Am.  Jour.  Pharm.  \ 
April,  19 1 7.  ■> 
Assay  Processes. 
169 
In  the  assays  of  mercurous  chloride  and  iodide,  an  interesting 
phase  is  introduced.  The  salt  is  mixed  in  a  flask  with  water,  iodine 
solution  is  added,  and  potassium  iodide,  and  the  mixture  allowed  to 
stand,  with  occasional  agitation,  until  complete  solution  has  taken 
place,  and  the  excess  of  iodine  is  titrated.  With  mercurous  chloride 
the  potassium  iodide  converts  it  into  mercurous  iodide,  and  it  is  sub- 
sequently changed  into  mercuric  iodide-  by  the  oxidation  of  the 
iodine : 
HgCl     +     KI     =     Hgl     +  KC1. 
Mercurous       Potassium  Mercurous  Potassium 
Chloride           Iodide  Iodide  Chloride 
Then 
Hgl     +     I  =  Hgl2, 
Mercurous      Iodine    Mercuric  Iodide 
Iodide 
which  is  soluble  in  potassium  iodide. 
In  the  assay  of  mercury  salicylate,  the  above  method  is  also  fol- 
lowed, but  a  preliminary  reduction  to  mercurous  chloride  is  made. 
The  salt  is  first  treated  with  sulphuric  and  nitric  acids  and  digested 
on  a  water  bath  until  dissolved.  This  produces  mercuric  sulphate 
and  nitration  products  of  salicylic  acid.  The  solution  is  diluted  with 
water  and  hydrogen  peroxide  added  to  oxidize  anything  that  may 
have  a  tendency  to  reduce  the  mercury  to  a  metallic  condition.  The 
solution  is  then  treated  with  hypophosphorous  acid,  followed  by 
sodium  chloride.  The  hypophosphorous  acid  reduces  the  salt  to  the 
mercurous  state,  and  the  sodium  chloride  then  converts  it  into  mer- 
curous chloride.  The  precipitate  is  thoroughly  washed,  and  the 
precipitate  and  filter  are  returned  to  the  flask,  and  the  method  is 
then  followed  as  under  mercuric  chloride. 
POTASSIUM  PERMANGANATE  METHOD. 
There  are  quite  a  number  of  the  official  compounds,  the  strengths 
of  which  are  ascertained  by  titration  with  n/10  potassium  permanga- 
nate. Some  of  these  are  by  direct  titration  or  by  an  excess  of  the 
permanganate,  and  titrating  such  excess  with  oxalic  acid.  Others 
are  first  converted  into  oxalate  by  the  addition  of  an  excess  of 
oxalic  acid,  and  this  excess  is  subsequently  titrated  with  perman- 
ganate, or  by  conversion  of  soluble  salts  into  oxalates  by  means  of 
ammonium  oxalate.  To  the  first  class  belong  the  ferric  salts,  hy- 
drogen peroxide,  sodium  nitrite  and  perborate.    In  the  latter  class 
