438  PREPARATION  OF  IODIDE  OF  POTASSIUM. 
phosphorus  is  converted  into  the  amorphous  variety,  which  how- 
ever in  no  way  interferes  with  the  process,  as  the  amorphous 
phosphorus  reduces  the  iodine  just  as  well,  though  not  quite  so 
fast.  The  colorless  liquid,  which  consists  of  a  solution  of  phos- 
phoric and  hydriodic  acids, 
P+5I+4H20  =  H3P04-f5HI. 
is  then  poured  off  from  any  excess  of  phosphorus,  and  milk  of 
lime  is  added  until  the  mixture  becomes  alkaline  ;  the  whole  is 
now  brought  upon  a  linen  filter,  and  the  precipitate,  which  con- 
sists of  phosphate  of  calcium,  is  pressed  and  washed  ;  the  filtrate 
contains  nothing  but  iodide  of  calcium,  and  is  now  to  be  boiled 
down  with  twelve  parts  of  sulphate  of  potassium  in  powder  (this 
quantity  is  not  quite  sufficient  to  decompose  the  iodide  of  cal- 
cium). When  concentrated  to  about  half,  the  whole  is  allowed 
to  cool  and  remain  at  rest  for  six  hours,  and  then  filtered,  the 
precipitate  being  pressed  as  before.  A  small  quantity  of  pure 
carbonate  of  potassium  is  then  added  to  the  filtrate,  which  re- 
moves every  trace  of  lime,  and  the  filtered  solution  now  yields, 
on  evaporation,  crystals  of  pure  iodide  of  potassium.  A  quantity 
of  iodide  of  potassium,  prepared  in  this  way  by  Pettenkofer, 
yielded  almost  the  theoretical  quantity.  By  a  modification  of 
this  process  we  may  prepare  the  iodides  of  sodium,  lithium, 
barium,  strontium,  calcium,  magnesium,  manganese,  ammonium, 
etc.  etc. 
In  repeating  Liebig's  experiments  I  find  the  iodides  prepared 
in  this  way  very  easily  acquire  a  peculiar  pinkish  hue,  and  that 
it  is  very  difficult  to  get  them  to  crystallize  properly;  what  the 
cause  of  this  is,  I  do  not  know,  but  the  objection  is  completely 
removed  by  fusing  the  salt  before  crystallization, — the  iodides 
after  this  treatment  crystallize  beautifully,  and  retain  their 
color  remarkably  well.  A  specimen  of  iodide  of  ammonium 
prepared  in  this  way,  by  fusing  the  iodide  of  barium,  was  found 
to  have  no  superior  as  an  iodizer  for  photographic  purposes ; 
and  this  is  perhaps  the  best  test  of  its  purity.  With  this  slight 
modification,  I  think  that  the  process  of  Baron  Liebig  is  one  of 
the  best  ever  proposed  for  the  manufacture  of  iodide  of  potas- 
sium ;  the  potassium  salt  used  is  at  once  the  cheapest  and  the 
purest,  and  the  only  other  item  of  cost  in  the  materials  is  the 
