AMTunei5T872RM'}  Analysis  of  Commercial  Samples  of  Iodine.  271 
solution  of  the  nitrate  of  silver  in  presence  of  an  excess  of  ammonia 
to  keep  chloride  of  silver  from  being  thrown  down.  All  this  is  ex- 
ceedingly simple  in  theory,  but  it  requires  a  number  of  minute  precau- 
tions for  its  successful  execution. 
1.  Weighing. — Iodine  cannot  be  weighed  in  an  open  capsule,  since 
it  evaporates  so  rapidly  that  the  loss  of  weight  would  be  appreciable. 
A  quantity  is  therefore  placed  in  a  small  tube  closed  at  one  end  and 
capable  of  being  stoppered  with  a  cork  at  the  other.  This  is  then 
carefully  weighed.  The  tube  is  then  rapidly  opened,  and  a  portion 
of  the  contents  shaken  into  the  solution  of  sulphurous  acid.  The 
cork  is  then  quickly  re-inserted  and  the  tube  re-weighed.  The  differ- 
ence between  the  first  and  second  weighing  shows  the  quantity  of  the 
sample  actually  taken  for  analysis. 
2.  Determination. — Prepare  beforehand  a  large  glass  capable  of 
holding  a  litre.  Pour  into  it  40  cubic  centimetres  of  a  solution  of  sul- 
phurous acid,  concentrated  and  recently  prepared.  When  the  iodine 
has  been  thrown  in,  it  is  stirred  with  a  glass  rod  till  entirely  dissolved. 
Should  there  remain  an  appreciable  residue  of  insoluble  matter,  it  be- 
comes needful  to  filcer  the  solution.  This  is  performed  by  means  of  a 
funnel  fitting  into  a  flat-bottomed  phial.  The  funnel  should  be  cov- 
ered with  a  plate  of  glass  during  this  process,  which,  however,  is  not 
generally  necessary.  Pour  into  the  glass  at  least  half  a  litre  of  boil- 
ing distilled  water.  Then  add  ammonia  in  excess,  and  lastly  a  solu- 
tion of  the  nitrate  of  silver.  Iodide  of  silver  is  formed,  and  falls 
down  as  a  yellowish  precipitate,  whilst  chloride  of  silver  remains  in 
solution.  The  precipitate,  on  stirring,  collects  at  the  bottom  of  the 
glass  when  the  liquid  is  hot  enough.  The  beaker  is  then  covered 
over  with  a  plate  of  glass  and  set  aside  for  half  an  hour.  The  pre- 
cipitate is  then  washed  by  decantation,  with  abundance  of  hot  water, 
the  liquid  being  allowed  to  pass  through  a  small  filter  of  the  best 
Swedish  paper,  without  folds.  It  is  then  thrown  upon  the  filter,  and 
collected  as  far  as  possible  at  the  bottom.  When  the  precipitate  is 
perfectly  washed,  i.  e.,  when  a  drop  of  the  liquid,  on  being  tested  with 
hydrochloric  acid,  is  found  to  contain  no  silver,  the  filter  is  taken  out 
of  the  funnel  and  carefully  dried  at  110°  C.  Before  weighing  it  is 
necessary  to  fuse  the  precipitate,  but  it  is  also  necessary  to  avoid 
heating  it  in  contact  with  the  carbon  of  the  filter,  which  might  reduce 
an  appreciable  quantity  of  silver.  When  the  filter,  therefore,  is  dry, 
it  is  laid  on  a  sheet  of  glazed  paper,  the  precipitate  of  iodide  of  silver 
