Am.  Jour.  Ptiarm.  \ 
November,  1897.  J 
Official  Lead  Preparations. 
573 
As  in  the  case  of  the  acetate,  calcium  is  present  in  the  lead  oxide 
of  commerce,  and  is  liable  to  interfere,  if  normal  sulphuric  or  oxalic 
acid  is  employed  as  precipitant. 
A  gravimetric  estimation,  by  means  of  sulphuric  acid  and  weigh- 
ing the  dried  precipitated  sulphate,  from  a  solution  of  the  litharge 
in  acetic  acid,  was  originally  employed  by  the  writer,  but  was  dis- 
carded for  the  less  tedious  volumetric  estimation  with  decinormal 
potassium  bichromate  solution. 
Before  directing  the  proper  precipitant,  however,  the  Pharma- 
copoeia must  fix  a  standard  and  require  a  definite  per  cent,  of  lead 
oxide  in  commercial  litharge. 
The  pharmacopceial  limit  as  to  the  amount  of  moisture  and 
carbonate  is  2  per  cent.,  that  of  insoluble  impurities  •  1*5  per  cent. 
If,  in  addition  to  this,  another  allowance  of  1-5  per  cent,  is  made 
for  impurities  soluble  in  acetic  acid,  such  as  zinc,  calcium,  iron, 
aluminum,  etc.,  an  actual  percentage  of  95  of  lead  oxide  can  be 
demanded  in  litharge. 
If  decinormal  potassium  bichromate  solution  is  employed,  -95 
gramme  of  absolute  lead  oxide,  when  in  solution,  should  require 
for  complete  precipitation  42- 8  c.c. 
As  each  c.c.  is  equivalent  to  -02064  Pb,  it  is  equivalent  to  022236 
of  lead  oxide. 
We  have,  consequently,  the  following  division  : 
_£5      =  427 
•022236 
and  as  a  slight  excess  of  the  dichromate  solution  is  necessary,  the 
above  amount  can  be  demanded. 
The  following  addition  to  the  pharmacopceial  description  of  lead 
oxide  may  be  introduced  : 
One  gramme  of  lead  oxide,  dissolved  in  5  c.c.  of  acetic  acid, 
diluted  with  5  c.c.  of  water,  by  means  of  a  gentle  heat,  should 
require  for  complete  precipitation  at  least  42  8  c.c.  of  the  decinormal 
potassium  bichromate  solution,  using  silver  nitrate  solution  as 
indicator. 
In  the  following  examination  of  twenty-five  specimens  of  litharge, 
the  pharmacopceial  directions  were  followed,  except  in  the  determi- 
nation of  carbonate  and  moisture. 
As  volumetric  determinations  were  made,  this  was  not  deemed 
necessary. 
