304  Preparation  of  Arsphenamine.     {Am'MTa°y  r'ijih9arm" 
residue  found  in  some  hydrosulphites,  no  further  filtering  or  removal 
of  extraneous  coloring  matter  is  neecssary. 
2.  Preparation  of  the  Dihydro chloride. — The  crude  diaminodi- 
oxyarsenobenzene,  after  filtering  and  washing  with  distilled  water  at 
o°  C,  is  transferred  to  a  porcelain  dish  and  suspended  in  400  Cc. 
of  distilled  water,  likewise  at  o°  C,  where  it  is  dissolved  in  the  least 
quantity  of  two  normal  sodium  hydroxide,  usually  about  150  Cc, 
which  should  be  at  o°  C.  We  find  it  convenient  to  prepare  ail  the 
necessary  solutions  two  or  three  days  ahead  of  time  and  to  keep 
them  in  an  ice-box  or  refrigerator,  at  the  end  of  which  time  they 
will  be  cold  enough  for  the  purpose. 
The  alkaline  solution  of  the  base  is  now  filtered  so  as  to  remove 
any  insoluble  matter,  like  filter  paper ;  any  fibers  of  dust,  and  also, 
a  small  amount  of  the  mineral  precipitate,  mentioned  before,  which 
is  present  when  some  hydrosulphites  are  used.  For  this  purpose  we 
have  constructed  a  simple  form  of  anaerobic  filter,  which  is  shown 
in  Fig.  1. 
To  the  perfectly  clear  alkaline  solution  of  the  arsphenamine,  150 
Cc.  of  strong  hydrochloric  acid  (1-1)  at  o°  C.  are  added,  which 
throws  out  and  re-dissolves  the  yellow  base.  The  clear  solution, 
resulting  from  the  addition  of  the  hydrochloric  acid,  contains  the 
dihydrochloride,  and,  for  the  purpose  of  getting  a  fine  precipitate, 
the  solution  is  further  diluted  with  distilled  water  at  o°  C.  in 
order  to  bring  the  volume  of  the  final  solution  up  to  1,^00  Cc. 
Into  a  stirring  apparatus,  3,250  Cc.  of  hydrochloric  (acid  1—1, 
i.  e.,  1  part  concentrated  acid  plus  1  part  water)  at  o°  C."  are 
placed.  While  vigorously  stirring,  the  cold  aqueous  solution  of 
dihydrochloride  of  arsphenamine  is  run  slowly  into  this  hydro- 
chloric acid.  The  grey-white  precipitate,  now  formed,  is  allowed 
to  settle  for  an  hour.  It  is  then  filtered  in  thin  layers  and  dried 
in  a  vacuum  desiccator  at  a  low  pressure,  fused  calcium  chloride 
and  solid  sodium  hydroxide  in  sticks  or  flakes12  being  used  as 
absorbents.  It  is  preferable  to  place  the  solid  alkali  separately  in  a 
dish  surrounded  by  the  calcium  chloride.  The  heat  of  neutralization 
of  the  hydrochloric  acid,  undoubtedly,  helps  to  a  considerable  degree 
to  dry  the  substance  quickly.  After  twelve  or  more  hours  hydrogen 
is  introduced  into  the  desiccator  to  equalize  the  pressure  and  the 
12  Flakes,  containing  97  per  cent,  of  sodium  hydroxide  can  now  be  ob- 
tained for  about  6  cents  a  pound. 
