266 
MEDICAL  ANALYSIS. 
2.  The  fluid  in  which  lime  water  has  failed  to  produce  a  pre- 
cipitate, or  the  filtrate  from  the  above  precipitate  is  acidulated 
with  hydrochloric  acid,  and  chloride  of  barium  added. 
A  white  precipitate  determines  the  presence  of  sulphuric  acid. 
The  presence  of  lead  was  strikingly  brought  before  the  author 
by  a  Dublin  friend  having  taken  out  of  a  sample  of  citric  acid  a 
considerable  per  centage  of  metallic  lead,  independently  of  that 
held  in  solution.* 
The  presence  of  a  trace  of  sulphuric  acid,  which  is  almost  al- 
ways present,  may  be  overlooked  in  a  great  measure, — but  the 
occurrence  of  lead  and  the  large  quantities  found  in  commercial 
samples  is  a  matter  of  much  greater  importance. 
The  pieces  of  metal  found  in  citric  acid  are  very  small,  and 
are  imbedded  in  the  substance  of  the  crystals. 
Citric  acid  should  leave  no  perceptible  ash  when  burned.  After 
ignition  the  crucible  should  be  washed  out  with  diluted  nitric 
acid  and  evaporated  to  dryness ;  the  residue  is  again  dissolved  in 
water  and  tested  with  sulphide  of  ammonium  for  lead. 
Sixty-seven  grains  of  the  crystals  are  neutralized  by  100 
measures  of  the  volumetric  solution  of  soda  of  the  Pharmacopoeia.f 
*  Citrate  of  lead  is  soluble  mfree  citric  acid,  and  is  not  precipitated 
from  this  solution  by  ammonia, 
f  Volumetric  solution  of  soda,  B.P. 
Soda  NaO  =  31.  Take  of  solution  of  soda  a  sufficiency,  distilled  water 
a  sufficiency.  Fill  the  volumetric  tube  to  0  with  the  solution  of  soda,  and 
drop  this  into  sixty-three  grains  of  oxalic  acid,  dissolved  in  two  fluid 
ounces  of  the  water  until  the  acid  is  exaetly  neutralized,  as  indicated  by 
litmus.  Note  the  number  of  measures  (N)  of  the  solution  used,  and  having 
then  taken  forty  ounces  of  the  solution  of  soda,  augment  this  quantity  by 
the  addition  of  distilled  water  until  it  becomes 
4000 
 =  fluid  ounces. 
N 
If,  for  example  N  =  93,  the  forty  ounces  of  solution  of  soda  should  be 
diluted  so  as  to  become 
4000 
 =  43-01  fluid  ounces. 
93 
The  quantity  of  this  solution  which  fills  the  volumetric  tube  to  O  includes 
31  grains  of  soda,  and  will,  therefore,  neutralize  an  equivalent  in  grains  of 
any  monobasic  acid.  It  is  generally  necessary  to  recrystallize  the  oxalic 
acid  for  titrating  this  volumetric  solution.    Mr.  Maumene  finds  the  first 
