176 
ON  LIQUOR  FERRI  IODIDI. 
it  will  precipitate  some  very  finely-divided  sulphur,  which  may 
be  redissolved  by  the  addition  of  a  little  iodine.  The  solution, 
before  filtering,  is  best  left  standing  over  night,  when  all  excess 
of  hyposulphite  will  have  been  decomposed  by  the  free  hy- 
driodic  acid  present. 
The  excess  of  iodine  and  the  hyposulphite  of  soda  are  added 
in  equivalent  proportion,  in  which  they  form  hydriodic  acid 
and  tetra-thionate  of  soda,  the  salt  of  the  acid  which  does  not 
precipitate  the  salts  of  baryta.  This  soda-salt  is  perfectly  inert, 
but  insures  the  permanency  of  the  proto-iodide  to  a  remarkable 
degree.  It  may  be  passed  through  a  number  of  filters  without 
being  oxidized.  It  may  be  sweetened  by  substituting  syrup  for 
part  of  the  water,  which,  however,  does  not  improve  its  appear- 
ance or  permanency.  The  dose  need  not  be  reduced  from  that 
of  the  officinal,  though  containing  slightly  more  iodine. 
In  addition  to  the  above  remarks,  the  writer  will  give  what 
appears  to  him  much  needed  in  our  Pharmacopoeia;  that  is, 
simple  qualitative  and  quantitative  tests  for  iodine  and  iodide 
of  potassium. 
The  presence  of  water  and  cyanide  of  iodine  in  commercial 
iodine  is  found  by  the  tests  described  in  the  Dispensatory.  The 
presence  of  chlorine  must  be  suspected  when  the  iodine  imparts 
a  brown  color  to  water.  It  is  found  by  heating  a  strong  solution 
of  a  neutral  persulphate  of  iron,  (iron  alum,)  free  from  chlorine, 
with  the  iodine,  until  no  iodine  appears  to  be  dissipated,  when 
the  solution  of  iron  will  give  a  precipitate  with  nitrate  of  silver 
if  chlorine  was  present  in  the  iodine. 
We  have  two  very  accurate  tests  for  free  iodine,  founded  on 
the  oxidation  of  arsenite  of  potassa  to  arseniate,  and  of  hypo- 
sulphite of  soda  to  tetra-thionate. 
A  test  solution  of  arsenite  of  soda  or  potassa,  for  the  use  of 
the  pharmaceutist,  is  prepared  after  the  manner  directed  for 
Fowler's  Solution,  using  70  grains  of  acid  and  140  of  bicarbon- 
ate of  potassa  to  one  quart  or  two  pounds  (avoirdupois)  of  test- 
liquid.  The  iodine  to  be  examined,  say  five  grains  accurately 
weighed,  is  placed  in  a  porcelain  mortar  with  a  little  bicarbon- 
ate of  potassa  or  soda,  and  a  drachm  or  so  of  the  solution  of 
arsenite  from  a  vial  previously  tared  on  a  balance.  A  few  drops 
of  a  solution  of  starch  are  then  added,  and  rubbed  up  with  the 
