Si2  Methods  for  Determination  of  Calomel.  4Am-  J(T-  p£Sf?1' 
°  1  J  \   November,  1914. 
method  it  is  necessary  to  oxidize  the  mercury  in  order  to  get  it  into 
solution,  and  for  this  oxidation  strong  acids,  aqua  regia,  free  chlorine, 
or  some  other  suitable  reagents  are  commonly  used.  When  the 
mercury  is  completely  oxidized  the  strong  acids  are  nearly  neutralized 
and  the  clear  solution  treated  with  hydrogen  sulphide.  The  mer- 
curic sulphide  thus  precipitated  always  contains  free  sulphur,  and 
this  must  be  removed  before  the  sulphide  can  be  dried  and  weighed. 
The  precipitate  is  therefore  washed  thoroughly  with  dilute  hydrogen 
sulphide  water  and  finally  washed  repeatedly  with  carbon  disulphide 
to  remove  the  free  sulphur. 
There  are  at  least  three  objectionable  points  in  this  method.  First, 
the  length  of  time  required  for  a  determination  (which  depends,  of 
course,  upon  the  modification  used)  is  much  too  long  for  the  average 
analyst.  Second,  the  difficulty  of  obtaining  complete  oxidation  is 
very  great.  Certain  fillers  seem  to  add  to  this  difficulty,  and  talc, 
if  it  is  present,  remains  suspended  in  the  solution.  In  such  a  case 
there  is  no  way  of  determining  when  the  reaction  is  complete.  The 
use  of  strong  acids  for  the  oxidation  is  inadvisable  because  of  the 
difficulty  in  removing  the  excess  of  acids  before  the  precipitation  of 
the  sulphide,  and,  according  to  Treadwell,  the  nitric  acids  in  the 
aqua  regia,  if  this  is  used,  can  not  be  removed  by  evaporation  be- 
cause of  the  volatility  of  the  mercuric  chloride.  It  was  found  that 
oxidation  with  free  chlorine  in  alkaline  solution  is  the  most  successful, 
as  the  proper  degree  of  acidity  for  complete  precipitation  with 
hydrogen  sulphide  is  more  easily  attainable  here  than  in  the  methods 
employing  concentrated  acids.  Third,  although  a  number  of  modifi- 
cations of  this  method  have  been  suggested  for  the  extraction  of  the 
free  sulphur  with  carbon  disulphide,  none  of  these  have  been  found 
to  give  very  satisfactory  results.  It  is  apparent  that  it  is  very 
difficult  to  wash  out  occluded  sulphur  from  precipitates  unless  they 
are  finely  divided.  Mercuric  sulphide  precipitates,  under  ordinary 
circumstances,  in  clots  which  can  not  be  easily  disintegrated,  and 
considerable  experience  has  shown  that  extraction  with  the  Soxhlet 
apparatus  for  many  hours  does  not  remove  the  sulphur  so  that  the 
results  obtained  could  be  looked  upon  with  confidence. 
According  to  Merrill,2  mercury  can  be  determined  by  the  vola- 
tilization of  the  mercurous  chloride.  In  this  method  the  procedure 
is,  roughly,  to  triturate  the  tablets  in  warm  water  and  transfer  the 
2  E.  C.  Merrill,  Bureau  of  Chemistry,  Washington,  D.  C. 
