AmDe°c"i£9.arm'}      Formation  of  Hydrogen  Peroxide.  621 
filtered,  the  filtrate  contains  hydrogen  peroxide  (=  7 '4  milligrams  of 
available  oxygen) ;  this  experiment  shows  that  Richarz's  view  {loo.  cit.) 
that  hydrogen  peroxide  is  only  formed  from  persulphuric  acid  in  70 
per  cent,  sulphuric  acid  solution,  is  untenable. 
Violet  chromic  sulphate  is  not  oxidized  to  chromic  acid  by  a  40  per 
cent,  sulphuric  acid  solution  of  persulphuric  acid ;  when  chromic  acid 
is  treated  with  persulphuric  acid  in  70  per  cent,  sulphuric  acid  solu- 
tion, it  is  reduced  to  chromic  oxide  in  consequence  of  the  formation  of 
hydrogen  peroxide. 
Persulphuric  acid  also  differs  from  other  powerful  oxidizing  agents 
in  this,  that  in  70  per  cent,  sulphuric  acid  solution  it  does  not  decom- 
pose, but  actually  gives  rise  to  hydrogen  peroxide.  Quantitative 
experiments  showed  that  whea  hydrogen  peroxide  and  persulphuric 
acid  are  present  together  in  40  per  cent,  sulphuric  acid  solution,  they 
are  both  very  slowly  decomposed,  equal  molecules  of  the  two  com- 
pounds being  destroyed,  but  a  similar  solution  of  either  compound 
alone  undergoes  no  considerable  change  within  48  hours. 
Persulphuric  acid  does  not  act  on  organic  compounds,  or  has,  at  the 
most,  only  a  very  slight  action.  In  40  per  cent,  sulphuric  acid  solu- 
tion, it  does  not  decolorize  indigosulphonic  acid  until  after  half  an 
hour's  time,  and  a  solution  in  dilute  sulphuric  acid  (about  10  percent.) 
can  be  filtered  through  paper  without  any  reduction  taking  place. 
Neutral  solutions  of  persulphuric  acid  are  only  very  slowly  reduced 
by  alcohol,  but  arsenious  acid  is  completely  oxidized  within  a  short 
time.  Berth  el  ot  (1878,)  has  stated  that  arsenious  acid  is  not  acted 
on  by  persulphuric  acid. 
The  following  reactions  show  that  persulphuric  acid  belongs  to  the 
class  of  oxygen  molecule  compounds  (holoxides).  Platinum,  either  in 
the  form  of  wire  or  as  platinum  black,  decomposes  persulphuric  acid 
with  evolution  of  oxygen,  but  only  in  the  presence  of  acid.  A  neutral 
or  slightly  acid  solution  of  persulphuric  acid  oxidizes  indigosulphonic 
acid  only  slowly,  but  immediately  on  adding  ferrous  sulphate.  Per- 
sulphuric acid  and  hydrogen  peroxide,  when  present  together,  behave 
like  two  holoxides.  Persulphuric  acid  sometimes  behaves  as  a  reduc- 
ing agent ;  a  solution  of  40  per  cent,  sulphuric  acid,  free  from  hydro- 
gen peroxide,  reduces  lead  peroxide,  the  hydrate  of  manganese  diox- 
ide, and  silver  peroxide. 
The  article  concludes  with  a  discussion  of  the  existence  of  oxygen- 
molecule  compounds,  which  is  principally  controversial. 
