246 
Assay  of  Iron  Filings. 
Am.  Jour.  Pharm. 
April,  1920. 
equation:  SKMnOi  +  12H2SO4  +  5PH3  =  4K2SO4  +  8MnS04  + 
12H2O  +  5H3PO4  it  was  considered  likely  that  traces  of  phosphine 
(and  probably  some  hydrocarbon)  were  responsible  for  the  abnormally 
high  results  obtained.    It  will  be  seen  from  the  above  equation  that 
PH3  =  8  Fe 
8KMn04  =  5PH3  =  40FeO  —  i.  e., 
35  =  447 
so  that  phosphine  has  the  reducing  power  of  about  thirteen  times  its 
weight  of  iron;  consequently,  a  little  in  the  solution,  if  calculated  as 
iron,  would  produce  the  high  result.  So  little  as  0.000437  Gm.  of 
phosphine  reduces  i  Cc.  of  N/io  KMn04.  Sulphuretted  hydrogen 
would  act  similarly,  but  the  proportion  of  phosphorus  in  iron  (which 
may  be  0.7  per  cent,  in  cast  iron),  judging  from  published  analyses 
of  iron,  is  from  four  to  seven  times  that  of  sulphur.  That  the  gas 
evolved  was  phosphine  was  proved  by  dissolving  5  Gms.  iron  filing 
in  dilute  sulphuric  acid  in  a  flask  and  passing  the  gas  generated 
through  a  solution  of  potassium  permanganate  in  another  flask. 
The  permanganate  solution  was  then  decolorized  »vith  tartaric  acid 
and  tested  with  excess  of  ammonium  molybdate  in  nitric  acid, 
when  ample  evidence  of  phosphate  was  obtained.  It  might  be 
expected  that  if  phosphine  is  formed  it  would  be  precipitated  from 
the  solution  as  copper  or  iron  compounds.  In  the  case  of  small 
amounts,  however,  and  under  certain  conditions,  such  as  the  reducing 
influence  to  the  copper-iron  couple,  it  is  possible  that  precipitation 
may  not  take  place.  It  was  notable  that  the  odor  of  phosphine 
was  not  always  perceptible,  though  some  may  have  been  formed 
in  further  experiments  on  the  sample  which  had  previously  yielded 
it  under  apparently  the  same  conditions.  In  fact,  odor  of  phosphine 
was  not  the  rule.  Assuming,  phosphine,  however,  to  be  the  dis- 
turbing factor,  prolonged  boiling  was  tried  to  eliminate  it  from  the 
solution,  and  this  was  found  successful.  The  following  process  was 
employed : 
0.025  Gm.  of  iron  filings  and  1.25  Gms.  of  copper  sulphate  were 
ground  together  in  a  small  mortar  to  thoroughly  subdivide  the  filings, 
then  rinsed  into  a  narrow  4,000  Cc.  glass  beaker  with  about  60  Cc. 
of  cold  water.  The  contents  of  the  beaker  were  then  brought  to 
the  boil  and  kept  briskly  boiling  till  reduced  to  about  15  Cc,  then 
rapidly  filtered,  preferably  at  the  pump,  through  a  Gooch  crucible 
or  through  a  small  Buchner  funnel.    The  time  of  filtration  is  thus 
