Am.  Jour.  Pharm.  ) 
May,  1879.  ) 
Chemical  Notes. 
231 
CHEMICAL  NOTES. 
By  Prof.  S.  P.  Sadtler. 
Inorganic  Chemistry.— E.  Schering,  the  chemical  manufacturer  or 
Berlin,  makes  the  following  comments  upon  the  manufacture  of  potas- 
sium iodide :  The  three  methods  in  actual  use  are:  (1)  Decompo- 
sition of  barium  iodide  (obtained  from  barium  sulphide  and  iodine)  with 
potassium  sulphate.  (2)  Introduction  of  iodine  into  caustic  potassa  ; 
evaporation  to  dryness  and  fusion  with  carbon  in  order  to  reduce  iodate. 
(3)  Decomposition  of  ferroso-ferric  iodide  with  potassium  carbonate. 
Satisfactory  results  can  be  obtained  by  any  of  these  methods,  and  the 
choice  must  be  determined  by  local  considerations. 
In  method  No.  I,  the  preparation  of  a  barium  sulphide  of  high  and 
regular  strength  is  not  unattended  with  nuisance,  and  the  lixiviation  of 
the  barium  sulphate  requires  much  time.  On  the  other  hand,  potassium 
sulphate  can  be  obtained  cheaper  and  purer  than  the  corresponding  car- 
bonate, whilst  the  barium  sulphate  can  be  readily  utilized  for  the  repro- 
duction of  sulphide. 
Method  No.  2  obviates  the  necessity  for  washing  a  precipitate,  and 
yields  at  once  a  very  strong  solution  of  potassium  iodide  ;  but  the  prep- 
aration of  pure  caustic  potassa,  and  the  concentration  and  subsequent 
fusion  are  circumstantial  and  tedious.  The  author  therefore  prefers 
the  third  method,  as  ferroso-ferric  iodide  is  easily  prepared,  and  the  car- 
bonate of  iron  is  readily  washed. 
To  obtain  cubic  crystals  of  a  porcelain-like  appearance  it  is  essen- 
tial, in  the  first  method,  to  ensure  the  complete  decomposition  of  the 
barium  sulphide  by  the  iodine  ;  if  alkaline  sulphides  are  mixed  with  the 
potassium  iodide  the  crystals  are  paltry.  If  the  lye  contains  iron  sul- 
phide, which  is  soluble  in  hot  and  concentrated  potassium  iodide,  the 
crystals  take  a  blue  appearance.  An  excess  of  iodine  dissolves  foreign 
metals  present  in  the  barium  sulphide,  and  the  crystals  may  then  be 
discolored.  In  the  case  of  the  second  method,  irregularity  in  melting 
may  produce  iodic  acid,  and  a  caustic  potassa  not  free  from  sulphates 
causes  the  presence  in  the  lye  of  alkaline  sulphide.  Both  these  injurious 
impurities  must  be  removed  prior  to  crystallizing. 
In  the  third  method  these  annoyances  are  excluded.  Salts  of  sodium 
must  in  all  cases  be  avoided.  Some  manufacturers,  to  avoid  the  pres- 
ence of  sulphides,  leave  purposely  a  trace  of  iodate  in  the  lye.  The 
result  is  that  the  crystals  turn  yellow.     The  presence  of  lead  in  the 
