ii4  Assay  of  Medicinal  Plasters.  ■[ 
Am.  Jour.  Pharm. 
March,  1910. 
of  alkaloid  required  in  the  plaster,  and  by  this  method  of  plaster 
assay  the  results  have  shown  that  in  mixing  several  hundred  pounds 
the  assayed  plaster  corresponds  with  the  calculated  amount,  often 
to  the  second  and  third  decimal. 
,  SALICYLIC  ACID  PLASTER. 
The  method  which  we  use  for  the  analysis  of  salicylic  acid  plas- 
ter is  a  colorimetric  method  with  ferric  chloride.  The  depth  of 
color  imparted  to  a  ferric  chloride  solution  by  a  measured  amount  of 
a  solution  of  the  plaster  is  compared  with  that  of  a  measured 
quantity  of  a -standard  solution  of  salicylic  acid  of  known  strength. 
Colorimetric  methods  are  always  subject  to  some  variation,  chiefly 
on  account  of  errors  of  eyesight ;  but  because  of  the  presence  of 
resinous  materials  in  rubber  plaster  masses,  no  other  method  has 
been  found  to  be  satisfactory  and  we  have  found  that  in  our  hands, 
by  careful  manipulation,  this  color  comparison  method  has  given 
sufficiently  satisfactory  results. 
Plaster  Solution. — Weigh  out  accurately  about  5  Gm.  of  the 
plaster  cut  into  rather  small  strips.  Place  on  the  table  two  beakers 
of  about  1 50  c.c.  capacity  each.  Designate  them  as  No.  1  and  No.  2. 
Place  the  weighed  strips  of  plaster  in  No.  1.  Add  to  it  50  c.c.  of 
chloroform.  Stir  gently  until  all  the  compound  is  removed  from  the 
plaster-cloth  and  in  solution.  Pour  the  syrupy  solution  into  beaker 
No.  2.  Add  to  this  in  No.  2  beaker,  40  c.c.  ordinary  94  per  cent, 
alcohol,  stir  thoroughly  to  precipitate  and  coagulate  the  rubber  and 
allow  it  to  stand.  Pour  off  all  possible  of  the  liquid  into  a  glass 
stoppered  graduate  250  c.c.  flask.  The  rubber  should  be  worked 
up  into  a  compact  mass  so  that  no  particles  are  carried  over  when 
the  liquid  is  poured  off  and  all  possible  liquid  should  be  pressed  out 
of  the  mass  with  a  glass  rod.  To  the  plaster-cloth  in  beaker  No.  1 
add  25  c.c.  chloroform.  Stir  carefully  and  thoroughly  until  all  the 
remaining  plaster-mass  is  dissolved  from  the  cloth  and  sides  of  the 
beaker.  Again  pour  off  the  solution  into  beaker  No.  2,  which  con- 
tains the  precipitated  rubber.  Work  up  with  glass  rod  until  all  of 
the  rubber  mass  is  again  in  solution  in  the  chloroform.  Now  re- 
precipitate  the  rubber  from  this  solution  with  20  c.c.  alcohol,  work- 
ing up  with  rod  and  pouring  off  as  before,  mixing  the  fluid  with 
the  first  portion  in  the  flask.  Once  again  wash  the  cloth  in  beaker 
No.  1  with  25  c.c.  chloroform.  Pour  off  into  No.  2  beaker,  dissolv- 
ing again  the  rubber  mass  in  it.    Re-precipitate  the  rubber  from 
