Am."  Jour.  f'Phann.  1 
March, 1910.  J 
Assay  of  Medicinal  Plasters. 
117 
beaker  until  all  the  gray  metallic  mercury  has  settled  to  the  bottom. 
This  usually  is  accomplished  in  about  twenty-four  hours.  Chloro- 
form can  be  substituted  for  benzol  in  this  dissolving  operation,  but 
benzol  is  used  in  our  work  on  account  of  its  relative  lightness,  be- 
cause of  which  the  mercury  settles  more  quickly. 
When  the  gray  mercury  has  settled  out  pour  off  carefully  the 
supernatant  benzol  which  contains  the  rubber  and  resins  in  solu- 
tion. On  account  of  the  high  gravity  of  the  mercury  the  benzol 
can  be  decanted  until  nothing  remains  but  a  slime  of  mercury  pow- 
der. To  this  add  at  once  one  or  two  c.c.  of  aqua  regia,  warm  it, 
stir,  and  let  stand.  If  all  gray  color  is  not  removed  in  about  an 
hour  add  another  c.c.  of  aqua  regia,  stir,  warm,  and  let  stand  again, 
repeating  the  operation,  if  necessary,  to  dissolve  all  of  the  mercury. 
Use,  however,  as  little  acid  as  possible  so  as  to  avoid  all  but  a  slight 
excess,  for  much  excess  will  interfere  with  the  subsequent  precipi- 
tation of  the  mercury  by  means  of  H2S.  When  the  acid  solution 
has  lost  all  gray  color,  indicating  the  complete  solution  of  the  mer- 
cury, add  about  50  c.c.  water.  Stir,  and  filter  through  a  paper  filter 
containing  a  tuft  of  absorbent  cotton.  The  cotton  catches  the  rloc- 
culent  particles  of  resinous  matter  and  prevents  stoppage  of  the 
filtration  by  clogging.  Rinse  out  the  beaker  with  repeated  portions 
of  water,  which  is  poured  through  the  filter.  Continue  until  about 
200  c.c.  of  filtrate  is  secured.  Place  this  filtrate  in  an  Erlenmeyer 
flask  of  ample  capacity,  pass  H2S  through  it  until  the  mercury  is  all 
precipitated  as  black  HgS.  Let  it  settle  a  few  minutes  and  filter 
at  once  through  a  weighed  filter  paper  of  fine  texture,  wash  the 
precipitate  on  the  filter  with  a  little  water  and  dry  at  100 0  C,  cool, 
and  weigh.  Subtract  the  weight  of  the  filter  paper  and  calculate 
the  resultant  weight  of  the  mercury  sulphide  to  mercury,  viz. : 
232  :20O  =  wt.    HgS  :  x  wt.  mercury. 
The  weight  of  mercury  found  multiplied  by  100  and  divided  by 
the  weight  of  the  compound  used  gives  the  per  cent,  of  mercury  in 
the  compound. 
AMMONIAC  AND  MERCURY  PLASTER. 
The  requirement  of  the  U.S. P.  is  that  this  plaster  shall  contain 
18  per  cent,  metallic  mercury. 
Method  of  Assay  for  Mercury. — This  is  the  same  as  with  mer- 
curial plaster. 
