^'"ocT'iSr'"  }         Commercial  Iodide  of  Potassium.  501 
Germ.,  1882.  Giving  a  neutral  or  very  slightly  alkaline  solution,  not 
made  turbid  by  lime-water,  Ph.  Germ.,  1872.  Biltz  found  that  ^  per 
cent,  of  potassium  carbonate  would  cause  a  violet-blue  color  to  show 
at  once  on  placing  a  crystal  upon  wet  litmus  paper,  while  J  per  cent, 
would  give  a  pure  blue  color  at  once,  and  1  per  cent,  a  deep  blue  color 
at  once.  Also  that  the  lime-water  test  does  not  reveal  less  than  1  per 
cent,  of  the  alkali  carbonate,  when  the  solution  is  concentrated,  and 
even  more  than  1  per  cent,  will  escape  recognition  when  the  solution  is 
dilute.  It  is  an  easy  task  to  determine  the  percentage  of  alkali  volu- 
metrically.  In  so  doing  I  prefer  to  weigh  out  6*9  grammes  of  the 
iodide  to  be  tested,  and  use  decinormal  solutions  of  acid  and  alkali, 
when  each  cc.  of  decinormal  acid  (beyond  that  taken  up  by  the  deci- 
normal alkali)  equals  O'l  per  cent,  of  potassium  carbonate.  Excess  of 
the  acid  is  first  added,  from  the  burette,  when  the  solution  is  quickly 
brought  to  boiling,  and  immediately  titrated  back  to  neutral  point  with 
the  decinormal  alkali.  With  phenol-phthalein  as  an  indicator,  the 
number  of  cc.  of  acid  required  to  take  out  the  red  color  in  presence  of 
bicarbonate  formed  will  be  about  one-half  the  number  of  cc.  of  acid 
required  to  keep  the  reaction  acid  after  driving  off  the  carbon  dioxide. 
It  may  be  assumed  that,  unless  in  some  exceptional  case,  the  alkali 
found  is  alkali  monocarbonate.  Some  degree  of  alkaline  reaction  is  a 
pretty  general  feature  of  medicinal  iodide  of  potassium,  all  over  the 
world,  and  is  accepted,  and  now  more  definitely  limited  by  the  phar- 
macopoeias. Alkali  in  fraction  of  1  per  cent,  probably  will  not  be 
objected  to  by  the  physicians.  But  it  must  be  admitted  that  any  alkali 
at  all  is  liable  to  be  an  inconvenient  pharmaceutical  imcompatibility. 
Nearly  all  iodide  of  potassium  offered  as  "  chemically  pure  is  neutral 
in  reaction,  whatever  its  other  defects.  A  neutral  iodide,  free  from 
iodate,  if  kept  moderately  air-dry,  will  not  acquire  free  iodine  from 
atmospheric  action. 
(3.)  The  Limit  of  Chloride  and  Bromide. — If  1  gm.  of  the  salt  be 
dissolved  in  10  cc.  of  water  of  ammonia  (of  10  per  cent),  then  skaken 
with  a  solution  of  I'l  gm.  of  nitrate  of  silver  in  20  cc.  of  water,  and 
the  filtrate  be  super-saturated  with  7  cc.  of  nitric  acid  (s.  g.  1*42),  no 
cloudiness  should  make  its  appearance  within  ten  minutes  (absence  of 
more  than  than  0*5  ]jer  cent,  of  chloride  or  bromide)"  U.  S.  Ph.,  1880. 
^'  Solution  of  nitrate  of  silver  added  in  excess  forms  a  yellowish-white 
precipitate  which,  when  agitated  with  ammonia,  yields  by  subsidence 
a  clear  liquid  in  which  excess  of  nitric  acid  causes  no  turbidity,"  Br. 
