268  NOTE  ON  THE  PURIFICATION  OF  PLATINUM. 
NOTE  ON  THE  PURIFICATION  OF  PLATINUM. 
By  Mr  E.  Sonstadt. 
The  tendency  of  platinum  to  alloy  with  other  metals  at  a 
temperature  far  below  its  fusing  point  is  sufficiently  well  knowh 
to  every  user  of  platinum  crucibles.  It  is  equally  well  known 
that  iron,  &c,  which  has  been  absorbed  by  platinum,  cannot  be 
removed,  except  superficially,  by  the  action  of  hydrochloric  acid 
for  instance,  nor  even  by  heating  in  acid  sulphate  of  potassium. 
Stas,  in  his  memoir  on  the  atomic  weight  of  silver,  &c,  states 
that  he  purified  his  platinum  vessels  from  iron  by  causing  them 
to  come  in  contact,  at  a  red  heat,  with  the  vapor  of  chloride  of 
ammonium.  The  process  had  to  be  repeated  as  often  as  any 
yellow  sublimate  was  formed.  This  process  is  less  effectual,  or 
less  conveniently  and  speedily  effectual,  than  the  modification  of 
it  that  I  have  to  propose  ;  because  if  the  vapor  of  the  sal  ammo- 
niac is  generated  from  the  solid  salt  in  the  vessel  to  be  purified, 
the  heat  absorbed  in  the  vaporization  of  the  salt  tends  to  keep 
the  vessel  at  a  temperature  below  that  at  which  volatile  metallic 
chlorides  are  most  readily  formed.  Instead  of  chloride  of  am- 
monium, I  put  dry  double  chloride  of  ammonium  and  magnesium 
in  the  platinum  vessel  intended  for  purification.  The  vessel  is 
then  heated  to  about  the  fusing  point  of  cast  iron  for  about  an 
hour.  I  find  a  Gore's  furnace  convenient  for  this  purpose.  In 
this  process,  not  only  is  chloride  of  ammonium  vapor  given  off 
for  a  long  while  with  the  double  salt,  at  a  temperature  much 
above  that  at  which  chloride  of  ammonium  alone  volatilizes,  but 
when  that  salt  is  completely  expelled,  the  chloride  of  magnesium 
remaining  is  perpetually  decomposed  with  evolution  of  free  chlo- 
rine, and,  frequently,  the  formation  of  a  crystalline  crust  of 
periclase  lining  the  crucible.  Platinum  thus  purified  is  softer 
and  whiter  than  ordinary  commercial  platinum.  The  method  is 
not  available  solely  for  the  removal  of  iron,  but  retrieves  crucibles 
that  have  become  dark  colored  and  brittle  from  exposure  to  gas 
flame,  as  well  as  crucibles  that  have  been  attacked  by  silicates 
during  fusion  of  these  with  carbonate  of  sodium.  I  cannot  con- 
clude this  note  without  remarking  on  the  extreme  facility  with 
which  platinum  becomes  impure  by  heating  in  contact  with  mat- 
ters containing  only  a  very  small  proportion  of  substance  capable 
