436 
Black  and  White  Mustard  Seed.  {\ 
upward  the  starch  test  by  this  method  is  exceedingly  plain  and  quite 
permanent.  An  excess  of  iodine  will  cause  the  precipitate  to  be- 
come green,  which  must  be  especially  borne  in  mind  when  testing 
for  small  quantities  of  starch. 
B.  Copper  Sulphate  and  Potassium  Iodide. 
Solutions. — (i)  Copper  sulphate  solution  (0-4  per  cent.).  Dissolve 
4  grammes  of  pure  crystallized  copper  sulphate  in  water  to  make 
1,000  c.c. 
(2)  Potassium  iodide  solution  (5  per  cent.),  5  grammes  of  crystal- 
lized pure  potassium  iodide  are  dissolved  in  water  to  make  100  c.c. 
Directions. — Put  1  gramme  of  the  powdered  black  mustard  seed 
into  a  perfectly  dry  test  tube  ;  add  10  c.c.  of  the  copper  solution  and 
boil.  Cool,  and  carefully  add,  by  means  of  a  burette,  about  y2  c.c.  of 
the  above  solution  of  potassium  iodide.  After  a  short  time,  an  exceed- 
ingly plain  and  decidedly  permanent  starch  reaction  will  be  devel- 
oped near  the  bottom  of  the  tube  in  the  presence  of  as  little  as  0  3 
per  cent,  starch. 
Remarks. — This  test  is  based  on  the  interesting  reaction  that 
takes  places  when  copper  sulphate  and  potassium  iodide  are  brought 
in  contact.  Insoluble  white  cuprous,  iodide  is  formed  and  iodine  is 
liberated  according  to  the  following  equation  : 
The  iodine  liberated  produces  the  starch  reaction. 
Yet,  the  chemical  reaction  does  not  take  place  in  too  dilute  solu- 
tions. The  concentration  of  the  liquids  and  probably  other  factors 
influence  the  speed  and  completeness  of  the  reaction. 
The  above-named  proportions  gave  the  most  satisfactory  results 
and  were  established  by  a  lengthy  series  of  experimentation. 
It  was  found  during  experimentation  with  the  foregoing  test  that 
on  boiling  powdered  mustard  seed  with  potassium  iodide  solution, 
the  formation  of  mustard  oil  is  likewise  entirely  avoided.  The  idea 
that  potassium  iodide  be  used  in  the  place  of  copper  sulphate  in  the 
same  manner  as  in  the  test  II  A.  then  suggested  itself.  On  testing 
the  boiled  liquid,  after  cooling,  with  iodine  U.S. P.  test  solution,  the 
result  proved  to  be  eminently  satisfactory.  To  such  an  extent  is 
this  method  in  our  favor  that  we  now  give  it  the  preference  over  ail 
others  in  the  testing  of  powdered  black  mustard  seed  for  starch. 
2Cu  SOd  +  4KI  =  2K2  S04  +  Cu2  I2  +  I2 
hi. 
POTASSIUM   IODIDE  METHOD. 
