^n;m*RM'}       Bromide  in  Iodide  of  Potassium.  465 
THE  MEANS  OF  DETECTING  AND  ESTIMATING  BROMIDE  IN 
IODIDE  OF  POTAS81UM.* 
By  Alfred  E.  Tanner. 
It  occurred  to  me  that  the  above  subject  would  be  well  worthy  at- 
tention at  the  present  time,  for  not  only  are  the  processes  for  detect- 
ing and  estimating  bromide  in  iodide  few  and  imperfect,  but  it  also 
seems  to  me  a  very  probable  adulterant,  inasmuch  as  there  is  a  great 
difference  in  price  between  these  two  salts,  and  the  difficulties  attend- 
ing the  detection  of  bromide  when  mixed  with  iodide  are  considerable. 
CI,  Br,  and  I,  so  resemble  one  another  in  their  chemical  characters 
and  reactions  that  it  becomes  a  difficulty  by  no  means  easily  sur- 
mounted to  distinguish  them  when  in  the  presence  of  one  another, 
and  this  is  especially  the  case  with  the  two  latter,  and  as  a  sample  of 
KI  may  contain  75  or  more  per  cent,  of  KBr  and  yet  be  indis- 
tinguishable from  the  pure  article  when  tried  by  the  pharmacopoeia 
tests,  it  needs  little  further  to  point  out  the  desirability  of  investi- 
gating this  subject;  and  before  I  go  further,  I  must  confess  that  I 
fear  1  have  accomplished  little  towards  doing  away  with  the  difficul- 
ties. What  we  require  is  a  test  presenting  no  great  difficulties  of 
application  by  the  ordinary  pharmacist,  and  one  which  shall  indicate 
with  a  fair  degree  of  accuracy  the  object  sought  to  be  attained.  Of 
course  it  is  well  known  to  chemists  that  PdC)2>  when  added  to  a  neu- 
tral solution  of  an  iodide  containing  bromide,  will  remove  the  whole 
of  the  I  without  affecting  the  Br,  but  PdCi2  is  a  rare  and  most  expen- 
sive reagent  to  use,  and  would  scarcely  pay  the  pharmacist  who  ex- 
amines usually  but  small  parcels  of  iodide  at  a  time.  This,  although 
I  believe  to  be  the  most  accurate,  we  must  consider  out  of  the  ques- 
tion. Recent  chemical  works  tell  us  that  a  mixture  of  FeS04,  two 
parts,  and  CuiS04,  one  part,  added  to  a  neutral  solution  of  an  iodide, 
in  the  presence  of  bromide  and  chloride,  and  the  mixture  neutralized 
with  JN 11 3,  will  remove  the  whole  of  the  I  without  affecting  the  Br  or 
CI.  In  my  hands,  at  least,  the  practice  of  this  process  has  been  at- 
tended with  only  partial  success,  for  I  have  found  it  impossible  to 
remove  the  whole  of  the  I ;  the  difficulty  therefore  remains  as  great 
as  ever ;  it  is  probable,  however,  that  further  experiments  with  this 
test  may  yet  prove  it  adequate  to  the  purpose.  1  rather  suspect  the 
Cu2I2  to  be  slightly  soluble  in  the  solution  from  which  it  is  precipi- 
tated ;  we  must  therefore  seek  some  salt  to  add  to  the  mixture  to 
prevent  this.    The  following  is  the  reaction  stated  to  occur : — 
*  Paper  read  before  the  Liverpool  Chemists'  Association,  May  8th,  1873. 
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