n6  Assay  of  Medicinal  Plasters.  j 
Am.  Jour.  Pharm. 
March,  1910. 
contains  the  same  amount  of  salicylic  acid  as  was  used  in  tube  No.  i. 
That  quantity  is  already  determined  by  means  of  the  standard 
solution.  Therefore  250  (total  amount  of  plaster  solution)  divided 
by  the  number  of  c.c.  plaster  solution  used  in  tube  No.  2  and  the 
result  multiplied  by  grammes  acid  found  to  have  been  used  in  tube 
No.  1  gives  the  weight  of  acid  in  the  plaster  compound.  Multiply 
the  weight  of  the  salicylic  acid  found  by  100  and  divide  the  result 
by  the  weight  of  the  plaster  compound  used.  The  result  is  the  per 
cent,  of  salicylic  acid  present. 
This  method  has  been  used  chiefly  with  plasters  containing  20, 
25,  and  30  per  cent,  of  salicylic  acid.  When  very  small  percentages 
are  present  it  will,  of  course,  necessitate  the  addition  of  a  much 
larger  quantity  of  plaster  solution  to  tube  No.  2  in  order  to  secure 
the  same  color  as  that  of  No.  1  tube  and  consequently  the  color  of 
tube  No.  2  will  be  unduly  diluted.  If  by  trial  this  is  found  to  be 
the  case,  it  will  be  necessary  to  prepare  a  weaker  standard  solution 
or  else  a  more  concentrated  plaster  solution.  This  matter  can 
readily  be  determined  by  trial. 
In  making  these  assays  it  is  well  to  make  three  of  each  and  to 
take  the  average.  By  doing  this  we  have  been  able  to  get  satisfac- 
tory results,  the  natural  variations  of  color  judgment  balancing  one 
another  and  the  result  being  nearly  correct  in  the  average  of  the 
three  determinations  from  one  lot  of  plaster  solution. 
MERCURIAL  PLASTER. 
The  requirement  of  the  U.S. P.  is  that  this  plaster  shall  contain 
30  per  cent,  metallic  mercury. 
Method  of  Assay. — Weigh  out  about  5  Gm.  of  the  plaster,  cut 
it  into  rather  small  strips.  Place  these  in  a  beaker  and  add  about 
50  c.c.  benzol.  Stir  well  for  some  time  to  soften  and  dissolve 
the  compound.  Pour  the  solution  oft  into  a  beaker  of  about  200  c.c. 
capacity,  allowing  the  cloth  to  remain  in  the  first  beaker.  To  the 
cloth  in  the  first  beaker  add  repeated  portions  of  about  50  c.c. 
benzol,  stirring  well  and  pouring  oft  each  time  into  the  second  beaker 
until  the  mixed  solutions  amount  to  about  200  c.c.  The  clean 
cloth  is  now  allowed  to  dry  in  the  air,  weighed,  and  its  weight  sub- 
tracted from  the  weight  of  the  plaster  taken.  This  gives  the  weight 
of  the  compound  used. 
The  mixed  solutions  of  the  compound,  measuring  about  200  c.c, 
are  now  well  stirred  and  th«n  allowed  to  stand  covered  in  a  tall 
