358       CARBONATE  OF  POTASH  IN  IODIDE  OF  POTASSIUM. 
sale  declarations  which  have  recently  been  put  forth  about  the 
adulteration  of  drugs  and  chemicals. 
In  this  notice  all  mention  of  impurities  other  than  the  above 
is  omitted.  Although  the  great  bulk  of  iodide  of  potassium, 
prepared  ifor  medicinal  use,  is  sufficiently  pure  to  be  "  adapted"  for 
medical  purposes,  it  rarely  happens  that  it  is  entirely  free  either 
from  carbonate  of  potash  or  iodate  of  potash,  or  both.  The 
manufacturer  has  a  difficulty  to  perform — to  produce  an  article 
not  only  pure,  but  in  well-defined  crystals  and  free  from  color. 
A  compound  consisting  of  nothing  more  than  iodine  and  potas- 
sium, united  in  their  equivalent  proportions,  is  not  very  stable 
in  respect  of  color ;  being  somewhat  hygroscopic,  water  as  well 
as  carbonic  acid  is  absorbed  from  the  air,  hydriodic  acid  and 
carbonate  of  potash  are  formed,  and  the  salt  acquires  a  yellow 
tinge,  a  change  which  is  the  more  noticeable  if  it  be  in  a  state 
of  powder,  in  which  form  it  is  used  in  some  establishments?  having 
been  prepared  by  the  purification  of  the  commercial  article,  the 
solution  thus  obtained  being  evaporated  to  dryness. 
In  purchasing  the  salt,  although  it  may  be  always  expected 
that  the  bodies  before  mentioned  will  be  present,  it  is  of  course 
desirable  to  select  the  sample  which  contains  them  in  the  smallest 
proportion — any  test  which  shall  enable  us  to  do  this  quickly  and 
with  certainty,  may  not  be  unwelcome. 
The  reaction  on  litmus  paper,  effervescence  with  a  dilute  acid, 
turbidity  of  the  solution  with  chloride  of  barium  or  lime  water 
(which,  when  "slight,"  according  to  the  late  Mr.  Philips,  "need 
not  be  noticed"),  and  the  formation  of  a  bluish  precipitate  with 
protosulphate  of  iron, — indicating  potash  or  its  carbonate  ;  and 
dilute  hydrochloric  acid  for  the  iodate,  really  leave  little  to  de- 
sire. The  protosulphate  of  iron,  although  an  excellent  test — 
the  most  delicate  of  those  enumerated — has  not  the  advantage 
of  the  chemical  constitution  and  relationship  possessed  by  the 
compound  now  proposed — the  protiodide  of  iron,  in  the  form  of 
the  recently  and  carefully  prepared  Syrupus  Ferri  lodidi  P.  L. 
The  usual  precautions  as  to  equal  quantities  and  similar  cir- 
cumstances being  observed,  a  drop  of  the  syrup  is  to  be  added  to 
each  sample  dissolved  in  distilled  water  ;  a  bluish  precipitate  is,  in 
most  cases,  formed  at  once,  which  retains  this  color  for  some  little 
time — the  peroxidation  of  the  iron  being  somewthat  retarded  by 
