116 
Tincture  of  Ferric  Chloride, 
/Am.  Jour.  Pharm. 
t      March,  1885. 
during  the  process,  leaving  an  excess  of  6  parts  of  officinal  hydro- 
chloric acid,  thus : 
324-2  :  37-8  ::  (218-4     -319  =)  684*6  :  79-82. 
Concerning  the  nitric  acid,  we  say  that,  if  972*6  parts  of  ferric  chlo- 
ride demand  for  its  oxidation  from  the  ferrous  state  181*5  parts  of  69*4 
per  cent,  nitric  acid,  then  37-8  parts  of  ferric  chloride  would  require 
less,  or  when  found  7*05  parts.  The  amount  of  HNO3  officinally 
ordered  is  8  parts,  leaving  a  minimum  unused  excess  of  1  part,  thus : 
972-6  :  37-8  ::  (126  ^  -694  =)  181-5  :  7-05. 
Now,  as  the  amount  of  nitric  acid  used  in  the  iron  valeation  is  theo- 
retically, and  in  practice,  admittedly  in  excess  of  the  amount  actually 
demanded  for  that  purpose  alone,  it  is  clearly  evident,  upon  examina- 
tion, that,  independently  of  the  facts  involved  in  the  valeation,  so 
called,  there  is  another  totally  separate  chemical  change  taking  place, 
which,  apparently,  has  failed  to  receive  the  notice  or  excite  the  atten- 
tion that  its  high  value  demands.  The  real  underlying  feature  of  the 
whole  case  seems  to  be  that  immediately  after  we  have  oxidized  the 
iron  compound  we  also  change  the  chemical  character  of  the  excess  of 
hydrochloric  acid  present,  in  the  same  manner  as  we  do  when  making 
nitrohydrochloric  acid,  through  the  presence  of  the  unremoved  excess 
of  nitric  acid.    The  reaction  is,  first : 
2HNO3  +  6HC1      NACI2  +  2CI2  +  4H2O ; 
or,  plainly  expressed,  the  change  consists,  essentially,  in  the  mutual 
decomposition  of  both  nitric  and  hydrochloric  acids,  with  the  forma- 
tion of  chloronitrous  anhydride,  NgOgCla  (a  chlorine  derivative  of 
nitrous  anhydride,  (N2O3),  or  the  so-called  nitrous  acid  gas,  formed 
by  the  simple  replacement  in  the  molecule  of  one  bivalent  oxygen 
atom  with  two  univalent  chlorine  atoms),  free  chlorine  gas  and  water. 
The  free  chlorine  is  volatilized  by  heat,  the  solution  being  a  diluted 
one ;  and,  since  chloronitrous  anhydride,  (N2O2CI2),  readily  decomposes 
on  dilution  into  nitrons  anhydride  and  free  hydrochloric  acid,  the  fol- 
lowing secondary  change  ensues : 
2N2O2CI2  H-  2H2O  =  2N2O3  -j-  4HC1 : 
or,  in  other  words,  the  chloro-anhydride  is  decomposed  by  water,  with 
the  abstraction  of  its  chlorine  atoms  to  form  hydrochloric  acid  with 
