38  Analysis  of  the  Gastric  Contents.  {^a2rry5.m' 
after  shaking.  The  presence  of  the  red  color  shows  that  all  the 
acid  present  has  been  neutralized. 
By  multiplying  the  number  of  cubic  centimetres  of  decinormal 
soda  solution  used  {x)  by  0  00365  gramme,  we  obtain  the  total 
acidity,  in  equivalents  of  hydrochloric  acid,  of  the  10  cubic  centi- 
metres of  contents,  and  from  this  result  the  per  cent,  or  per  mille 
amount  of  acid  is  estimated. 
The  estimation  of  the  total  acidity  due  to  free  acids  and  acid  salts 
(&)  is  performed  in  the  same  manner,  using  Congo  red  as  the  pre- 
liminary indicator  and  alizarin  as  the  final  indicator. 
Congo  red  paper,  prepared  by  dipping  filter  paper  into  a  strong 
aqueous  solution  of  Congo  red,  turns  a  clear  blue  color  in  the  pres- 
ence of  free  HC1,  purple  in  the  presence  of  organic  acids,  and  a  dull 
brown  with  acid  salts.  In  the  presence  of  combined  acids  it  does 
not  change  color.  In  this  test  the  decinormal  solution  is  added  to 
a  given  amount  of  the  contents  until  a  drop  of  the  mixture  fails  to 
give  any  change  of  color  to  the  Congo  red.  The  reading  taken  at 
this  point  is  recorded  as  the  preliminary  record  of  the  amount  of 
free  acids  plus  acid  salts  present. 
An  aqueous  solution  of  alizarin  monosulphonate  of  soda  (1  per 
cent.)  of  a  yellowish  color  gives  no  change  of  color  when  added  to 
a  solution  containing  free  acids  or  acid  salts,  but  gives  a  purple  color 
in  alkaline  solution  or  in  an  acid  solution,  the  acidity  of  which  is 
due  to  combined  acids  alone.  Decinormal  soda  solution  is  added 
to  a  portion  of  the  contents,  to  which  two  or  three  drops  of  a  I  per 
cent,  solution  of  alizarin  has  been  added,  until  a  purple  color  is 
obtained.  The  appearance  of  this  color  marks  the  neutralization  of 
the  free  acids  and  acid  salts.  It  is  the  final  record  of  the  amounts 
of  these  substances. 
The  alizarin  test  is  the  more  accurate,  and  should  be  taken  as  the 
final  index,  where  it  is  possible  to  get  an  accurate  result  with  it. 
The  reason  for  employing  both  the  Congo  red  and  alizarin  in 
this  estimation  is  that  it  is  impossible  in  some  cases  to  make  an 
accurate  reading  by  the  alizarin  test,  the  point  of  transition  from  a 
yellow  to  a  purple  color  not  being  abrupt.41 
In  such  cases  the  record  by  the  Congo  red  test  must  be  used- 
The  reading  by  this  test  will  always  be  less  than  the  alizarin  test  by 
about  o-i  to  0-3  c.c.  decinormal  solution  to  10  c.c.  contents,  as  the 
41  Mohr  :  Zeitsch.f.  Physiolog.  Chemie,  Bd.  XIX,  H.  6,  s.  67. 
