48 
The  Action  of  Zinc,  etc. 
Am,  Jour.  Pharm 
Jan.,  1894. 
liquids  and  with  the  weight  of  the  metal  employed.  Another  cause 
has  also  a  remarkable  influence  upon  this  proportion — the  degree 
of  purity  of  the  metal.  It  is  thus  that  the  weights  of  platinum 
displaced  by  equal  weights  of  different  specimens  of  zinc  in  equal 
volumes  of  one  and  the  same  solution  of  platinum  chloride,  have 
been  10092,  100-39,  119-12  per  cent.;  that  is  to  say,  the  platinum 
has  been  precipitated  with  excesses  of  0-92,  10-39  and  19- 12 
per  cent.  The  first  specimen  of  zinc  had  been  obtained  by  the 
electrolysis  of  an  ammoniacal  solution  of  pure  zinc  sulphate ;  the 
second  was  a  distilled  zinc,  containing  no  impurities  except  traces  of 
sulphur  not  determinable  by  the  balance ;  the  third  was  commercial 
zinc,  containing  i-i  per  cent,  of  impurities,  of  which  0-44  was  fixed 
matter. 
These  results  show  that  zinc  cannot  be  used  for  the  determination 
of  platinum  even  after  a  correction  for  the  impurities.  When  the 
zinc  is  impure,  the  presence  of  the  impurities,  even  in  an  infinitely 
slight  quantity,  occasions  the  fixation  of  a  very  notable  quantity  of 
this  metal  upon  the  platinum.  Electrolytic  zinc  gives  results  which 
are  merely  approximate. 
Magnesium,  as  it  is  found  in  commerce  in  the  form  of  ribbons,  is, 
on  the  contrary,  quite  suitable  for  the  determination  of  copper, 
gold  and  platinum  ;  and,  consequently  of  potassium,  after  a  precipi- 
tation as  double  platinum-potassium  chloride. 
Determination  of  Copper. — When  copper  is.  accompanied  by  no 
other  metals  except  the  alkaline  and  earthy  metals,  it  may  be  deter- 
mined very  quickly  and  easily  in  the  metallic  state  by  treating  its 
solutions  with  magnesium.  The  copper  is  thus  liberated,  in  a 
slightly  acid  liquid,  in  the  form  of  a  granular  precipitate  very  easy 
to  wash.  The  washing  is  finished  with  alcohol ;  the  metal  is  dried 
at  1000  and  weighed. 
We  may  make  use  of  a  weighed  filter,  or  more  simply  deposit 
the  reduced  copper  in  a  small  weighed  capsule. 
The  process  is  not  applicable  in  presence  of  metals  like  zinc  easily 
acted  on  by  hydrochloric  acid ;  there  are  produced  alloys  even  if 
the  liquids  are  very  acid. 
Determination  of  Potassium. — If  we  determine  potassium  in  the 
double  platinum  chloride,  the  precipitate  is  collected,  after  desicca- 
tion, on  a  weighed  filter.  Or  we  incinerate  the  filter,  ignite  the 
precipitate  in  hydrogen,  and  determine  the  platinum  in  the  residue, 
