272    Analysis  of  Commercial  Samples  of  Iodine.  { A  jUn°eDi,"  im*" 
is  detached  with  a  small  platinum  spatula,  and  the  paper  carefully 
scraped.  Still  a  little  iodide  of  silver  remains  on  the  lower  part  of 
the  filter.  This  portion  is  cut  out  with  scissors,  and  ignited  in  a  small 
porcelain  capsule  of  about  12  millimetres  diameter,  the  weight  of 
which  must  previously  be  carefully  determined.  When  the  filter  is  , 
burnt  and  the  ash  is  perfectly  white,  the  iodide  of  silver  is  thrown 
into  the  capsule  and  heated  till  it  begins  to  fuse.  It  is  then  cooled 
and  weighed.  The  excess  of  weight  gives  the  iodide  of  silver,  of  which 
54  per  cent,  is  iodine. 
Determination  of  Chlorine. — The  mother  liquor,  decanted  from  the 
iodide  of  silver,  contains  all  the  chlorine  held  in  solution  by  the  am- 
monia. It  is  mixed  with  pure  nitric  acid  in  excess,  filtered  and 
weighed  in  the  usual  manner. 
Ash. — Weigh  out  about  five  grammes  of  the  sample  of  iodine  by 
means  of  the  tube,  as  described  above.  Put  it  in  a  small  porcelain 
capsule  and  volatilize  it  by  exposure  to  a  moderate  heat.  The  residue 
is  then  weighed.  It  is  generally  very  small,  and  consists  of  silica^ 
alumina  and  traces  of  alkaline  chlorides. 
Moisture. — This  may  amount  to  20  per  cent.,  and  even  upwards.. 
It  is  generally  determined  as  difference,  as  the  moisture  cannot  be 
driven  off  by  heat  without  at  the  same  time  volatilizing  the  iodine  also. 
The  following  method  may  be  adopted,  which,  though  not  absolutely 
accurate,  is  useful  as  a  check.  Weigh  out  1  gramme  of  the  iodine,, 
and  put  it  in  a  glass  tube  of  narrow  bore,  graduated  to  tenths  of  cubic 
centimetres.  Pour  into  the  tube  20  cubic  centimetres  of  the  bisul- 
phide of  carbon,  which  will  of  course  occupy  200  of  the  divisions. 
Shake  the  tube  until  all  the  iodine  is  dissolved,  keeping  the  aperture 
closed  with  the  finger.  Then  let  it  stand  two  or  three  hours,  well 
corked.  The  water  present  in  the  sample  separates  out  and  floats 
above  the  bisulphide  of  carbon  as  a  slightly  yellow  liquid.  If  it  oc- 
cupies the  space  between  two  divisions  of  the  tube  it  is  1-10  of  a  cubic 
centimetre  in  bulk,  and  weighs  consequently  1  decigramme.  The 
iodine  therefore  in  this  case,  if  exactly  1  gramme  was  operated  upon, 
contains  10  per  cent,  of  water.  A  fair  average  sample  of  commercial 
iodine  contains  about : — 
Iodine,  88-61 
Chlorine,  ......  0*52 
Ash,  0-72 
Water,  1045 
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