A"jtn7i, mT'}  New  Process  for  Detecting  Bromide,  etc.  249 
the  same  conditions.  Isomorphous  salts  behave  differently.  Von 
Hauer*  proved  by  interesting  researches  that,  the  physical  conditions 
being  identical,  a  given  weight  of  a  solution  of  mixed  isomorphous 
salts  contains  the  same  quantity  of  solid  matter  which  is  contained  in 
a  like  weight  of  a  saturated  solution  of  the  most  soluble  salt. 
100  parts  of  water  dissolve,  at  16°  C,  140*10  p.  iodide  of  potas- 
sium. The  author  found  that  the  same  quantity  of  water  dissolves, 
at  the  same  temperature,  63-39  p.  bromide  of  potassium.  At  this 
temperature  all  the  following  experiments  have  been  made. 
When  an  excess  of  a  mixture  of  bromide  and  iodide  of  potassium 
is  treated  with  water,  100  p.  of  it  dissolve  140  p.  of  the  mixture,  and 
the  analysis  of  the  dissolved  portion  proves  it  to  be  solely  iodide  of 
potassium.  Von  Hauer's  proposition  may,  for  this  case,  be  rendered 
as  follows  :  If  a  mixture  of  bromide  and  iodide  of  potassium  is  treated 
with  water,  the  latter  salt  alone  is  dissolved,  if  its  quantity  is  suffi- 
cient to  saturate  the  water. 
It  might  be  supposed  that  100  p.  of  water  saturated  with  bromide 
would  dissolve  140-10  —  63-39  =  76*71  iodide  of  potassium ;  such 
is,  however,  not  the  case.  Only  13*15  p.  of  iodide  are  taken  up,  and 
if  more  is  added,  bromide  of  potassium  is  precipitated.  If  double  the 
weight  of  KBr,  soluble  in  100  water  (2  x  63-39  ==  126*78),  is  deducted 
from  the  weight  of  KI  soluble  in  the  same  quantity  (140*10),  the 
resulting  figure  (13*32)  closely  approaches  13-15  found  by  experi- 
ment, and  represents  the  maximum  solubility  of  KI  in  100  water 
saturated  with  KBr,  which  is  equal  to  about  10  parts  of  the  former 
salt  in  100  of  the  saturated  solution. 
If  pure  iodide  and  bromide  of  potassium  be  dissolved  separately  to 
saturation  in  water,  the  temperature  falls  21°  and  15°  C.  This  fall 
in  the  temperature  must  be  taken  into  consideration  in  making  the 
saturated  solution  of  KBr,  and  in  adding  thereto  the  iodide,  particu- 
larly if  larger  quantities  are  operated  upon.  The  bromide  of  potas- 
sium is,  therefore,  dissolved  in  warm  distilled  water,  the  solution  is 
allowed  to  cool,  and  after  crystallization  decanted  or  filtered. 
To  10  c.  c.  of  this  solution  10  drops  of  distilled  water  are  added  in 
a  test-tube,  and  afterwards,  in  small  quantities,  under  repeated  agita- 
tion, 1  gram,  of  the  suspected  iodide  in  coarse  powder.  If  free  from 
bromide,  it  will  dissolve  almost  instantly,  while  this  impurity,  if  pres- 
ent, will  remain  undissolved. 
*  Journal  fur  Praktische  Chemie,  vols,  xcviii  and  ciii. 
