476  Volumetric  Determination  of  Mercury .{^m^t;S^em 
VOLUMETRIC  DETERMINATION  OF  MERCURY. 
By  Rod.  Namias. 
The  volumetric  process  to  be  explained  is  applicable  only  to  mer- 
curic chloride,  into  which  every  other  compound  must  be  previously 
converted.  The  nitrate  must  be  treated  with  hydrochloric  acid  in 
excess,  and  evaporated  to  dryness  ;  the  mercurous  salts  are  sub- 
mitted to  evaporation  in  presence  of  hydrochloric  acid  and  of  potas- 
sium chlorate. 
The  evaporation  must  be  conducted  with  care  at  a  temperature 
below  that  of  ebullition,  so  as  to  avoid  any  loss  of  mercuric  chloride 
by  volatilization. 
The  process  depends  upon  the  following  principle  :  If  to  a  solu- 
tion of  mercuric  chloride,  slightly  acidified  with  hydrochloric  acid, 
we  add  a  solution  of  stannous  chloride,  the  mercuric  chloride  is 
reduced  first  to  the  mercurous  state  and  then  to  metallic  mercury. 
But  whilst  the  first  reaction  is  nearly  instantaneous,  the  second  takes 
a  longer  or  shorter  time,  so  much  the  longer  as  the  excess  of  the 
stannous  chloride  is  less. 
If  we  have  a  'reagent  by  which  we  can  recognize  the  moment 
when  the  stannous  chloride  is  in  excess  for  the  first  reaction,  and 
begins  to  produce  the  second,  we  may  by  this  means  determine  the 
mercury  volumetrically. 
The  reagent  for  detecting  the  presence  of  stannous  chloride  in 
excess  is  sodium  molybdate.  I  dissolve  a  small  quantity  of  molyb- 
dic  anhydride  in  a  solution  of  sodium  hydrate  or  carbonate  ;  I  steep 
in  this  liquid  a  morsel  of  filter-paper,  which  I  spread  out  whilst  wet 
upon  a  plate  of  porcelain.  The  solution  of  molybdate  ought  to  be 
freshly  prepared,  and  the  paper  of  good  quality  ;  it  must  not  take 
a  yellow  tint  on  immersion  in  the  alkaline  liquid.  The  paper  must 
be  steeped  in  the  molybdate  only  a  little  time  before  the  experi- 
ment, so  that  it  may  not  have  time  to  dry. 
The  paper  thus  prepared  shows  a  relatively  slight  excess  of  stan- 
nous chloride,  which  communicates  to  it  according  to  its  quantity 
a  color  varying  from  the  lightest  sky-blue  to  an  intense  blue.  This 
color  is  due  to  the  reduction  of  the  molybdic  acid.  Ammonium 
molybdate,  less  stable  than  sodium  molybdate,  is  less  fit  for  use. 
The  determination  of  the  value  of  the  stannous  solution  may  be 
made  by  means  of  a  standard  solution  of  iodine  ;  but  it  is  preferably 
