"^""'AS'ian*'""  }  Quantitative  Methods.  187 
Method  No.  2. 
Suggested  by  Mr.  L.  A.  Brown. 
(Merck's  Report,  1908,  page  57.) 
Applicable  to  mercuric  chloride,  iodide,  cyanide,  nitrate,  oxide, 
ammoniated  mercury,  metallic  mercury,  and  preparations  of  mercury 
such  as  ointments  of  mercury,  ammoniated  mercury,  nitrate,  and 
oxide ;  solution  of  mercuric  nitrate,  Donovan's  solution,  plaster  of 
mercury,  etc. 
Dissolve  one  gramme  of  the  sample  {e.g.,  HgCls)  using  one  or 
two  grammes  of  potassium  iodide  if  necessary,  in  sufficient  water 
to  make  100  c.c.  of  solution. 
Take  20  c.c.  aliquot,  add  i  gramme  KI,  5  to  10  c.c.  of  10 
per  cent.  KOH  sol.,  and  10  c.c.  of  water  containing  2  or  3  c.c. 
of  formaldehyde  solution.  Mix  thoroughly  and  place  on  water-bath 
for  about  10  minutes,  or  until  supernatant  liquid  settles  clear ;  then 
decant  off  through  a  small  filter,  washing  residue  with  two  portions 
or  more  of  water,  decanting  through  filter  as  before. 
Dissolve  the  small  amount  of  metallic  mercury  off  the  filter  by 
means  of  a  few  drops  of  hot  diluted  nitric  acid  ( i  :  i ) ,  washing 
the  filter  with  a  few  c.c.  of  water  to  remove  all  traces  of  mercury. 
Collect  filtrate  and  washings  in  the  beaker  containing  the  Hg, 
adding  more  nitric  acid  if  necessary  to  secure  solution  of  the  mer- 
cury. Evaporate  to  about  2  or  3  c.c.  on  a  water-bath,  then  dilute 
with  water  and  transfer  to  a  100  c.c.  flask,  rinsing  out  beaker 
with  successive  amounts  of  water  sufficient  to  make  100  c.c.  of 
the  solution. 
Take  an  aliquot  representing  about  o.i  gramme  of  HgCls,  add 
25  c.c.  of  water,  then  a  slight  excess  of  5  per  cent,  iodic  acid  solution, 
5  c.c.  being  enough.  This  is  added  drop  by  drop,  agitating  all 
the  while  to  secure  complete  agglutination  of  the  curdy  precipitate. 
As  soon  as  the  supernatant  Hquid  is  clear,  filter  and  wash  precipitate 
with  three  or  four  portions  of  water. 
Dissolve  precipitate  off  the  filter  with  a  few  drops  of  diluted 
HQ,  wash  filter  thoroughly,  add  i  or  2  grammes  of  KI,  allow  to 
stand  for  about  5  minutes,  then  titrate  the  liberated  iodine  with 
N/to  Na,S,0,. 
Each  c.c.  of  N/to  Thiosulphate  =  0.0022405  gramme  HgCU. 
In  the  case  of  ointments,  such  as  official  ointments  of  mercury 
