i8 
Note  on  Lactic  Acid. 
(  Am.  Jour.  Pharm. 
\    January,  IBll. 
It  appears  from  the  results  given  in  the  above  Tables  that  the 
titration  of  lactic  acid  at  a  boiling  temperature  is  not  only  avoidable, 
but  may  even  yield  varying  and  considerably  lower  results  than  by 
the  residual  titration  process.  Further,  the  residual  titration  process 
is  applicable  even  to  dilute  (about  tenth-normal)  aqueous  solutions 
of  the  lactic  acid  when  the  amount  of  alkali  added  is  about  twice 
the  amount  necessary  to  neutralize  all  the  lactic  acid,  and  that  prac- 
ticably as  close  results  may  be  obtained  even  if  the  mixture  is  not 
boiled  but  simply  allowed  to  stand  at  ordinary  temperature  for  half 
an  hour  before  titrating  the  excess  alkali ;  all  of  the  samples  of  lactic 
acid  examined  showing  a  greater  acid  strength  even  by  the  latter  pro- 
cedure than  when  titrating  at  a  boiling  temperature  and  taking  the 
end-reaction  as  that  point  when  the  color  of  the  indicator  will  not 
completely  disappear  even  by  continuing  the  boiling  for  ten  minutes 
longer.  These  results  also  show  that  the  residual  titration  process 
indicates  an  acidity  which  is  quite  in  harmony  with  the  U.S. P. 
specific  gravity  requirement;  while  the  fact  that  the  present  U.S. P. 
acidity  requirement  of  75  per  cent,  is  only  comparatively  little 
above  that  which  most  of  the  samples  examined  showed  (about 
72  per  cent.)  on  direct  titration  at  ordinary  temperature  (procedure 
No.  i)  and  considerably  below  that  shown  by  procedures  Nos.  2 
and  3  (about  83  to  86  per  cent.),  would  indicate  that  the  first 
appearance  of  neutrality  was  taken  as  the  end-reaction  by  the  author 
of  the  present  U.S. P.  method.  It  is  also  seen  that  none  of  the 
eight  samples  examined  showed  an  acid  strength  of  less  than  87 
per  cent,  when  assayed  by  the  residual  titration  process.  A  require- 
ment of  not  less  than  85  per  cent,  instead  of  the  present  requirement 
of  not  less  than  75  per  cent,  would,  therefore,  come  nearer  the 
actual  strength  of  the  lactic  acid  on  the  market  and  would  also 
fairly  harmonize  this  requirement  with  the  specific  gravity  require- 
ment. Finally,  it  may  be  mentioned  in  this  connection  that,  as  shown 
by  Utz,^  lactic  acid  is  volatile  with  water  vapor  and  hence  for  this 
reason  also  it  would  seem  best  to  avoid  boiling  its  aqueous  solution 
when  it  is  desired  to  estimate  its  quantity.  Likewise,  from  the  point 
of  view  of  the  time  required  for  the  assay,  the  residual  titration 
process  is  certainly  not  at  a  disadvantage  in  the  comparison.  For 
when  titrating  at  a  boiling  temperature,  from  about  twenty  minutes 
(procedure  No.  2)  to  about  forty  minutes  (procedure  No.  3)  was 
''Chem.  Zeit,  29,  363-364  (iQOS). 
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