466  Bromide  in  Iodide  of  Potassium .     { AMoi?™'  fmf M" 
2  KI+2  CuS04-f2  FeS04=Cu2I2+K2S044-Fe23S04. 
A  test  proposed  by  Van  Melckebeke  (Journ.  Pharm.  cV Anversr 
xxviii,  49,  1872)  seemed  to  promise  well,  and  from  its  great  simplicity 
would  have  been  a  valuable  one  if  successful.  It  depended  on  the 
fact  that  a  saturated  solution  of  one  salt  is  capable  of  dissolving  ap- 
preciable quantities  of  another  salt.  A  saturated  solution  of  KBr 
was  therefore  used,  and  to  this  the  sample  of  KI,  in  powder,  was 
added  in  small  quantities  at  a  time,  when,  if  pure,  it  dissolved  readily, 
but  if  KBr  were  present,  the  liquid  being  already  saturated  with  this 
salt,  it  would  remain  undissolved.  Repeated  trials  with  this  test  have 
proved  to  me  that  it  is  quite  useless.  The  author  recommends  you 
to  take  10  c.c.  of  the  saturated  KBr  solution,  and  to  add  to  this  10 
drops  of  distilled  water ;  1  gram  of  the  suspected  salt,  in  powder,  is 
then  added,  small  portions  at  a  time,  which,  if  the  iodide  be  pure, 
should  at  once  dissolve ;  but  10  drops  of  distilled  water  is  quite  suf- 
ficient to  dissolve  5  or  more  grains  of  KBr,  and  if  no  water  be  added, 
some  KBr  is  very  liable  to  be  thrown  out  of  solution  by  the  shaking 
necessary.  The  next  test  I  tried  was  one  by  M.  Personne,  published 
in  the  Journal  de  Pharmacie.  It  depends  on  the  property  possessed 
by  Hg012  of  precipitating  a  solution  of  iodide  but  not  one  of  bromide, 
bromide  of  mercury  being  soluble.  It  is  necessary  to  the  success  of 
this  test  that  the  iodide  be  free  from  KI03,  KC1,  and  K2C03. 
I  may  mention  here  that  KI03  is  much  more  frequently  present 
than  is  generally  supposed,  and  traces  of  it  may  generally  be  detected' 
in  the  best  samples  of  iodide,  and  as  this  salt  (KI03)  is  stated  on 
pretty  good  authority  to  be  of  a  poisonous  nature,  it  behoves  us  to 
be  on  our  guard  against  it ;  it  is  fortunately  easy  to  detect  by  adding 
a  little  starch  solution  to  the  iodide  to  be  tested,  and  then  adding  a 
small  quantity  of  tartaric  acid  ;  a  blue  color  is  developed  more  or  less 
rapidly,  by  the  liberated  iodide  acting  on  the  starch,  if  the  KI  con- 
tain KI03,  thus  : — 
5  KI+KIOs+H3C4H406=K2C4H406+5HI+HIOs 
and 
5  HI+HI03=I2+3H20. 
1  gram  HgCl2  is  dissolved  to  20  c.c.  with  distilled  water,  of  this  solu- 
tion 16  c.c.  is  capable  of  removing  the  whole  of  the  I  from  1  gram 
KI.    If  therefore  the  KI  be  mixed  with  KBr  a  proportionably  less 
quantity  of  the  mercuric  solution  will  be  required. 
It  is  a  somewhat  curious  fact,  and  one  which  I  have  nowhere  seen, 
recorded,  that  when  exactly  half  the  mercuric  solution  is  added  a. 
