512 
A  NEW  WET  ASSAY  OF  IODINE. 
Commercial  iodide  of  potassium  to  be  examined  must  be  free 
from  carbonates,  which,  if  present,  are  removed  by  the  careful 
addition  of  sulphuric  acid. 
Liquids  containing  hydrothion,  free  or  combined,  as  is  fre- 
quently the  case  with  the  kelp  or  varec-lyes,  are  evaporated  to 
dryness  with  an  excess  of  hydrochloric  acid.  (Penny). 
I.  One  equivalent  I  (126  grammes  of  the  weed-lyes,  12.6  of 
richer  liquids,  1.26  of  iodine  or  iodides  in  substance,)  is  treated 
with  a  sufficient  quantity  of  the  solutions  of  sulphates  (160 
grammes  of  Cu  OS03  (anhydrous,)  and  at  least  150  of  FeO  S03). 
The  precipitate  of  protoiodide  of  copper,  which  is  almost  white  in 
the  cold,  turns  to  light  brown  by  heating,  and  settles  readily, 
when  the  supernatant  liquid  may  be  decanted  from  the  precipi- 
tate, which  is  to  be  well  washed.  A  filtration  is,  in  most  cases, 
not  needed.  To  the  clear  liquid  a  solution  of  permanganate  of 
potassa  is  added  from  a  burette,  containing  in  100  cubic  centi- 
meters, one-fifth  of  an  equivalent  of  the  salt  to  every  2  equiva- 
lents FeO  S03  added,  :  150.  The  reaction  is  finished  when 
the  color  of  the  liquid  changes  suddenly  to  a  bright  red,  that  is 
when  all  FeO  left  is  converted  into  Fe2Oa.  The  number  of  cubic- 
centimetres  of  the  manganic  solution  not  added,  gives  directly  the 
per  centage  of  iodine  in  the  substance  to  be  tested. 
II.  The  well  washed  precipitate  of  protoiodide  of  copper  is  now 
treated  with  a  solution  of  caustic  potash,  100  cc.  of  which  are 
accurately  neutralized  by  the  same  volume  of  a  solution  of  oxalic 
acid,  containing  one  equivalent  (63)  of  the  latter.  The  difference 
in  the  amount  of  caustic  alkali  for  and  after  the  reaction  gives 
in  direct  numbers  the  per  centage  of  iodine  in  the  original  sub- 
stance. 
III.  The  brownish  protoxide  of  copper  obtained  in  II.,  which 
may  be  separated  from  the  alkaline  solution  of  iodide  of  potassium 
by  filtration  or  decantation,  is  treated  in  the  water-bath  (or  over 
the  spirit  lamp)  with  a  measured  volume  (or  weight)  of  a  con- 
centrated solution  of  perchloride  of  iron  and  hydrochloric  acid, 
avoiding  too  large  an  excess  of  either.  The  clear  liquor  is  then 
tested  by  means  of  permanganate  of  potassa,  and  if  the  strength 
of  the  solution  of  the  latter  be  such  that  the  quantity  of* FeO  (72) 
which  is  reduced  from  (80)  1  At.  Fe203  (or  Fe2  Cl3)  by  (72) 
1  At.  Cu20,  is  accurately  reoxidized  by  one  hundred  cc.  :  the 
