Amka°rchr;  iP9hia7rm"}     AssaV  Processes  of  US. P.  IX.  n3 
With  aromatic  sulphuric  acid  the  same  method  is  followed,  but 
it  is  first  refluxed  six  hours,  the  object  of  which  is  to  decompose 
the  ethyl-sulphuric  acid  of  the  compound  under  examination  into 
alcohol  and  sulphuric  acid,  and  thereby  obtain  its  full  acid  content. 
In  the  assay  of  phosphoric  acid  and  phosphates  a  departure  from 
the  general  method  is  resorted  to.  About  i  Gm.  of  the  acid  is  ac- 
curately weighed  and  diluted  with  water  to  make  100  mils;  10  mils 
of  this  solution  are  transferred  to  a  100  mil  flask  and  neutralized 
with  special  KOH  T.S.  (one  free  from  chlorides),  phenolphthalein 
being  used  as  an  indicator.  To  the  solution  50  mils  n/10  silver 
nitrate  are  added  and  then  zinc  oxide,  free  from  chloride,  is  added 
in  small  portions  to  neutralize  the  liquid.  Sufficient  distilled  water 
is  added  to  make  exactly  100  mils,  and  the  whole  agitated.  The 
mixture  is  filtered  through  a  dry  filter,  and  to  50  mils  of  the  filtrate 
2  mils  of  nitric  acid  and  2  mils  iron  and  ammonium  sulphate  T.S. 
are  added,  and  the  excess  of  silver  used  is  titrated  with  n/10  KCNS. 
In  the  old  U.  S.  P.,  lactic  acid  was  assayed  with  normal  KOH, 
in  the  cold.  The  new  directions  are  that  after  the  addition  of  the 
alkali,  the  solution  be  boiled  20  minutes.  This  is  to  convert  the 
lactones,  or  inner  anhydrides,  into  the  lactate.  The  purity  rubric  of 
the  old  U.  S.  P.  was  75  per  cent.  By  the  new  method  a  strength 
of  85  per  cent,  results,  including  both  the  lactic  acid  and  its  an- 
hydrides. 
In  the  assay  of  benzoic  and  salicylic  acids,  n/10  barium  hy- 
droxide is  substituted  for  the  KOH,  phenolphthalein  being  used 
as  an  indicator.  The  end  reaction  with  alkali  hydroxide  is  not 
sharp,  owing  to  the  carbonate  it  usually  contains  and  because  the 
titration  is  conducted  in  the  cold.  Barium  hydroxide  has  been  sub- 
stituted to  overcome  this. 
There  are  several  salts  of  the  pharmacopoeia  that  are  assayed 
by  acidimetry  or  alkalimetry:  by  direct  titration  without  inter- 
mediate steps.  Under  the  acidimetric  method  is  potassium  bitar- 
trate.  Under  the  alkalimetric  method  are  the  following:  am- 
monium carbonate  and  hydroxide;  potassium  hydroxide,  bicarbo- 
nate and  carbonate;  magnesium  oxide,  carbonate  and  hydroxide; 
sodium  borate,  carbonate,  hydroxide,  cacodylate  and  glycerophos- 
phate; zinc  stearate,  carbonate  and  oxide. 
With  compounds  such  as  zinc  stearate  the  process  slightly  varies. 
They  are  first  boiled  with  an  excess  of  n/10  sulphuric  acid,  which 
