Am.  Jour.  Pharm. 
June,  1915. 
The  Estimation  of  Free  Acid. 
247 
If  the  liquid  is  alkaline,  it  is  rejected  and  the  operation  repeated 
after  adding  1  Cc.,  or  as  much  as  may  be  required,  of  decinormal 
hydrochloric  acid  before  adding  the  other  reagents. 
As  the  free  thiosulphuric  acid  decomposes  with  notable  rapidity, 
even  in  the  cold  solution,  the  titration  should  be  effected  rapidly. 
The  change  of  color  can  best  be  observed  by  looking  from  above 
through  the  liquid  onto  a  white  foundation.  Side  light  must  be  shut 
out. 
The  following  results  will  illustrate  the  value  of  the  method: 
With  soluble  starch 
Without  starch 
No. 
Ferric  chloride  solution 
employed 
Decinormal  Aspect  of  the  Decinormal  Aspect  of  the 
solution       mixture  ten       solution       mixture  ten 
required     minutes  after     required      minutes  after 
Prepared    from  sublimed 
ferric  chloride 
f Solution  47.5  Cc.l 
1  Normal  NaOH.  .  2.5  Cc.  I 
Solution   2.00  Cc.  I 
Water  20.00  Cc.  1 
Decinormal 
NaOH   2.50  Cc. 
Complete  after  30  min- 
utes' standing  J 
99  Cc.  Xo.  5  with  1  Cc. 
\    hydrochloric    acid    of  > 
(    1. 1 9  specific  gravity  j 
[48  Cc.  No.  5  with  2  Cc.  \ 
\    normal  NaOH 
2.55  Cc.  Unchanged  2.55  Cc.  Very  turbid 
1.37  Cc.    Unchanged    1.70  Cc.    Very  turbid 
0.02  Cc.    Unchanged    0.12  Cc.    Slightly  tur- 
bid 
0.53  Cc.    Unchanged    0.70  Cc.   Very  turbid 
0.08  Cc.    Unchanged    0.12  Cc.     Almost  un- 
changed 
2.45  Cc.    Unchanged    2.55  Cc.    Very  turbid 
0.68  Cc. 
Alkaline 
reaction 
0.25  Cc.  ;  Very  turbid 
Decinormal  sodium  hydroxide  was  employed  in  the  first  six  double 
assays  ;  in  the  last  double  assays  the  mixtures,  being  alkaline,  were 
titrated  with  decinormal  hydrochloric  acid. 
All  the  solutions  were  according  to  the  P.  X.  IV.,  sp.  gr.  1.470- 
1.482. 
Solution  one,  prepared  from  sublimed  ferric  chloride,  yields  a  clear 
liquid  with  the  test  of  the  P.  N.  IV.  Examined  by  the  tests  of  the 
other  three  pharmacopoeias,  free  sulphur  is  separated.  These  two 
tests  are  thus  in  accordance  with  the  results  in  the  table  and  show 
that  the  solution  contains  free  acid. 
This  may  result  from  the  hydrolysis  of  the  ferric  chloride  and  the 
