500  Commercial  Iodide  of  Potassium.  {"^"'ocT'^iS?'^"'* 
tions.  The  German  Pharmacopoeia  prudently  specifies  the  strength  of 
the  solution  (1  to  20).  Biltz^  found  that  in  solution  of  1  to  20,  the 
addition  of  dilute  sulphuric  acid,  without  starch,  gave  a  pale  yellow 
color  when  only  O'l  to  0*01  per  cent,  of  iodate  was  present,  while  in 
much  more  dilute  solutions  as  large  a  quantity  as  0*5  per  cent,  could 
be  present  before  a  full  brown  color  was  developed  on  addition  of  the 
acid.  Iodate  of  potassium  is  well  known  to  be  poisonous.  Most 
reducing  agents,  including  many  organic  bodies,  liberate  iodine  from 
an  iodate,  and  with  help  of  an  iodide  all  acids  at  once  cause,  as  in  the 
test,  the  liberation  of  iodine  in  amount  six  times  that  in  the  iodate.^ 
Even  atmospheric  carbonic  acid  gradually  effects  this  change.  And  1 
part  of  potassium  iodate,  with  excess  of  iodide  and  with  free  acids, 
furnishes  3*5  parts  of  free  iodine.  Free  iodine,  as  an  impurity  in  the 
salt  in  hand,  or  as  a  product  in  the  stomach,  is  alike  objectionable.  In 
one  method  of  manufacture  of  iodide  of  potassium,  iodate  is  formed 
and  afterward  reduced.  Also,  pure  iodide,  exposed  for  an  hour  to 
melting  heat,  acquires  a  trace  of  iodate  and  an  alkaline  reaction. 
Manufacturers  are  not  likely  to  be  so  careless  as  to  neglect  due  reduc- 
tion of  iodate  when  this  is  a  considerable  intermediate  product  of  their 
process,  but  they  may  neglect  needful  precautions,  or  additional  work, 
to  remove  final  traces  of  this  impurity,  especially  when  only  such 
traces  are  formed  in  the  method  employed.  When  potassium  iodide  is 
manufactured  free  from  iodate,  I  do  not  believe  this  impurity  is  ever 
acquired  by  atmospheric  action  in  keeping,  even  if  in  solution.  The 
pure  salt  is  not  alfected  by  the  air  when  dry,  and  when  moist  is  aifected 
no  further  than  a  possible,  very  slight  liberation  of  hydriodic  acid,  and 
then  of  free  iodine,  with  corresponding  formation  and  preservation  of 
alkali  carbonate. 
(2.)  The  Limit  of  Alkalinity. — "  .  .  .  Generally  .  .  .  having  a 
faintly  alkaline  reaction ;  but  single  crystals  laid  upon  moistened  red 
litmus  paper  should  not  at  once  produce  a  violet-blue  stain  (absence  of 
more  than  about  0.1  per  cent,  of  alkali),''  U.  S.  Ph.,  1880.  "  Produces 
no  change  in  the  color  of  litmus,  and  little  if  any  in  that  of  turmeric," 
U.  S.  Ph.,  1870.  "It  commonly  has  a  feeble  alkaline  reaction.  .  .  . 
Its  aqueous  solution  is  only  faintly  precipitated  by  the  addition  of  sac- 
charated  solution  of  lime,"  Br.  Ph.  Crystals  laid  upon  moistened  red 
litmus  paper  should  not  immediately  give  a  violet-blue  color.  Ph. 
1  Notizen  zur  Pharmacopoea  Germanica  (1878),  p.  181. 
2  SKI  +  KIO3  +  6HC1  =  61  +  6KC1  +3H2O. 
