Am.  Jour.  Pharm. 
Dec.,  1889. 
IJ [fusion  of  Acids  and  Bases. 
615 
DIFFUSION  OF  ACIDS   AND  BASES   INTO  ONE 
ANOTHER.1 
By  J.  Stefan. 
When  very  dilute  ammonia  is  placed  over  a  column  of  dilute  hy- 
drochloric acid,  the  surface  of  separation  between  the  acid  and  the 
alkaline  liquids  remains  well  defined,  as  can  be  easily  observed  if 
the  solutions  are  colored  with  litmus,  but  the  region  occupied  by  the 
acid  gradually. extends  upwards.  In  one  experiment,  a  solution  con- 
taining 1  gram-molecule  of  hydrochloric  acid  per  litre  was  covered 
with  a  solutiou  of  ammonia  containing  one-sixteenth  of  a  gram-mole- 
cule; the  surface  of  separation  rose  8*2,  Vro,  and  24'5  mm.  in  one, 
four,  and  nine  hours  respectively,  the  rise  being  proportional  to  the 
square  root  of  the  time  during  which  diffusion  takes  place.  With 
the  same  hydrochloric  acid,  employing  a  solution  of  ammonia  contain- 
ing J  gram-mol.,  the  surface  of  separation  rose  o,  9*9,  and  14- 7  mm. 
in  one,  four,  and  nine  hours  respectively. 
When  a  solution  containing  1  gram-mol.  of  ammonia  is  employed, 
the  region  occupied  by  the  acid  extends  upwards,  rising  1*2,  2 '3,  and 
3*5  mm.  in  one,  four,  and  nine  hours  respectively,  but  when  the  am- 
monia is  four  times  as  concentrated,  the  surface  of  separation  gradu- 
ally sinks,  and  the  depression  is  1*2,  2,o,  and  3*8mm.  in  one,  four,  and 
nine  hours  respectively.  If  ammonia  of  a  certain  concentration  were 
employed,  the  surface  of  separation  would  remain  stationary.  The 
concentration  of  the  ammonia  which  would  fulfil  this  condition  would 
lie  somewhere  between  that  of  the  solutions  employed  in  the  last  two 
experiments  ;  the  exact  strength  cannot  be  ascertained  experimentally 
except  with  great  difficulty,  but  it  can  be  shown  mathematically 
that  the  surface  of  separation  would  remain  stationary  if  solutions  of 
1  raol.  of  hydrochloric  acid  and  1*8  mol.  of  ammonia  respectively  were 
employed. 
The  phenomena  described  above  are  determined  by  the  relative 
concentrations  of  the  acid  and  alkaline  solutions;  they  are  inde- 
pendent of  their  absolute  values,  at  any  rate  as  long  as  the  latter  are 
not  very  large. 
Similar  experiments  were  made  with  solutions  containing  2  graui- 
mols.  of  hydrochloric  acid  and  O'O  gram-mol.  of  ammonia  respec- 
tively. The  region  occupied  by  the  acid  extended  5,  9'9,  and  14*8 
mm.  upwards  in  one,  four,  and  nine  hours'  time. 
'Monatsh.,  x,  201-219  ;  reprinted  from  Jour.  Chem.  Soc,  1839,  p.  1016. 
