3§4 
Zinc  and  Copper  in  Gelatin. 
Am.  Jour. 
June, 
.  Pharm. 
1919. 
from  the  filtrate  containing  the  zinc,  ammonium  chloride  is  added 
along  with  ammonium  hydroxide  to  make  the  solution  alkaline. 
Enough  hydrochloric  acid  is  added  to  render  the  solution  acid  to 
methyl  orange.  After  adding  a  large  excess  of  sodium  or  am- 
monium acetate,  the  zinc  is  precipitated  with  hydrogen  sulphide  and 
filtered.  The  zinc  sulphide  is  dissolved  in  hydrochloric  acid  and  the 
resulting  nitrate  is  boiled  to  remove  the  hydrogen  sulphide.  A  small 
amount  of  ferric  chloride  is  added  and  a  basic  acetate  precipitation 
is  made  in  the  usual  manner  in  order  to  separate  any  phosphates 
present  from  the  zinc.  The  zinc  is  precipitated  from  the  filtrate  as 
sulphide  and  after  filtration  is  ignited  to  the  oxide.  It  is  well  known 
that  the  digestion  of  gelatin  with  nitric  and  sulphuric  acids  requires 
almost  constant  attention  during  the  entire  process  which  generally 
takes  two  hours,  and  sometimes  longer  for  completion.  In  addition 
to  the  digestion,  the  long  analytical  procedure  makes  the  method 
unsatisfactory. 
Several  years  ago  C.  R.  Smith  decomposed  gelatin  with  hydro- 
chloric acid  in  connection  with  the  determination  of  arsenic2  and 
suggested  that  this  method  of  decomposing  gelatin  could  be  used 
to  advantage  for  the  determination  of  the  other  metals  in  place  of 
the  tedious  digestion,  process.  Mr.  Smith  and  a  number  of  other 
chemists  have  employed  the  hydrolysis  method  for  the  determina- 
tion of  zinc  and  copper  in  gelatin.  Since  the  method  of  separating 
the  zinc  from  the  hydrolyzed  solution  is  somewhat  different  from 
that  employed  in  the  digestion  method,  it  will  be  briefly  described. 
A  small  amount  of  magnesia  mixture  and  an  excess  of  sodium 
phosphate  solution  are  added  along  with  enough  ammonium  hy- 
droxide to  make  the  hydrolyzed  gelatin  solution  alkaline.  Then 
the  zinc  and  copper  are  precipitated  together  as  sulphides  and  fil- 
tered. The  crystalline  precipitate  of  ammonium  magnesium  phos- 
phate serves  to  collect  the  metallic  sulphides  so  that  they  can  be 
readily  filtered.  The  precipitate  is  treated  with  a  cold  solution  of 
1  :  10  hydrochloric  acid  which  has  been  saturated  with  hydrogen 
sulphide  in  order  to  dissolve  the  zinc  sulphide  and  leave  the  copper 
sulphide  on  the  filter.  The  zinc  and  the  copper  are  determined  as 
described  above. 
It  is  believed  that  the  following  method  will  be  found  much  sim- 
pler than  those  now  used.  The  size  of  the  sample  taken  for  analysis 
varies  from  20  to  50  Gm.,  according  to  the  amount  of  zinc  and  cop- 
per present.    The  samples  are  weighed  into  500  Cc.  beakers  and 
