20 
Note  on  Sulphur  Dioxide. 
( Am.  Jour.  Pharm. 
(    January,  1911. 
the  method  of  Kastle  and  McHargue.^  This  method  depends  on 
the  reaction  expressed  by  the  equation 
I2  +  SO,  +  2H,0  =  2HI  +  ; 
from  which  it  is  seen  that  for  every  equivalent  of  iodine  two  equiva- 
lents of  acid  result.  Hence  by  determining  both  the  amount  of 
iodine  used  up  in  the  reaction  and  also  the  total  acidity  after  the 
reaction,  we  can  estimate  both  the  SO,  and  SO3,  since  the  found 
excess  acidity  over  that  required  by  the  above  equation  would  be 
a  measure  of  the  SO3  originally  present.  In  the  work  of  Kastle  and 
McHargue,  the  exact  strength  of  the  thiosulphate  solution  was 
determined  by  means  of  Kahlbaum's  resublimed  iodine  immediately 
before  each  experiment  and  the  exact  strength  of  the  iodine  solution 
determined  by  means  of  the  thiosulphate.  The  exact  strength  of  the 
sodium  hydroxide  was  frequently  determined  by  means  of  weighed 
amounts  of  pure  oxalic  acid. 
In  this  connection,  it  occurred  to  the  writer  that  the  above  reac- 
tion between  SO,  and  iodine  might  be  used  with  advantage  in 
checking  the  equivalencies  of  the  standard  iodine  and  hydroxide 
solutions ;  and  if  instead  of  using  an  approximate  amount  of 
potassium  iodide  ("about  18  Gm."  ^  per  litre)  in  the  preparation 
of  the  iodine  solution,  an  accurately  weighed  amount  of  KI  (say 
16.602  Gm.^  per  litre)  were  used,  the  purity  of  which  has  been 
accurately  ascertained  by  means  of  the  standard  silver  solution,  the 
values  of  the  iodine  and  hydroxide  solutions  could  be  directly  com- 
pared, in  the  same  solution,  with  the  standard  silver  solution; 
which,  as  pointed  out  by  the  writer,^  might  be  used  with  advantage 
in  the  standardization  of  all  of  the  volumetric  solutions  ordinarily 
employed.  And,  since  the  value  of  the  permanganate  solution  can 
be  ascertained  by  titrating  with  thiosulphate  (the  strength  of  which 
is  measured  by  the  iodine  solution)  the  amount  of  iodine  which  a 
definite  amount  of  the  KMnO^  can  liberate  from  a  solution  of  KI 
acidified  with  sulphuric  acid,  if  we  prove  the  accuracy  of  our  iodine 
solution  in  terms  of  the  silver  solution  we  can  thus  also  check  the 
accuracy  of  the  value  assigned  to  the  permanganate  solution.  In 
Amer.  Chem.  Jour.,  38,  465-475  (1907);  and  Chem.  News,  96,  236-238 
(1907). 
"  Sutton :  Volumetric  Analysis,  9th  ed.,  p.  133. 
^  This  amount  of  KI  per  litre  would  yield  a  solution  exactly  tenth-normal 
(using  the  1910  international  atomic  weights). 
^  Amer.  Jour.  Pharm.,  82,  203-211  (1910). 
