510 
A  NEW  WET  ASSAY  OF  IODINE. 
2d.'  The  protiodide  of  copper,  Cu2  I,  on  being  heated  with  a 
solution  of  caustic  potash  or  soda,  or  their  carbonates,  forms 
iodide  of  potassium  or  sodium,  and  protoxide  of  copper.  (Cu2I 
+  KO=  KI  +  Cu20). 
3d.  The  protoxide  of  copper,  treated  with  a  solution  of  per- 
chloride  of  iron  and  hydrochloric  acid,  is  converted  into  per- 
oxide of  copper,  which  dissolves  in  the  acid,  the  oxygen  neces- 
sary being  taken  from  an  equivalent  amount  of  the  peroxide  of 
iron  (Fe2  Cl3  Aq.)  present. 
On  either  of  these  reactions  a  centigrade  analysis  of  soluble 
iodides  or  iodine  may  be  founded,  which  is  very  simple  and  the 
more  accurate  since  in  itself  it  furnishes  the  means  for  three 
distinct  determinations,  each  one  serving  as  control  to  the  other, 
and  at  the  end  of  the  operation  leaves  all  the  iodine  acted  on  in 
the  state  of  iodide  of  potassium  ready  for  crystallization. 
The  reagents  employed  are  the  following : 
1.  Crystallized  permanganate  of  potassa,  which  is  prepared  in 
the  most  convenient  manner  from  binoxide  of  manganese,  potash 
and  chlorate  of  potassa,  according  to  Gregory  (v.  Gmelin's  Hand- 
book, Vol.  II.),  and  which  may  be  dissolved  to  any  desired  strength 
in  distilled  water. 
2.  A  solution  of  the  sulphates  of  peroxide  of  copper  and  prot- 
oxide of  iron.  Both  the  commercial  salts  commonly  contain,  as 
their  principal  adulteration,  one  of  the  oxides  of  iron.  Neither 
will  in  the  least  endanger  the  success  of  the  assay,  if,  before 
adding  it  to  the  iodide,  the  amount  of  protoxide  of  iron  be  as- 
certained by  means  of  a  standard  solution  of  permanganate  of 
potassa.  A  perfectly  pure  protosulphate  of  iron,  however,  is 
that  obtained  in  the  preparation  of  sulphuretted  hydrogen  from 
sulphide  of  iron  and  sulphuric  acid.  Commercial  bluestone, 
again,  is  readily  purified  by  igniting  the  salt  in  the  form  of  a 
coarse  powder,  moistened  with  common  nitric  acid  in  a  Hessian 
crucible  at  a  red  heat  and  lixiviating  when  cool.  But  the 
easiest  method  of  preparing  those  sulphates  in  the  manner  re- 
quired is  to  boil  them  for  a  short  time  in  a  sufficient  quantity  of 
water,  slightly  acidulated  by  sulphuric  acid,  with  some  copper 
filings,  when  all  the  peroxide  of  iron  present  will  be  reduced  to 
protoxide  by  the  metallic  copper,  and  the  peroxide  of  copper 
formed  be  dissolved  by  the  excess  of  acid. 
