324  Solution  and  Tincture  of  Ferric  Chloride.  {AmjJa°i^i£5.arm* 
in  specific  gravity  and  Fe2Cl6\  No.  10  is  lowest  in  specific  gravity, 
containing  also  the  lowest  percentage  of  Fe2Cl6,  yet  corresponding 
almost  proportionately  to  No.  4  or  the  official. 
Then  again,  by  taking  the  average  of  both  columns  we  have  for 
specific  gravity  1-390,  and  for  Fe2C16,  36-95  per  cent.,  and  again  the 
proportion  corresponds  practically  to  that  of  the  official — 37-8  per 
cent. 
The  average  of  Fe2Cl6  is  -85  per  cent,  below  that  of  the  official, 
which  would  imply  a  good  collection  of  samples ;  but  when  the 
extreme  variation  is  observed  to  be  5  per  cent.,  this  would  at  once 
indicate  some  weakness  and  imperfection  in  the  process,  which  is  to 
be  attributed  to  the  later  part  in  which  the  U.S  P.  directs  after 
effervescence,  etc.,  add  enough  water  to  make  a  certain  definite 
weight.  If  pure  iron  wire  is  used,  hydrochloric  acid  containing  31-9 
per  cent,  of  absolute  acid,  sufficient  time  allowed  for  complete  reac- 
tion, the  tests  for  nitric  acid,  ferrous  salt  and  oxychloride  cautiously 
applied,  and  the  work  carefully  manipulated,  it  will  result  in  a  relia- 
ble product.  But  the  iron  is  not  always  pure,  hydrochloric  acid  is 
often  of  indefinite  strength,  the  reaction  not  always  completed, 
besides  taking  into  consideration  the  liability  to  error  in  applying 
the  tests,  and  the  manner  in  which  the  work  is  generally  conducted ; 
hence,  the  process  will  not  result  in  products  of  uniform  strength.  If,, 
instead  of  diluting  the  solution  to  a  definite  weight,  it  be  diluted  to 
a  definite  specific  gravity,  we  then  might  disregard  slight  impuri- 
ties in  iron  or  HC1,  or  other  sources  of  errors  likely  to  be  encoun- 
tered, and  secure  decidedly  better  results. 
Another  noteworthy  fact  in  connection  with  the  table  is  this : 
Had  the  solutions  with  a  specific  gravity  below  that  of  1*405  been 
evaporated,  and  those  containing  a  higher  specific  gravity  further 
diluted  so  that  they  would  all  conform  with  the  official  in  specific 
gravity,  the  percentage  of  Fe2Cl6  would  vary  only  about  3  per  cent. 
The  fact  above-mentioned,  with  others  involved  in  the  table,  for- 
cibly impress  some  excellent  advantages  over  that  of  the  official 
method. 
The  Pharmacopoeia  directs  free  hydrochloric  acid  to  be  present  in 
the  solution  of  ferric  chloride,  but  does  not  furnish  a  method  whereby 
the  amount  contained  may  be  determined.  The  U.  S.  P.  assay  for 
anhydrous  ferric  chloride  is  not  in  itself  sufficient  to  insure  the  quality 
or  standard  of  the  preparation.    Every  pharmacist  should  be  fami- 
