Am'oSt.!iSo.arm"}      Action  of  Acids  on  Aluminium.  511 
ACTION  OF  SULPHURIC  AND  NITRIC  ACIDS  ON 
ALUMINIUM.1 
By  A.  Ditte. 
When  aluminium  foil  is  placed  in  sulphuric  acid  of  2-5  per  cent., 
it  at  first  seems  to  undergo  no  change,  but  after  a  time  the  surface 
loses  its  polish  and  becomes  matt,  and  bubbles  of  gas  appear  and 
increase  in  quantity  as  the  surface  becomes  rougher.  Eventually 
the  aluminium  dissolves  completely.  In  a  vacuum,  the  phenomena 
are  similar,  but  follow  one  another  more  rapidly,  the  air  condensed 
on  the  surface  being  given  off  first,  followed  as  soon  as  the  pressure 
is  low  by  the  hydrogen  liberated  by  the  action  of  the  acid.  Like 
amalgamated  zinc,  the  aluminium  is  attacked  immediately  on  its 
immersion  in  the  acid,  but  the  surface  becomes  covered  with  a 
film  of  hydrogen,  which  prevents  further  action. 
If  the  sulphuric  acid  is  mixed  with  a  small  quantity  of  any 
chloride  with  a  heat  of  formation  greater  than  that  of  aluminium 
chloride  (platinum,  gold,  copper,  mercury),  the  evolution  of  hydro- 
gen is  much  more  rapid.  The  chlorides  of  iron,  zinc,  etc.,  have  no 
such  effect. 
In  presence  of  traces  of  the  active  chlorides,  the  evolution  of 
hydrogen  is  at  first  rapid,  but  after  a  time  it  slackens,  and  a  white 
deposit  of  basic  aluminium  sulphate,  2A1203,S03,  is  formed,  and 
increases  in  quantity  until  the  whole  of  the  aluminium  has  been 
dissolved. 
Aluminium,  under  atmospheric  pressure  is  not  attacked  by  a 
solution  of  aluminium  sulphate,  but  in  a  vacuum,  air  is  first  disengaged 
from  the  surface  of  the  metal,  and  the  latter  is  then  converted  into 
the  basic  sulphate  with  evolution  of  hydrogen.  The  same  change 
takes  place  readily  even  under  atmospheric  pressure  if  a  trace  of 
one  of  the  active  chlorides  is  added. 
The  displacement  of  copper  from  copper  sulphate  by  aluminium 
is  an  exothermic  change,  but  does  not  take  place  under  ordinary 
conditions  even  in  presence  of  free  acid.  After  prolonged  contact, 
the  aluminium  is  slowly  attacked,  the  rapidity  of  the  reaction  in- 
creasing as  the  surface  of  the  metal  becomes  roughened.  In  pres- 
ence of  a  trace  of  one  of  the  active  chlorides,  the  deposition  of 
copper  is  much  more  rapid,  even  in  absence  of  free  sulphuric  acid. 
1  Compt.  rend.,  HO,  573  and  782.  Reprinted  from  Jour.  Chem.  Soc,  July, 
1890,  p.  701. 
