252  Estimation  of  Hydrochloric  Acid.  {Am']&0ayVi?89arm 
the  lactic  acid ;  this  is  separated,  and  cinchonine  is  added  to  the  re- 
mainder until  the  reaction  is  neutral,  then  it  is  shaken  four  or  five 
times  with  a  large  excess  of  chloroform,  which  takes  up  the  cinchonine 
hydrochloride ;  the  chloroform  is  distilled  off,  and  the  chlorine  esti- 
mated in  the  residue.  Another  method  devised  by  Cahn  and  v.  Mer- 
ing  consists  in  distilling  the  contents  of  the  stomach  with  water  three 
times.  The  volatile  fatty  acids  are  estimated  in  the  distillate.  The 
residue  is  shaken  six  times  with  500  cc.  of  ether,  and  this  is  evapo- 
rated to  dryness,  and  the  lactic  acid  estimated  by  titration.  The  resi- 
due contains  the  hydrochloric  acid,  and  this  also  can  be  estimated  by 
titration. 
Another  method  used  by  Seemann  (Zeit.  hlin.  Med.,  v),  and  Hebner 
(Zeit.  anal.  Chem.,  xvii,  236),  consists  in  neutralizing  the  stomach-con- 
tents by  titrating  with  sodium  hydroxide,  evaporating  to  dryness,  and 
carefully  incinerating.  The  ash  is  extracted  with  water,  and  in  the 
extract  the  alkali  present  is  estimated  by  titrating  with  an  acid  ;  the 
difference  between  the  amount  of  alkali  added,  and  the  amount  of 
alkali  found,  gives  the  amount  which  must  have  combined  with  hy- 
drochloric acid. 
This  method,  however,  gives  too  high  results ;  and  the  other  meth- 
ods take  too  long,  and  too  large  a  quantity  of  the  necessary  reagents 
to  be  available  for  clinical  work. 
The  method  now  recommended  gives  absolutely  accurate  results, 
and  is  sufficiently  simple  to  use  clinically.  The  contents  of  the  sto- 
mach are  evaporated  to  dryness  with  barium  carbonate  and  then  in- 
cinerated ;  barium  chloride  remains  unchanged,  and  the  salts  of  the 
organic  acids  are  burnt  to  barium  carbonate.  The  barium  chloride  is 
then  extracted  with  water,  and  the  quantity  of  barium  dissolved  is  a 
measure  of  the  original  amount  of  free  hydrochloric  acid.  The  bar- 
ium may  be  estimated  by  Mohr's  titration  method.  In  this  method, 
potassium  dichromate  is, added  to  the  barium  solution,  by  which 
means  a  precipitate  insoluble  in  water  and  acetic  acid  is  formed;  the 
indicator  of  the  end  of  the  reaction  is  the  yellow  color  which  the 
smallest  excess  of  the  dichromate  gives  to  the  liquid  which  floats  over 
the  precipitate.  A  more  delicate  test  for  excess  of  the  dichromate  is, 
however,  Wurster's  tetramethylparaphenylenediamine  paper.  Potas- 
sium dichromate  in  an  acetic  acid  solution  acts  in  the  same  way  as 
ozone,  to  test  for  which  the  paper  was  originally  used ;  it  turns  it 
blue. 
