u8 
Assay  of  Medicinal  Plasters. 
{Am.  Jour.  Pharm. 
March,  1910. 
STRENGTHENING  PLASTER. 
Assay  for  Iron. — Weigh  out  accurately  two  pieces  of  plaster  of 
about  5  Gm.  each,  cutting  them  side  by  side  from  the  same  piece 
in  order  that  the  relative  amount  of  compound  to  cloth  will  be  the 
same  in  each  piece. 
Dissolve  off  the  compound  from  one  of  the  pieces  with  repeated 
portions  of  chloroform  until  the  cloth  is  clean.  Allow  the  cloth  to 
dry  spontaneously  and  weigh  it.  Multiply  its  weight  by  100  and 
divide  by  the  total  weight  of  plaster  used.  This  gives  the  per  cent, 
of  cloth  in  the  plaster,  the  figure  so  obtained  to  be  used  later. 
The  second  piece  of  plaster  of  known  exact  weight  is  placed  in 
small  clippings  in  a  porcelain  crucible  and  ignited.    Dissolve  the 
residue  in  warm  concentrated  hydrochloric  acid,  dilute  to  about  200 
c.c.  with  water,  and  filter.    Pour  the  solution  into  a  porcelain  dish, 
heat  nearly  to  boiling,  and  add  ammonia  in  excess  to  precipitate 
the  iron  as  Fe2OH6.    Allow  the  precipitate  to  settle,  decant  on  to  a 
paper  filter  of  known  low  ash  value,  wash  the  precipitate  with  hot 
water,  and  dry.    This  is  now  ignited  in  a  weighed  porcelain  crucible 
with  lid,  the  iron  hydroxide  being  first  carefully  scraped  from  the 
filter  and  ignited  alone  in  the  crucible  and  the  paper  containing 
only  a  little  adherent  hydroxide  is  ignited  separately  on  the  lid 
of  the  crucible.    When  the  paper  is  completely  reduced  to  ashes, 
the  lid  is  placed  on  the  crucible  and  both  are  transferred  to  a  desic- 
cator to  cool.    When  cooled,  weigh  and  subtract  the  weight  of  the 
crucible  and  lid  from  the  weight  of  the  same  with  contents.  This 
gives  the  weight  of  iron  oxide  (Fe2Os)  contained  in  the  crucible. 
This  is  calculated  to  metallic  iron,  viz. : 
160:  ii2==wt.    Fe203:^r  wt.  Fe. 
The  percentage  of  cloth  in  the  plaster  has  been  determined  as 
above  in  other  piece  of  plaster,  therefore  the  weight  of  the  piece  of 
plaster  ignited  is  multiplied  by  the  per  cent,  figure  for  cloth,  deter- 
mined in  the  other  piece  of  plaster.  The  weight  so  found  is  sub- 
tracted from  the  weight  of  plaster  ignited,  which  result  is  the  weight 
of  compound  ignited. 
The  weight  of  Fe  found,  multiplied  by  100,  and  divided  by  the 
weight  of  compound  ignited  gives  the  per  cent,  of  Fe  in  the 
compound. 
To  Arthur  W.  Clark  and  Powell  Hampton  acknowledgment  is 
due  for  work  in  the  elaboration  of  these  processes. 
