A  NEW  WET  ASSAY  OF  IODINE. 
511 
A  solution  of  this  kind,  slightly  acid  and  with  some  metallic 
copper  immersed  in  it,  will  keep  any  length  of  time  without 
showing  the  least  formation  of  peroxide  of  iron,  all  that  might 
be  formed  by  atmospheric  influences  being  immediately  upon  its 
formation  reduced  by  the  copper  ;  besides,  the  amount  of  sul- 
phate of  copper  does  not  come  into  consideration,  since  only 
that,  of  protoxide  of  iron  is  required  to  be  constant. 
3.  A  solution  of  caustic  potash,  the  strength  of  which  is  as- 
certained on  alkalimetric  principles  (by  oxalic  acid,  see  below).  It 
may  be  kept  perfectly  free  from  carbonic  acid  on  proceeding  in 
the  following  manner,  as  proposed  by  Mohr.  The  bottle  con- 
taining the  liquor  is  closed  with  a  good  cork  boiled  in  wax.  This 
holds  a  common  chloride  of  calcium  tube,  open  at  both  ends  and 
filled  with  a  mixture  of  caustic  lime  and  glauber  salt,  which,  ac- 
cording to  Graham,  entirely  prevents  the  passage  of  atmospheric 
air,  or,  what  in  this  case  is  the  same,  carbonic  acid,  to  reach  the 
caustic  liquor.  From  this  bottle  any  desired  quantity  of  potash 
may  be  drawn  without  opening  it,  by  fitting  into  it  a  glass  tube, 
bent  so  as  to  form  a  syphon,  the  inside  arm  of  which  reaches  to 
the  bottom  of  the  bottle,  and  the  outer  somewhat  below  that, 
having  one  of  Mohr's  brass-clamps  affixed, — for  a  description  of 
which  the  reader  is  referred  to  an  elaborate  paper  on  improve- 
ments in  alkalimetry  by  the  inventor  in  Liebig's  Annalen  der 
Chemie  u.  Pharmacie,  Ixxxvi.,  129,  Dingler's  Polyt.  Journal, 
cxxxii.,  42,  also  London  Pharm.  Journ.  xiii.,  287. 
4.  A  solution  of  oxalic  acid.  For  the  advantage  of  this  over 
sulphuric  acid  we  refer  as  above. 
5.  A  solution  of  perchloride  of  iron,  which  is  prepared  free 
from  nitric  acid  or  protochloride  of  iron  by  dissolving  the  caput 
mortuum  vitrioli,  the  ferri  peroxidum  of  the  Dublin  Pharmaco- 
poeia, to  saturation  in  strong  muriatic  acid.  The  quantity  of 
peroxide  of  iron  it  contains  is  ascertained  and  regulated  by 
means  of  the  permanganate  of  Potassa,  according  to  Marguerite. 
The  following  are  the  manipulations  and  the  rationale  of  the 
assay  itself : 
When  iodine  is  to  be  tested,  it  is  obtained  in  solution,  by  heat- 
ing it  with  metallic  zinc  under  water,  in  the  form  of  iodide  of 
zinc. 
