474  Detection  of  Minute  Quantities  of  Lead.{Am-^;^vm- 
THE  DETECTION  AND  ESTIMATION  OF  MINUTE 
QUANTITIES  OF  LEAD  IN  THE  PRESENCE  OF 
COPPER  AND  IRON. 
By  Frank  L.  Teed,  E\Sc. 
I  propose  first  to  refer  to  the  detection  of  lead  in  sulphuric  acid. 
The  usual  method  is  by  dilution  with  water,  but  I  found,  some  years 
ago,  that  by  adding  hydrochloric  acid  to  the  sulphuric  acid,  and 
keeping  it  cold,  a  much  smaller  quantity  of  lead  could  be  detected 
than  by  adding  water,  or  even  by  adding  water  and  subsequently  an 
equal  volume  of  absolute  alcohol.  The  lead  is  precipitated  as  chlor- 
ide as  a  peculiar  pearly  opalescence,  I  patented  this  process  for 
the  purpose  of  removing  lead  from  sulphuric  acid,  which  it  does 
perfectly,  but  sulphuric  acid  manufacturers  assure  me  that  there  is  no 
commercial  value  in  any  process  that  would  effect  such  a  result. 
Although  leaving  something  to  be  desired  as  a  patent,  the  process 
is  still  of  use  as  a  test  on  account  of  its  delicacy.  It  can  be  applied 
to  the  detection  of  minute  quantities  of  lead  in  organic  substances. 
Take,  for  instances,  a  substance  which  is  generally,  I  may  say  invari- 
ably, contaminated  with  lead — tartaric  acid.  The  lead  is  a  little 
difficult  of  detection  by  the  ordinary  process,  because  sulphide  of 
lead,  as  we  -know,  is  more  or  less  soluble  in  tartaric  acid.  If  the  tar- 
taric acid  is  ignited,  a  large  proportion  of  the  lead  is,  of  course,  lost, 
but  some  is  left,  and  there  is  sufficient  in  the  small  quantity  of  ash, 
if  digested  with  pure  sulphuric  acid,  to  show  the  characteristic 
reaction  on  the  subsequent  addition  of  hydrochloric  acid.  While 
on  the  use  of  hydrochloric  acid  as  a  test  for  lead  in  sulphuric  acid,  I 
may  mention  that  nitric  acid  is  also  a  test  for  lead  in  sulphuric  acid  ; 
but  it  is  not  nearly  so  delicate  as  hydrochloric  acid.  If  to  a  sample 
of  commercial  sulphuric  acid  which  is  very  rich  (i.  e.,  very  impure) 
in  lead,  nitric  acid  is  added,  the  lead  mainly  settles  out,  but  still, 
after  treatment  with  nitric  acid,  lead  can  be  detected  by  the  addition 
of  hydrochloric  acid.  Hydrochloric  acid  gas  is  insoluble  in  sulphuric 
acid,  and  if  passed  through  strong  sulphuric  acid  which  is  highly 
contaminated  with  lead,  for  an  hour  or  more,  nothing  whatever  hap- 
pens, but  if  to  that  acid  in  a  test  tube  a  single  drop  of  the  solution 
of  hydrochloric  acid,  or  a  crystal  of  common  salt  is  added,  the  char- 
acteristic precipitate  at  once  appears. 
The  main  object  of  my  paper  was  to  draw  attention  to  the  diffi- 
culty of  detecting  and  estimating  minute  quantities  of  lead  in  pres- 
