Am.  Jour.  Pharm.  | 
January,  1911.  j 
Note  on  Sulphur  Dioxide. 
23 
8.  Found  that  50  c.c.  of  the  above  n/io  NaoSoOg  are  equivalent 
to  49.5  c.c.  of  the  n/io  iodine.  Therefore,  the  normaHty  of  the 
iodine  solution,  expressed  as  above,  is  99.55. 
Control. — Used  25  c.c.  of  the  above  n/io  iodine.  This  was  just 
decolorized  by  means  of  a  dilute,  freshly  prepared  aqueous  solution 
of  SO2.  On  titrating-  the  total  acidity  with  the  above  N/^^oNaOH.  it 
was  found  to  require  49.7  c.c.  The  solution  was  then  acidified  with 
5  c.c.  of  10  per  cent,  nitric  acid,  55  c.c.  of  the  above  n/io  AgNO^ 
added,  and  the  excess  silver  in  the  filtrate  determined  by  means 
of  the  above  n/io  NH^CNS.  In  this  way,  it  was  found  that  the 
total  iodide  in  the  solution  was  equivalent  to  49.85  c.c.  of  the  above 
n/io  AgNOs  (or  49-9  c.c.  of  an  exact  n/io  AgNOg).  Subtracting 
from  this  24.96  c.c.  (the  calculated  value,  in  terms  of  n/io  AgNO., 
of  the  added  KI  in  the  25  c.c.  of  the  iodine  solution),  it  would 
make  the  25  c.c.  of  iodine  equivalent  to  24.94  c.c.  of  an  exact 
n/io  AgNOg. 
Expressed  in  percentage  of  an  exact  n/io  solution,  we  would 
have  as  the  value  of  the  iodine  solution  by  the  SO2  control,  99.76. 
Corresponding  value  as  found  through  the  use  of  the  above- 
mentioned  intermediary  solutions,  99.55. 
On  the  basis  of  the  iodine  solution  being  99.76  per  cent,  tenth- 
normal, it  should  have  required  49.88  c.c.  of  an  exact  n/io  NaOH. 
Now,  since  only  49.7  c.c.  of  the  above  n/io  NaOH  was  required,  it 
would  show  that  the  latter  is  a  little  stronger  than  tenth-normal. 
Expressed  in  percentage  of  an  exact  n/io  solution,  we  would 
have  as  the  value  of  the  NaOH  solution  by  the  SOo  control,  100.36. 
Corresponding  value  as  found  through  the  use  of  the  above- 
mentioned  intermediary  solutions,  100.54. 
These  results  may  be  considered  as  showing  a  fair  agreement 
between  the  first  standardization  by  the  silver  solution,  through  the 
use  of  the  above-mentioned  intermediary  solutions,  and  the  SO^  con- 
trol, especially  when  it  is  remembered  that  an  error  of  one  drop  in 
any  of  the  measurements  or  titrations  would  correspond  to  a  differ- 
ence of  about  0.1  to  0.2  per  cent.  It  would  seem,  therefore,  that  the 
reaction  between  sulphur  dioxide  and  iodine  might  be  used  as  a 
convenient  check  on  the  found  equivalencies  of  the  iodine  and  hy- 
droxide solutions  in  terms  of  the  substance  chosen  as  the  ultimate 
standard — pure  metallic  silver.  • 
