666  Estimation  of  Lead  in  Food.      /Am.  jour.  Pharm. 
J  <-     October,  1919. 
ESTIMATION  OF  SMALL  QUANTITIES  OF  LEAD  IN 
FOOD  AND  SUBSTANCES  CONTAINING 
CALCIUM  PHOSPHATE.1 
By  B.  W.  J.  Warren,  F.I.C. 
In  the  B.P.  method  for  the  estimation  of  lead,  the  substance  to 
be  tested  is  dissolved  either  in  water  or  a  dilute  solution  of  ammonia. 
With  foods  it  is  necessary  to  first  destroy  the  organic  matter  and 
estimate  the  lead  in  the  ash.  The  ash  can  be  dissolved  in  dilute  nitric 
acid,  and  the  solution  rendered  alkaline  with  ammonia,  a  precipitate 
of  calcium  and  magnesium  phosphate  being  obtained.  If  the  nitrate 
is  used  for  the  estimation  of  lead  (using  the  B.P.  method)  consid- 
erable quantities  of  lead  may  sometimes  be  overlooked,  as  the  lead 
is  occluded  with  the  precipitated  phosphates  (or  as  an  insoluble 
double  phosphate  of  calcium  and  lead). 
Wilkie  (/.  Soc.  Chem.  hid.,  1909,  28,  636)  has  pointed  out  that 
ferric  hydroxide  will  remove  lead  from  a  tartrate  solution. 
If  calcium  phosphate  containing  traces  of  lead  and  copper  is 
dissolved  in  dilute  nitric  acid  and  ammonia  be  added  the  precipi- 
tate contains  all  the  lead,  while  the  copper  is  in  solution.  If  this 
precipitated  phosphate  (containing  lead)  is  dissolved  in  dilute  nitric 
acid  the  lead  can  be  estimated  colorimetrically. 
In  the  absence  of  iron,  lead  and  copper  can  be  easily  and  accu- 
rately estimated  in  a  food.  Iron,  however,  presents  some  little  diffi- 
culty:  if  a  phosphate  precipitate  containing  lead  and  iron  (copper 
being  eliminated  as  shown  above)  is  dissolved  in  dilute  acetic  acid 
the  solution  is  turbid  owing  to  the  presence  of  phosphate  of  iron. 
If  this  precipitate  is  filtered  off  some  of  the  lead  is  removed  with 
the  precipitate  (with  the  material  with  which  I  was  working  about 
two  thirds  were  removed). 
It  is,  however,  possible  to  match  the  color  with  the  slightly  turbid 
solution  and  thus  estimate  the  lead. 
The  method  adopted  is  as  follows : 
Ten  Grms.  of  foodstuff  are  incinerated  in  a  silica  dish,  dissolved 
in  a  small  quantity  of  water  with  the  addition  of  1  Cc.  of  nitric  acid, 
filtered,  and  washed.  To  the  filtrate,  which  should  be  colorless,  a 
slight  excess  of  ammonia  is  added,  the  precipitate  filtered  and 
1  Reprinted  from  The  Analyst,  June,  1919. 
