Amsiptr,'i8?2arm  }   Volumetric  Determination  of  Mercury,  477 
effected  by  operating  upon  a  known  quantity  of  pure  mercuric 
chloride  obtained  by  sublimation. 
If  we  operate  with  iodine  we  obtain  a  slight  error  in  excess  due 
to  the  small  quantity  of  stannous  chloride,  which  must  be  used  in 
excess  to  mark  the  end  of  the  operation  by  spotting  upon  the 
molybdate  paper.  In  any  case  it  is  well  to  determine  the  standard 
of  the  stannous  solution  by  means  of  iodine  approximately,  if  not 
exactly  ;  this  serves  as  a  control,  and  facilitates  at  the  same  time 
the  exact  titration  by  means  of  mercuric  chloride. 
To  prepare  the  solution  of  stannous  chloride  I  dissolve  2  to  3 
gm.  tin  by  means  of  hydrochloric  acid,  and  dilute  it  to  I  litre. 
This  solution  is  preserved  in  an  apparatus  which  prevents  alteration 
by  contact  with  air.  I  determine  its  strength  as  follows  :  0-2  to 
04  gm.  of  mercuric  chloride  weighed  exactly  are  dissolved  in  50 
cc.  of  distilled  water,  acidulated  with  >£.cc.  of  concentrated  hydro- 
chloric acid.  Into  this  cold  solution  the  stannous  solution  is 
allowed  to  flow  from  a  burette.  If  the  determination  of  the  standard 
has  been  previously  made  with  iodine,  we  may  pour  in  at  once, 
without  fear  of  over-stepping  the  limit,  the  volume  as  determined 
by  calculation  from  the  titration  with  iodine ;  we  then  add  the 
liquid  drop  by  drop,  stirring  carefully,  and  each  time  putting  a  drop 
of  the  mixture  with  the  stirring-rod  upon  the  molybdate  paper. 
An  excess  of  a  few  drops  of  stannous  liquid  turns  the  paper  to 
a  pale  blue,  which  becomes  manifest  after  a  few  seconds,  and  which 
an  experienced  eye  easily  recognizes. 
To  obtain  a  well-marked  coloration,  it  is  necessary  to  use  an 
excess  of  from  0  3  to  0-5  cc.  for  50  cc.  of  liquid.  The  small  quan- 
tity of  mercurous  chloride  placed  upon  the  molybdate  paper  whilst 
"  spotting  "  has  no  injurious  influence  upon  the  result. 
The  proportion  of  free  hydrochloric  acid  contained  in  the  liquid 
should  be  about  0-5  cc.  to  50  cc.  of  liquid  ;  a  larger  proportion  may 
occasion  errors  from  its  action  upon  the  molybdic  acid  in  presence 
of  filter-paper. 
To  obtain  exact  results  we  must  operate  each  time  exactly  as 
when  standardizing  the  liquid.  We  must,  as  far  as  possible,  oper- 
ate  upon  the  same  volume  of  liquid,  in  presence  of  the  same  pro- 
portion of  hydrochloric  acid,  and  obtain  spots  of  the  same  intensity 
upon  the  molybdate  paper. — Revue  Universelle  ties  Mines ;  Client. 
News,  Aug.  19,  p.  90. 
