^tmbe/mt }  Hydrochloric  Acid  in  Ferric  Chloride.  501 
THE    DETERMINATION    OF    UNCOMBINED  HYDRO- 
CHLORIC ACID  IN  SOLUTION  OF  FERRIC  CHLORIDES 
By  C.  H.  Briggs.  , 
For  the  preparation  of  Tincture  Ferric  Chloride  U.  S.  P.  it  is 
essential  that  the  Solution  Ferric  Chloride  used  should  have  the 
proper  degree  of  acidity.  If  the  Ferric  Chloride  does  not  contain 
enough  free  acid,  the  tincture  will  become  cloudy  in  the  course  of 
time  and  some  of  the  iron  will  be  precipitated. 
The  United  States  Pharmacopoeia  gives  a  process  for  the  prep- 
aration of  Solution  Ferric  Chloride  and  prescribes  the  amount  of 
hydrochloric  acid  which  should  be  added,  but  it  does  not  give  any 
method  for  the  determination  of  free  acid  in  the  finished  product. 
Hence,  in  the  examination  of  this  solution,  it  is  necessary  to  depend 
entirely  on  the  manufacturer  for  the  free  acid  content,  and  if  the 
solution  has  not  been  properly  made,  this  may  be  the  source  of  con- 
siderable trouble  for  the  consumer. 
A  search  of  the  literature  failed  to  disclose  any  simple  method 
for  the  determination  of  free  hydrochloric  acid  in  solution  Ferric 
Chloride.  A  direct  titration  of  the  free  acid  with  a  standard  alkali 
solution  and  an  indicator  is  impossible  because  of  the  weakly  basic 
nature  of  ferric  hydroxide  and  its  failure  to  react  with  indicators. 
However,  the  amount  of  iron  in  the  solution  is  readily  determined, 
so  that  if  the  total  amount  of  chlorides  in  the  solution  were  known, 
the  amount  of  free  or  uncombined  acid  could  be  calculated.  It  was 
found  that  the  total  chlorides  could  be  readily  determined  by  titration 
with  N/10  Silver  Nitrate,  using  the  U.  S.  P.  method  for  titrating  acid 
solutions. 
The  percentage  of  iron  in  the  solution  multiplied  by  the  factor 
1.955  wnl  equal  the  amount  of  combined  hydrochloric  acid.  This 
subtracted  from  the  total  hydrochloric  acid,  previously  estimated 
as  chlorides,  will  give  the  percentage  of  free  hydrochloric  acid  by 
weight. 
The  calculated  amount  of  free  hydrochloric  acid  in  Solution 
Ferric  Chloride  U.  S.  P.  is  1.25  per  cent.  A  test  of  one  sample 
which  was  claimed  to  be  neutral  showed  0.18  per  cent,  free  hydro- 
1  Paper  presented  at  the  Milwaukee  Meeting  of  the  American  Chemical 
Society,  March,  1913. 
