  to  the  solution  in  the  flask.  Make 
up  to  the  500  cc  mark  with  50  per  cent,  alcohol,  shake  thoroughly. 
One  cc  of  this  solution  contains  .001  Gm.  salicylic  acid. 
Analysis  by  Color  Comparison. — For  this  work  we  use  two  large 
test-tubes  of  similar  internal  diameter  (1%  inches  by  6  inches  long). 
Any  pair  of  glass  cylinders  or  tubes  will  sufhce,  but  small  diameter 
test-tubes  do  not  give  sufficient  thickness  of  solution  to  secure  enough 
depth  of  color  with  transmitted  light.  Place  these  tubes  side  by  side. 
In  a  beaker  place  100  cc.  distilled  water  to  which  add  one  drop 
ferric  chloride  solution  U.S. P.  Stir  the  liquid  and  pour  into  each 
of  the  test-tubes  50  cc  of  it.  Designate  them  No.  1  and  No.  2. 
Now  add  to  No.  1  tube  from  burette,  sufficient  of  the  standard  sali- 
cylic acid  solution  to  give  a  strong  clean  wine  color.  Stir  with 
a  glass  rod  after  each  addition  of  the  acid  solution.  Multiply 
number  of  cc  used  by  .001,  which  will  give  the  weight  in  grammes 
of  salicylic  acid  used  in  tube  No.  i-^the  standard.  Now  add  to  tube 
No.  2  from  another  burette  sufficient  of  the  plaster  solution  exactly 
to  match  the  color  obtained  in  tube  No.  1.  This  plaster  solution 
must  be  added  a  little  at  a  time  and  the  solution  in  the  test-tube 
well  stirred  with  a  glass  rod  after  each  addition.  When  the  match- 
ing point  is  nearly  reached  it  may  be  necessary  to  filter  off  the 
contents  of  the  test-tube  No.  2.  Clean  the  tube  and  replace  the 
fluid,  proceeding  thereafter  to  add  the  plaster  solution  a  drop 
at  a  time.  The  reason  for  doing  this  is  that  the  small  amount 
of  resinous  matter  separated  from  the  solution  may  cloud  the  mix- 
ture in  No.  2  test-tube  and  interfere  with  the  color  judgment. 
By  closing  one  eye  and  observing  the  colors  while  holding  the  tubes 
side  by  side  between  the  eye  and  a  window  the  colors  can  be  matched 
very  closely.    It  is  obvious  that  the  quantity  of  plaster  solution  used 
