AnkJour;Jnharm I        A  Routine  Test  for  Sulphites. 
Dec,  1921.        )  J  1 
867 
A  ROUTINE  TEST  FOR  THE  PRESENCE  OF  SULPHITES.* 
By  Albert  E.  Parkes. 
The  following  method  has  been  found  to  be  a  useful  routine  one 
for  the  detection  of  sulphites  added  as  a  preservative  or  bleaching' 
agent  to  foodstuffs,  confectionery,  and  other  goods. 
It  is  a  modification  of  the  combined  methods  of  Schmidt  (Ar~ 
beiten  aits  dem  Kaiserlichen  Gesundheitsamte,  21,  226)  and  of  Win- 
ton  and  Bailey  (/.  Amer.  Chem.  Soc,  1907,  29,  1499),  and  in  prac- 
tice has  been  found  to  be  speedy,  sensitive,  and  efficient,  without  the 
disadvantages  of  the  better-known  method  of  reduction  by  means  of 
zinc  to  hydrogen  sulphide  (U.  S.  Dept.  Agr.  Bui.,  107,  A.  O.  A.  C). 
.  Ten  grms.  of  the  material,  such  as  dried  fruit  or  minced  meat 
or  fish,  are  incorporated  with  10  to  20  cc.  of  water,  by  means  of  a 
pestle  and  mortar,  and  transferred  to  a  small  conical  flask  of  about 
50  cc.  capacity.  In  the  case  of  fruit-pulp,  glucose-syrup  or  fruit 
juice,  10  cc.  may  be  diluted,  when  necessary,  with  10  to  20  cc.  of 
water  in  the  flask. 
Ten  cc.  of  dilute  sulphuric  acid  of  about  2N-strength  and  two 
or  three  small  fragments  of  marble  chips  are  now  introduced  into  the 
flask,  and  the  mouth  immediately  covered  with  a  piece  of  starch 
paper  (impregnated  with  a  1  per  cent,  starch  solution),  which  should 
be  screwed  round  the  neck  of  the  flask,  and  held  in  place  with  a 
rubber  ring.  The  reason  for  the  addition  of  the  marble  is  to  set 
up  a  gentle  current  of  carbon  dioxide  to  sweep  out  the  oxygen  and 
the  liberated  sulphur  dioxide.  The  top  of  the  paper  is  moistened 
with  1  drop  of  a  1  per  cent,  solution  of  iodine. 
In  the  presence  of  any  appreciable  quantity  of  sulphites  the  blue 
stain  on  the  starch  paper  will  be  immediately  discharged  by  the 
sulphur  dioxide.  If  traces  only  be  present,  it  may  take  a  fexw  min- 
utes. The  action  takes  place  in  the  cold;  it  may  be  hastened  by 
leaving  the  flask  in  a  warm  place. 
If  the  drop  of  iodine  solution  used  be  of  the  magnitude  of  0.1 
cc.  it  is  obvious  that  the  limit  of  sensitiveness  of  the  test  is  the  quan- 
tity of  sulphur  dioxide  necessary  to  reduce  the  iodine  and  discharge 
the  blue  color — namely,  0.00025  grm. ;  and  this  is  the  limit  usually 
found  when  using  known  amounts  of  sulphites,  showing  that  prac- 
*From  The  Analyst,  October,  1921. 
