Am.  Jour.  Pharm.  i 
March,  1890.  J 
Unofficial  Formulary. 
153 
of  Fe  is  sufficient  to  effect  the  decomposition,  yet,  by  taking  into 
consideration  the  conditions  of  the  experiment  and  the  various 
secondary  reactions,  he  considers  that  the  equation  :  —  KeCl3  -f 
.3KI  =  Fel2  -f  I  +  3KCI,  is  more  probably  the  correct  representa- 
tion of  the  reaction  than  the  equation  :  —  FeCl3  -f-  KI  =  FeCl2  -f- 
KC1  -f-  I.  For  the  volumetric  estimation  of  ferric  iron,  the  ratio 
Fe  to  liberated  iodine  is  alone  considered,  and  is  the  same  in  both 
equations.  The  distillation  method  is  considered  preferable  to  the 
digestion  process.  The  solution  of  potassium  iodide  is  placed  in 
the  flask,  saturated  with  carbonic  anhydride,  the  ferric  solution 
added,  and  distillation  proceeded  with  as  rapidly  as  possible  ;  the 
volatilized  iodine  being  caught  in  potassium  iodide  solution  saturated 
with  carbonic  anhydride  (to  neutralize  any  hydroxide  present).  For 
the  distillation,  it  is  convenient  to  have  the  delivery-tube  ground 
into  the  neck  of  the  flask,  so  as  to  permit  of  speedy  detachment ; 
for  the  delivery  of  the  thiosulphate,  the  author  employs  an  impro- 
vised "  stillimeter,"  on  the  principle  of  Mariotte's  bottle.  The 
"after-bluing"  of  the  starch,  sometimes  observed  in  the  titration, 
is  considered  as  due  to  the  sodium  iodide  formed  during  the  titra- 
tion, reinforcing  the  small  residue  of  potassium  iodide,  which,  in  its 
turn,  reacts  on  the  residual  ferric  chloride,  establishing  a  fresh 
equilibrium,  until  some  more  thiosulphate  is  added,  when  the  same 
reactions  take  place  again,  until  all  the  ferric  chloride  is  destroyed. 
It  is  pointed  out  that  commercial  potassium  iodide  nearly  invariably 
contains  sufficient  free  potash  to  vitiate  in  some  degree  all  iodo- 
metric  estimations  effected  with  its  aid.  The  potassium  iodide  solu- 
tion used  to  absorb  the  iodine  liberated  in  such  estimations  should 
be  supersaturated  with  carbonic  anhydride  previous  to  use. 
UNOFFICIAL  FORMULARY  ADDENDUM.1 
Acidum  Hydrocyanicum  (Scheele) — Hydrocyanic  Acid  (Scheele). 
Take  o£- 
Ferrocyanide  of  potassium,  2  '_,  oz. 
Sulphuric   acid,   1  fluid  oz. 
Distilled  water,  24  fl.  oz.,  or  a  sufficient  quantity. 
Dissolve  the  ferrocyanide  of  potassium  in  10  ounces  of  the  water,  then  add 
the  sulphuric  acid,  previously  diluted  with  4  ounces  of  the  water  and  cooled. 
Put  the  solution  into  a  flask,  to  which  are  attached  a  condenser  and  a  receiver 
1  Year-book  of Pharmacy \  1889]  published  by  the  British  Pharmaceutical  Conference. 
