5M 
Methods  for  Determination  of  Calomel. 
Am.  Jour.  Pliarm. 
November,  1914. 
of  calomel  received  for  analysis  and  the  limited  amount  of  time 
available.  A  method  was  devised,  which  was  afterward  found  to  be 
identical  with  the  method  recently  suggested  by  Kohn  and  Oster- 
setzer,4  which  seems  to  very  nearly  satisfy  the  conditions.  Kohn 
found  that  the  halogen  compounds  of  mercury  could  be  easily  oxi- 
dized and  made  soluble  by  means  of  sodium  peroxide,  and  the  halogen 
titrated  by  Volhard's  method  with  silver  nitrate.  This  method,  as  it 
has  been  used  for  the  results  below,  is  as  follows :  The  tablets,  in 
amount  corresponding  to  0.2  to  1.0  gm.  of  Calomel,  are  first  disinte- 
grated in  about  30  C.c.  of  water,  made  acid  with  nitric  acid  to  drive 
off  the  carbon  dioxide  from  the  sodium  bicarbonate,  which  is  often 
used  as  a  filler,  and  sodium  peroxide  added,  a  little  at  a  time,  with 
stirring,  until  the  gray  metallic  mercury  separates  out.  About  one 
gramme  of  sodium  peroxide  is  added  in  excess.  After  heating  for 
a  very  few  minutes,  the  precipitated  mercury  is  filtered  onto  a 
Gooch  crucible  and  washed  with  water.  The  filtrate  is  strongly 
acidified  with  nitric  acid,  tenth-normal  silver  nitrate  added  in  excess, 
and  the  solution  then  cooled  and  agitated,  causing  the  precipitate  to 
coagulate.5  The  excess  of  silver  nitrate  is  then  titrated  with  potas- 
sium sulphocyanate,  using  ferric  alum  as  an  indicator.  This  method 
is  rapid,  requiring  only  a  few  minutes  for  a  determination ;  it  is 
simple,  and  it  was  found  to  give  accurate  results  with  all  of  the 
samples  of  calomel  tablets  tried. 
The  first  table  below  gives  a  series  of  results  of  the  analyses  of 
known  amounts  of  pure,  carefully  dried  mercurous  chloride  by 
Kohn's  method,  using  certified  burettes  for  the  titrations. 
The  second  table  gives  the  results  with  mixtures  of  calomel  and 
sodium  carbonate.  This  will  give  an  idea  of  about  how  close  tablets 
consisting  of  these  constituents  could  be  determined  by  this  method. 
Table  I. 
No. 
Gms.  HgCl 
Cc.  N/10  AgN03 
Cc.  N/10  KCNS 
Gms.  HgCl  found 
Per  cent. 
I 
O.50OO 
25.OO 
3-83 
O.4986 
9972 
2 
O.50OO 
25.OO 
378- 
O.4998 
99.98 
3 
O.50OO 
25.OO 
3.83 
O.4986 
9972 
4 
O.50OO 
25.OO 
3.80 
O.4993 
99-86 
5 
O.50OO 
25.OO 
3.83 
O.4986 
99.72 
Average  per  cent 
99.80 
4  Zeits.  anorg.  Chem.,  80,  218. 
6  Rathmund  and  Burgstaller,  Zeits.  anorg.  Chem.,  63,  330  (1909). 
