386 
Zinc  and  Copper  in  Gelatin. 
Am.  Jour.  Pharm. 
June,  1919- 
crucible.  When  the  washings  have  thoroughly  drained,  the  Gooch 
crucible  is  placed  inside  of  a  porcelain  crucible  and  dried  over  a 
small  flame  for  a  few  minutes.  .  Then  the  flame  is  gradually  in- 
creased to  its  capacity.  At  this  point  the  outer  crucible  is  removed 
and  the  Gooch  crucible  heated  directly  in  the  oxidizing  part  of  the 
flame  for  15  min.  After  cooling  and  weighing,  the  crucible  is 
heated  again  as  hot  as  possible  for  5  min.  more  in  order  to  be  cer- 
tain that  all  the  copper  is  converted  into  the  oxide.  The  filtrate 
containing  the  zinc  is  heated  until  the  hydrogen  sulphide  is  ex- 
pelled. After  adding  about  5  Cc.  of  ammonia  in  excess  of  that  re- 
quired to  neutralize  the  sulphuric  acid  in  the  solution,  15  Cc.  of  50 
per  cent,  formic  acid  are  added  and  a  rapid  stream  of  hydrogen 
sulphide  is  passed  into  the  solution  for  5  min.  to  precipitate  the  zinc 
sulphide.  It  is  important  to  stir  the  solution  with  the  delivery  tube 
while  passing  in  the  hydrogen  sulphide  until  the  larger  part  of  tke 
zinc  is  precipitated.  The  solution  containing  the  zinc  sulphide  is 
heated  on  the  steam  bath  for  about  half  an  Hour,  The  zinc  sul- 
phide is  filtered  on  a  Gooch  crucible  and  washed  with  a  2  per  cent, 
solution  of  ammonium  thiocyanate.  During  the  filtration  and  wash- 
ing of  the  precipitate,  it  is  best  to  use  either  no  suction  or  at  most 
a  very  slight  suction.  When  all  of  the  precipitate  is  in  the  crucible 
and  the  wash  solution  has  largely  run  through,  the  suction  is  in- 
creased until  it  is  sufficient  to  drain  the  crucible  properly.  The  zinc 
sulphide  is  dried  and  ignited  to  convert  it  into  the  oxide  in  the  same 
manner  as  the  copper  sulphide  is  treated.  After  weighing,  the  cru- 
cibles containing  the  oxides  of  zinc  and  copper  are  treated  with 
hydrochloric  acid,  thoroughly  washed  with  water,  and  ignited  in 
order  to  prepare  them  for  subsequent  analyses. 
Several  samples  of  commercial  gelatins  of  various  grades  were 
analyzed  by  the  hydrolysis  method  described  above  as  well  as  by 
the  digestion  method  for  the  sake  of  comparison.  In  the  digestion 
analyses  it  should  be  noted  that  the  copper  was  determined  by 
weighing  the  oxide  in  place  of  the  volumetric  method  given  above. 
Also  the  zinc  was  precipitated  as  sulphide  in  the  presence  of  am- 
monium formate  and  formic  acid  instead  of  ammonium  acetate  and 
acetic  acid. 
In  conformity  to  the  usual  custom,  the  results  of  the  analyses 
are  stated  in  terms  of  milligrams  of  metal  per  kilo  of  gelatin  or 
parts  per  million. 
