Am Ap?iir" i9i5arm' }  Standardisation  of  Sodium  Thiosulphate. 
155 
exactly  1000  Cc.  Unless  freshly  prepared,  its  strength  should  always 
be  determined  anew  at  the  time  it  is  used.  Transfer  the  solution  to 
glass-stoppered  vials. 
"Preparation  of  Pure  Iodine. — Heat  powdered  iodine  (Iodum, 
U.  S.  P.)  in  a  porcelain  dish  placed  over  a  bath  of  boiling  water  for 
twenty  minutes,  and  stir  it  constantly  with  a  glass  rod,  so  that  adher- 
ing moisture,  cyanogen  iodide,  and  most  of  the  iodine  bromide  and 
iodine  chloride,  if  present,  may  be  vaporized.  Then  transfer  the 
iodine  to  a  porcelain  or  other  non-metallic  mortar,  and  triturate  it 
with  about  5  per  cent,  of  its  weight  of  dry  potassium  iodide,  so  as  to 
decompose  any  remaining  iodine  bromide  and  iodine  chloride.  Then 
return  the  mass  to  the  dish,  cover  it  with  a  glass  funnel,  and  heat  the 
dish  carefully  on  a  sand-bath.  Detach  the  sublimed,  pure  iodine, 
and,  after  pulverizing  and  drying  for  twenty-four  hours  over  calcium 
chloride,  keep  it  in  well-stoppered  bottles,  in  a  cool  place." 
Since  N/10  iodine  is  the  exact  equivalent  of  N/10  Na2S203,  the 
U.  S.  P.,  on  page  550,  gives  as  an  alternative  method  of  preparing 
the  N/10  iodine  the  following: 
"  Dissolve  about  14  Gin.  of  iodine  (Iodum,  U.S.  P.)  in  a  solution 
of  18  Gm.  of  potassium  iodide  (Potassii  Iodidum,  U.  S.  P.)  in  about 
300  Cc.  of  water,  diluting  finally  to  1000  Cc.  Of  this  solution  (which 
is  too  concentrated),  carefully  measure  from  a  burette  10  Cc.  into  a 
flask,  then  add  gradually  and  cautiously,  from  a  burette,  tenth- 
normal sodium  thiosulphate  V.  S.  (shaking  constantly)  until  the 
color  of  the  solution  is  discharged.  Note  the  number  of  cubic 
centimetres  of  the  sodium  thiosulphate  V.  S.  consumed,  and  then 
dilute  the  iodine  solution  so  that  any  known  volume  of  the  latter  will 
require  for  decolorization  exactly  the  same  volume  of  the  tenth- 
normal sodium  thiosulphate  V.  S." 
This  indicated  to  me  that  the  factor  for  Na,S203  V.  S.,  stand- 
ardized by  either  potassium  dichromate  or  resublimed  iodine,  would 
be  the  same,  and  to  determine  whether  such  was  actually  the  case  the 
following  work  was  undertaken.  A  N/10  K2Cr207  V.  S.  was  made 
up  by  the  following  method  on  page  551  of  the  U.  S.  P.,  employing 
Kahlbaum's  salt : 
"  Dissolve  4.8713  Gm.  of  pure  potassium  dichromate,  which  has 
been  pulverized  and  dried  at  1200  C.  (2480  F.)  (see  Reagent  No.  85), 
in  sufficient  water  to  measure,  at  250  C.  (770  F.),  exactly  1000  Cc." 
The  Na2S303  V.  S.  standardized  against  this  had  the  factor 
recorded  in  tabulation  below. 
