Assay  Processes. 
Am.  Jour.  Pharm. 
April,  1917. 
The  next  step  is  the  liberation  of  iodine  by  the  free  bromine: 
2KI     +     Br2     =     2KBr     +  I2. 
Potassium        Bromine  Potassium  Iodine 
Iodide  Bromide 
In  the  last  stage,  the  decolorization  of  the  iodine  by  the  sodium  thio- 
sulphate : 
2Na2S2Os     +     I2    =     2NaI     +  Na2S406. 
Sodium  Iodine  Sodium  Sodium 
Thiosulphate  Iodide  Tetrathionate 
Thymol  iodide,  C20H24O2I2.  The  assay  of  this  compound  differs 
from  general  methods.  The  thymol  iodide  is  mixed  with  sodium 
carbonate  and  heated  in  a  crucible  until  carbonized.  The  residue  is 
extracted  with  water  and  washed  on  a  filter  until  an  opalescence  is 
no  longer  formed  with  silver  nitrate.  The  solution  is  heated  and 
treated  with  potassium  permanganate  until  a  faint  pink  color  re- 
mains. Sufficient  alcohol  is  added  to  remove  the  pink  tint,  and  the 
solution  cooled.  Water  is  added  to  make  200  mils.  It  is  mixed 
and  filtered.  To  100  mils  of  the  nitrate,  potassium  iodide  is  added, 
and  an  excess  of  sulphuric  acid,  and  the  liberated  iodine  is  titrated. 
The  principles  underlying  the  above  are  the  following:  The  iodine 
of  the  thymol  is  converted  into  sodium  iodide  when  heated  with  the 
sodium  carbonate.  The  mixture  is  converted  into  sodium  iodate  by 
the  potassium  permanganate.  The  action  of  the  sulphuric  upon  the 
sodium  iodate  and  potassium  iodide  results  in  the  formation  of 
hydriodic  acid  and  iodic  acid,  and  these  reacting  upon  each  other 
liberate  iodine. 
Sodium  arsenate.  A  weighed  amount  of  the  salt  is  dissolved 
in  water,  and  heated  to  8o°  C.  HQ  is  added  and  potassium  iodide. 
After  standing  for  some  time,  the  liberated  iodine  is  titrated.  In 
this  assay  the  following  reactions  take  place :  The  HC1  decomposes 
the  salts,  forming  their  respective  acids : 
2KI  +  Na2HAs04  +  4HCI  ==  2KCI  +  2NaCl  +  2HI  +  H3As04. 
Potassium     Sodium    Hydrochloric  Potassium  Sodium  Hydriodic  Arsenic 
Iodide      Arsenate        Acid        Chloride  Chloride     Acid  Acid 
The  acids  then  react  upon  each  other: 
2HI     +     H3As04     =     I2     +     H3As03      +  H20. 
Hydriodic  Arsenic  Iodine  Arsenous 
Acid  Acid  Acid 
