Am.  Jour.  Pharm.  ) 
June  1,  1874.  / 
Analysis  of  Iodine. 
291 
it  is  impossible  to  weigh  it  upon  an  open  watch-glass  as  in  ordinary 
analyses.  Place  a  few  grammes  of  iodine  in  a  well-corked  specimen 
tube,  and  weigh.  Throw  a  few  decigrammes  into  the  sulphurous  acid 
solution  and  rapidly  close  the  tube.  The  difference  in  the  weight  of 
the  tube  gives  the  amount  of  iodine  taken  for  analysis. 
2.  Determination  of  the  Iodine. — Place  in  a  beaker  or  flask  of 
about  one  litre  capacity  40  c.  c.  of  concentrated  and  freshly  prepared 
solution  of  sulphurous  acid.  In  this  the  iodine  is  dissolved  by  stirring. 
If  an  appreciable  residue  remains,  this  is  filtered  off  and  weighed,  care 
being  taken  to  keep  the  funnel  covered  during  the  operation.  This, 
however,  is  seldom  necessary,  commercial  iodine  containing  generally 
but  small  traces  of  insoluble  substances. 
Pour  now  into  the  beaker  at  least  half  a  litre  of  boiling  water,  and 
an  excess  of  ammonia ;  and  then  add  nitrate  of  silver,  cover  the 
beaker,  and  allow  the  precipitate  to  stand  about  half  an  hour  in  a 
warm  place.  At  the  end  of  that  time  filter  through  Swedish  paper 
without  folds,  wash  by  decantation  with  boiling  water,  taking  care  to 
gather  the  precipitate  at  the  apex  of  the  funnel.  When  thoroughly 
washed,  giving  no  turbidity  with  hydrochloric  acid,  the  precipitate  is 
dried  in  an  air  bath  at  110°  C. 
Care  must  be  taken  prior  to  igniting  the  argentic  iodide  to  remove 
all  trace  of  carbon,  lest  the  precipitate  should  be  reduced  to  metallic 
silver.  Remove  the  iodide  from  the  filter  by  means  of  a  platinum 
spatula,  to  a  piece  of  glazed  paper,  scraping  the  paper  to  remove  the 
smallest  fragments  ;  in  spite  of  these  precautions  a  little  will  adhere 
to  the  apex  of  the  filter  ;  cut  this  off  and  calcine  it  in  a  weighed  por- 
celain capsule  of  about  12  mm.  diameter.  When  the  ash  is  perfectly 
white,  add  the  iodide,  and  heat  till  it  begins  to  fuse ;  cool  and  weigh. 
The  equivalent  of  iodide  of  silver  is  235,  and  of  silver  108,  so  that 
the  precipitate  contains  54  per  cent,  of  iodine. 
3.  Determination  of  the  Chlorine. — To  the  filtrate  from  the  last  de- 
termination add  an  excess  of  nitric  acid,  filter  and  weigh  the  result- 
ing  chloride  with  the  usual  precautions. 
4.  Determination  of  the  Ash. — Weigh  five  grammes  in  the  method 
already  described,  and  ignite  gently  in  a  porcelain  capsule  till  all  the 
iodine  has  volatilized  ;  the  residue  should  be  very  slight,  and  consists 
chiefly  of  silica  and  alumina,  with  trace  of  alkaline  chlorides. 
5.  Determination  of  the  Moisture. — The  quantity  of  moisture  in 
