AlAu£usrta895!"m'}       Solution  of  Potassium  Arsenite.  405 
By  the  first  of  these  reactions,  10  grammes  of  arsenous  acid 
would  require  11-329  grammes  of  potassium  hydrate,  while  by  the 
second  5-665  grammes  would  be  needed,  in  both  cases  the  amounts 
being  calculated  by  proportion.  Since  the  potassium  hydrate  of 
the  market  is  of  uncertain  strength,  the  definite  weight  required 
cannot  be  adjusted  without  resorting  to  a  valuation  of  the  article. 
In  view  of  this  existing  condition,  a  convenient  way  to  supply  the 
necessary  quantity  of  the  alkali  would  be  to  add  the  requisite 
amount  in  the  form  of  a  solution  of  known  strength,  as,  for  exam, 
pie,  a  normal  volumetric  solution  (55  99  grammes  of  KOH  in  1,000 
c.c ). 
If  a  normal  volumetric  solution  was  to  be  used,  10  grammes  of 
arsenous  acid  would  require,  for  the  fulfilment  of  the  reactions,  202 
c.c.  and  101  c.c,  respectively. 
That  the  reaction  involving  the  smaller  amount  of  alkali  might 
be  accomplished,  10  grammes  of  arsenous  acid  were  added  to  101 
c.c.  of  normal  potassium  hydrate  volumetric  solution,  in  which  it 
dissolved  completely  on  slight  agitation,  and  without  the  necessity 
of  applying  heat.  To  this  solution  was  added  sufficient  water  to 
make  970  c.c,  and  then  30  c.c  of  compound  tincture  of  lavender. 
The  product  of  this  rapid  and  easy  method  was  correct  in  arsenical 
strength,  this  fact  being  established  by  the  results  of  an  application 
of  the  pharmacopceial  process  of  estimation,  while  in  physical 
appearance  and  stability  it  was  identical  with  the  product  of  the 
official  directions.  An  objection  which  may  be  urged  against  the 
general  use  of  this  process  is  that,  although  it  is  easy  and  rapid,  the 
average  pharmacist  may  not  have  at  hand  the  apparatus  necessary 
for  adjusting  a  potassium  hydrate  solution  to  a  known  strength. 
But  any  such  difficulty  can  be  obviated  by  making  use  of  the 
official  liquor  potassae,  instead  of  the  normal  volumetric  solution,  in 
which  to  dissolve  the  arsenous  acid.  The  strength  of  liquor 
potassae  is  stated  by  the  Pharmacopoeia  to  be  about  5  per  cent.,  and 
that  of  the  normal  volumetric  solution  is  practically  5  6  per  cent. 
When  10  grammes  of  arsenous  acid  were  added  to  100  c.c  of  liquor 
potassae,  it  was  found  to  dissolve  as  readily  as  did  the  same  amount 
in  101  c.c  of  the  normal  volumetric  solution. 
In  order  to  determine  the  limit  of  this  ready  solubility  an  excess 
of  arsenous  acid  was  added  to  some  liquor  potassae,  the  mixture 
agitated  for  a  few  minutes  and  then  filtered.  Five  cubic  centimetres 
