368  COLLEGE   CHEMISTRY

The  Sulphate  and  Bisulphate.  —  Potassium  sulphate  K2S04
is  a  constituent  of  several  double  salts  found  in  the  Stassfurt  de-
posits. It  is  extracted  from  schoenite  MgSO4,K2S04,6H2O  and
kainite  MgS04,MgCl2,K2SO4,6H20.  The  former  is  treated  with
potassium  chloride  and  comparatively  little  water,  whereupon  the
relatively  insoluble  potassium  sulphate  crystallizes  out,  and  the
magnesium  chloride  remains  in  the  mother-liquor.  The  crystals
belong  to  the  rhombic  system,  contain  no  water  of  crystallization,
and  melt  at  1066°.  This  salt  is  employed  in  preparing  alum  (q.v.)
and  is  much  used  as  a  fertilizer.  Since  plants  take  up  so-
lutions through  their  cell  walls,  they  can  absorb  soluble  compounds
only.  They  are,  therefore,  dependent,  for  the  potassium  com-
pounds which  they  require,  upon  the  weathering  out  of  soluble
potassium  compounds  from  insoluble  silicates  containing  potas-
sium (p.  348)  found  in  the  soil.  The  weathering  takes  place  too
slowly  to  furnish  a  sufficient  supply  for  many  crops,  particularly
that  of  the  sugar-beet.  Hence  potassium  sulphate  is  mixed
directly  with  the  soil.
Potassium-hydrogen  sulphate  (bisulphate)  KHS04  is  made  by  the
action  of  sulphuric  acid  upon  potassium  sulphate:  K2SO4  +  H2S04  — »
2KHS04.  It  crystallizes  from  water,  in  which  it  is  very  soluble,  in
tabular  crystals.  Its  properties  are  similar  to  those  of  sodium
bisulphate  which  have  already  been  described  (p.  265).
Sulphides  of  Potassium.  —  By  the  treatment  of  a  solution  of
potassium  hydroxide  with  excess  of  hydrogen  sulphide,  a  solution  of
potassium-hydrogen  sulphide  is  obtained.  Evaporation  of  the  solu-
tion gives  a  deliquescent,  solid  hydrate  2KHS,H2O.  When  the
solution,  before  evaporation,  is  treated  with  an  equivalent  amount
of  potassium  hydroxide,  and  the  water  is  driven  off,  the  sulphide
K2S  remains  behind  (cf.  p.  253) :

KHS  +  KOH  <=±  K2S  +  H20.
