404 
Solution  of  Potassium  Arsenite. 
{ 
A.m.  Jour.  Pharm. 
August,  1895. 
grammes  of  arsenous  acid  and  20  grammes  of  potassium  bicarbon- 
ate with  100  c.c.  of  water,  until  solution  is  effected,  when  cold,  water 
is  added  to  make  970  c.c,  and  then  30  c.c.  of.  compound  tincture  of 
lavender. 
It  is  to  be  noted  in  this  formula  that  the  amount  of  potassium 
bicarbonate  is  double  that  of  the  Pharmacopoeia  of  1880.  The 
weight  directed  by  the  present  authority  practically  agrees  with  the 
quantity  required  for  a  reaction,  which  may  be  represented  thus  : 
The  view  generally  taken,  however,  as  to  the  nature  of  the  arseni- 
cal salt  present,  is  that  it  is  a  mono-potassium  arsenite,  the  reaction 
being  : 
And  with  the  quantities  required  for  this  reaction  those  which 
were  stated  in  the  Pharmacopoeia  of  1880  practically  coincide. 
It  is,  of  course,  well  understood  that  neither  of  these  reactions 
are  fully  realized,  and  upon  testing  a  volume  of  20  c.c.  of  a  freshly- 
made  official  solution  (in  the  production  of  which  -400  gramme  of 
potassium  bicarbonate  had  been  used)  in  a  carbon  dioxide  appara- 
tus for  excess  of  carbonate  undecomposed  by  the  action  of  the 
arsenous  acid,  a  loss  of  -115  gramme  of  that  gas  was  experienced. 
The  same  process  was  carried  out,  with  varying  results,  upon  solu- 
tions which  had  been  kept  for  longer  periods,  but  the  evidence  was 
abundant  proof  of  the  fact  that  solution  of  the  arsenic  trioxide  can 
take  place  even  before  boiling  has  converted  all  of  the  potassium 
bicarbonate  into  carbonate. 
Garraud's  recommendation  consists  in  the  use  of  potassium 
hydrate  instead  of  the  potassium  carbonate  directed  by  the  French 
Codex,  or  the  purer  salt,  potassium  bicarbonate,  ordered  in  the 
United  States  Pharmacopoeia. 
The  reactions  would  then  be  expressed  as  follows  : 
As203  +4KHCO3  =  2K2HAs03  +  4CO,  +  H20 
197-68  399-52 
As203  +  2KHCO3  +  HoO  =  2KH2\s03  + 
197-68  19976 
f  2C02 
As203  -f  4KOH  =  2K2HAs03  +  H20 
197-68  223-96 
or, 
As203  +  2KOH  +  H20  =  2KH2As03 
19768  111-98 
