1 66  Assay  Processes.  { ^ vJ «gf  *rm- 
ASSAY  PROCESSES  OF  THE  U.  S.  P.  IX. 
By  Philip  Asher,  Ph.G.,  M.D. 
(Concluded  from  page  121.) 
IODOMETRY. 
Iodometric  methods  are  followed  in  quite  a  number  of  U.  S.  P. 
assays.  These  may  be  divided  into  two  classes.  First,  those  in 
which  direct  addition  of  n/10  iodine  forms  with  the  substance  under 
examination  a  definite  compound,  and  the  completion  of  the  reaction 
is  shown  by  the  production  of  a  blue  color  with  starch.  Secondly, 
those  in  which  either  an  excess  of  iodine  is  added,  or  iodine  is 
liberated  by  the  addition  of  potassium  iodide,  and  the  liberated 
iodine  titrated  with  sodium  thiosulphate. 
To  class  one  belong  the  assays  of  tartar  emetic,  arsenic  trioxide 
and  sodium  thiosulphate. 
Antimony  and  potassium  tartrate.  .5  Gm.  of  the  salt  is  dis- 
solved in  30  mils  of  water,  to  which  are  added  25  mils  cold  saturated 
sodium  bicarbonate  solution,  and  starch  as  an  indicator.  This  is 
immediately  titrated  with  n/10  iodine. 
2KSbO(C4H4Oe)  +  I4  +  8NaHCOs 
Tartar  Emetic      Iodine  Sodium 
Bicarbonate 
=  2NaSbOs  +  4NaI  +  2KNaC4H406  +  8C02  +  5H.O. 
Sodium        Sodium  Potassium 
Meta  Antimonate  Iodine        Sodium  Tartrate 
Arsenic  trioxide.  .2  Gm.  arsenic  oxide  accurately  weighed  is 
dissolved  in  20  mils  of  hot  water  by  the  gradual  addition  of  NaOH 
T.  S.  This  is  neutralized  with  dilute  sulphuric  acid.  To  the  cooled 
solution,  sodium  bicarbonate  is  added  and  titrated  with  iodine. 
As2Os  +  H20  +  4I  +  4NaOH  =  2H3As04  +  4NaI. 
Arsenic  Trioxide  Iodine     Sodium         Arsenic  Sodium 
Hydroxide        Acid  Iodide 
In  the  second  class  are  the  assays  of  the  ferric  salts,  crude  cal- 
cium sulphide,  chromium  trioxide,  iodine,  acetone,  sodium  bisul- 
phite and  arsenate,  phenols,  thymol  iodide,  mercury  salicylate,  and 
mercurous  iodide  and  chloride. 
