PREPARATION  OF  IODIDE  OF  POTASSIUM. 
437 
ON  THE  PREPARATION  OF  IODIDE  OF  POTASSHM,  AND 
SOME  OTHER  IODIDES. 
By  W.  Stevens  Squire,  PH.D.,  F.C.S. 
I  propose  to  bring  under  your  notice  this  evening  a  beautiful 
process  for  the  preparation  of  iodide  of  potassium,  which  has 
lately  been  devised  by  BaronjLiebig,  and  I  shall  take  this  oppor- 
tunity of  making  some  practical  remarks  upon  this  process,  and 
on  some,  other  points  connected  with  the  iodides  employed  in 
medicine. 
Of  the  many  processes  which  have  been  recommended  for  the 
preparation  of  iodide  of  potassium,  I  believe  only  three  have 
been  found  at  all  practical: — 1.  A  solution  of  iodide  of  iron 
or  zinc  is  decomposed  by  a  solution  of  carbonate  of  potassium. 
2.  Iodine  is  added  to  a  warm  solution  of  hydrate  of  potassium  ; 
the  color  disappears  with  a  formation  of  iodide  and  iodate  of 
potassium,  the  solution  is  evaporated  to  dryness,  and  heated  with 
charcoal,  which  reduces  the  iodate  to  iodide.  3.  Iodine  is 
added  to  a  solution  of  sulphide  of  barium ;  in  this  case  sulphur 
is  precipitated,  and  iodide  of  barium  is  formed.  This  solution 
is  boiled  with  powdered  sulphate  of  potassium ;  iodide  of 
potassium  then  remains  in  solution,  and  may  be  filtered 
from  the  insoluble  sulphate  of  barium.  This  last  pro- 
cess has  the  great  disadvantage  that  the  product  is  rarely,  if 
ever,  free  from  sulphur  compounds,  and  is  rather  inclined  to 
blacken  when  heated  ;  in  other  respects  it  is  a  beautiful  prepara- 
tion, and  is  very  free  from  sulphates,  chlorides,  and  other  im- 
purities generally  found  in  iodide  of  potassium.  In  the  first 
two  processes  purified  pearl-ashes  are  used  instead  of  pure  car- 
bonate of  potassium,  which  is  too  expensive  for  the  purpose.  In 
this  way  the  various  impurities  of  the  potashes  find  their  way 
into  the  iodide  of  potassium,  or  the  mother-liquors  must  be  re- 
jected, and  the  iodine  recovered  from  them,  which  is  not  found 
to  be  an  economical  plan. 
In  order  to  meet  these  objections,  Baron  Liebig  has  proposed 
the  following  mode  of  preparation  :  1  part  of  phosphorus  is 
placed  in  a  basin,  and  covered  with  about  40  parts  of  hot  water, 
and  20  parts  of  iodine  are  gradually  added,  with  frequent  stir- 
ring ;  violent  action  takes  place,  and  a  great  portion  of  the 
