AmjJne r"i9ih9arm" )        Zinc  and<  C°PPer  in  Gelatin.  385 
treated  with  100  Cc.  of  water  and  15  to  30  Cc.  of  hydrochloric  acid, 
depending  upon  the  amount  of  gelatin  taken  for  analysis.  The 
covered  beakers  are  heated  for  an  hour  or  two  on  the  steam  bath. 
During  the  first  part  of  the  heating,  the  solutions  are  agitated  sev- 
eral times  in  order  to  loosen  any  lumps  adhering  to  the  bottom  of 
the  beakers.  After  hydrolysis  the  solutions  are  made  very  slightly 
alkaline  with  ammonium  hydroxide  and  allowed  to  cool  to  about 
400  C.  Then  hydrogen  sulphide  is  passed  into  the  solutions  for 
2  min.  -While  passing  in  the  hydrogen  sulphide,  the  solutions 
should  be  stirred  several  times  with  the  delivery  tube  in  order  to 
facilitate  the  separation  of  the  sulphides  in  the  form  most  suitable 
for  nitration.  When  the  sulphides  have  settled  for  about  10  min., 
they  are  filtered  on  a  9  Cm.  filter  and  washed  several  times  with 
a  very  dilute  solution  of  colorless  ammonium  sulphide.  The  wash 
solution  is  prepared  by  passing  hydrogen  sulphide  for  several  min- 
utes into  250  Cc.  of  water  which  contains  0.5  Cc.  of  1  :  1  ammonium 
hydroxide.  In  the  case  of  high-grade  gelatin  it  is  recommended 
that  about  2.5  Mg.  of  iron  should  be  added  to  the  hydrolyzed  gelatin 
before  making  the  solution  alkaline.  It  is  found  that  ferrous  sul- 
phide greatly  facilitates  the  precipitation  and  the  filtration  of  small 
quantities  of  zinc  and  copper  sulphides.  The  iron  is  conveniently 
added  in  the  form  of  a  standard  solution  which  contains  2.49  Gm. 
of  ferrous  sulphate  in  1,000  Cc.  (1  Cc.  =  0.5  Mg.  of  iron).  If  the 
directions  given  above  are  closely  followed  in  making  the  hydro- 
lyzed gelatin  solution  very  slightly  ammoniacal,  there  is  no  danger 
of  leaving  any  weighable  amount  of  copper  sulphide  dissolved  in 
the  ammonium  sulphide  in  the  filtrate.  The  sulphides  are  dissolved 
by  pouring  a  small  quantity  of  very  hot  1 :  1  nitric  acid  around  the 
upper  edges  of  the  niters.  The  filters  are  washed  very  thoroughly 
with  water  at  room  temperature.  The  nitrate  is  evaporated  with  10 
Cc.  of  1:3  sulphuric  acid  until  all  the  nitric  acid  is  expelled.  When 
the  sulphuric  acid  is  cool,  30  Cc.  of  water  are  added,  and  the  solu- 
tion is  filtered  to  remove  the  silica.  The  nitrate,  which  should  have 
a  volume  of  about  100  Cc.  is  warmed  to  about  500  C.  and  hydrogen 
sulphide  is  passed  into  the  solution  for  at  least  5  min.  in  order  to 
precipitate  the  copper  completely.  The  copper  sulphide  is  filtered 
on  a  Gooch  crucible  and  washed  with  hot  water  saturated  with 
hydrogen  sulphide.  During  the  filtration  and  washing  of  the  pre- 
cipitate a  very  gentle  suction  is  employed,  otherwise  there  is  the 
danger  of  having  some  of  the  copper  sulphide  pass  through  the 
