556  Inorganic  Chemistry  of  the  U.S.P.  {Aveclmbe*w™' 
of  other  inorganic  substances,  with  the  exception  of  water.  The 
1890  Pharmacopoeia  requires  it  to  contain  90  per  cent.,  as  does  also 
the  German  Pharmacopoeia,  in  which  it  is  called  Kali  causticum 
fusum.  Neither  the  new  Pharmacopoeia,  nor  that  of  1890,  nor  the 
German,  give  a  method  for  its  preparation,  though  at  least  three 
previous  decennial  revisions  of  the  U.S.P.  directed  it  to  be  made  by 
evaporating  the  liquor  potassae  in  iron  or  in  silver  vessels,  and  pour- 
ing the  fused  mass  into  suitable  molds.  Methods  for  making  liquor 
potassae  were  given  in  each  of  these  Pharmacopoeias  and  in  that  of 
1890.  The  method  of  preparing  this  solution  consisted  in  reacting 
upon  potassium  carbonate  with  slaked  lime,  and  adding  a  sufficient 
quantity  of  distilled  water  to  make  a  5  per  cent,  solution.  The  re- 
action is  thus  illustrated :  K2C03  -f  Ca(HO)2  =  2KHO  +  CaC03. 
In  the  British  Pharmacopoeia  potassium  carbonate  is  used  directly, 
but  in  the  U.S.P.,  1890,  and  before,  potassium  bicarbonate  was  em- 
ployed. The  solution  of  the  latter  was  heated  until  effervescence 
ceased,  i.  e.,  until  the  bicarbonate  was  converted  into  carbonate,  and 
then  poured  into  the  slaked-] ime  solution.  The  use  of  the  bicarbo- 
nate insured  a  purer  product.  The  carbonate  is  never  as  pure  a  salt. 
In  the  former  U.S.P.'s  the  potassa  was  made  from  the  liquor,  whilst 
in  the  present  the  liquor  is  made  from  the  dry  potassii  hydroxidum. 
A  very  pure  potassium  hydroxide  may  be  made  by  the  reaction 
between  potassium  sulphate  and  barium  hydroxide  K2S04  -f 
Ba(HO)2  =  2KHO  -f  BaS04.  When  made  this  way  it  is  called  pot- 
ash by  baryta.  The  fact  that  most  of  the  possible  impurities  of  potas- 
sium hydroxide  are  insoluble  in  alcohol,  while  the  hydroxide  itself  is 
readily  soluble,  makes  it  possible  to  obtain  a  pure  product  by  the 
use  of  alcohol.  The  commercial  product  being  digested  in  alcohol, 
the  solution  filtered,  and  the  alcohol  evaporated.  When  so  made 
it  is  called  potassa  by  alcohol. 
The  tests  for  identity  given  comprise  (1)  its  non-volatile  nature; 
(2)  its  violet-colored  flame ;  (3)  its  intense  alkaline  reaction ;  (4)  the 
yellow  precipitate  with  platinic  chloride  T.  S  ;  (5)  the  white  precipi- 
tate of  bitartrate  upon  treatment  with  excess  of  tartaric  acids  T.  S. 
The  tests  for  impurities  include  the  time-limit  test  for  heavy  metals, 
and  the  efflorescence  test  for  carbonates.  The  1890  U.S.P.  gave 
also  tests  for  calcium,  chloride,  sulphate,  silicate,  nitrate,  and  for 
soda.  The  volumetric  assay  method  is  much  more  satisfactory  than 
that  of  the  old  U.S.P.    In  the  old  process  0-56  gramme  of  potassa 
