AmFeb.?i?9h3arm-}   Rapid  Assay  of  Hydrogen  Peroxide.  67 
explanation  being  that  the  sulphate  of  manganese  produced  has  the 
power  of  decomposing  H202  and  hence,  the  slower  the  K2Mn2Og 
added,  the  greater  the  decomposition  by  the  sulphate  and  the 
less  K2Mn2Og  required.  By  carrying  out  the  assay  in  presence  of  a 
large  excess  of  water,  reliable  and  uniform  results  are  obtained  ; 
to500cc.  water  (river  water  will  answer)  in  a  capsule  add  5  cc.  dilute 
sulphuric  acid  and  sufficient  permanganate  solution  to  give  a  pink 
tint  (this  counteracts  any  reducing  action  which  the  river  water  may 
have  on  the  permanganate  solution) ;  now  add  5  cc.  of  the  hydrogen 
peroxide  and  then  (from  a  bottle  or  graduate  containing  175  cc.) 
allow  the  permanganate  solution  to  run  in  in  a  thin  stream,  stirring 
constantly  until  the  pink  color  is  no  longer  discharged  ;  the  pink 
color  after  a  short  time  is  replaced  by  a  brownish  color  or  precipi- 
tate (Mn02H20)  due  to  the  action  of  manganous  sulphate  upon  the 
slight  excess  of  permanganate  ;  measure  the  permanganate  solution 
remaining  in  the  bottle  or  graduate  and  divide  the  permanganate 
used  by  10,  the  result  will  be  the  volume  of  oxygen  liberated  by 
one  volume  H202. 
To  dispense  entirely  with  the  metric  system,  a  solution  can  be 
made  by  dissolving  38*5  grains  potassium  permanganate  in  a  quart 
of  distilled  water  (this  solution  (0-264  per  cent.  K2Mn2Og  is  almost 
of  the  same  strength  as  the  one  just  described  containing 
0-2625  per  cent.  K^MojO ;  one  fluidrachm  of  the  permanganate 
solution  liberates  one-half  fluidrachm  of  oxygen  at  200  C.  In  the 
assay  the  following  measures  should  be  substituted  for  the  ones 
described  :  one  fluidrachm  each  of  hydrogen  peroxide  and  dilute  sul- 
phuric acid,  one  pint  river  water  and  then  add  the  permanganate 
solution  from  a  bottle  or  graduate  containing  four  fluidounces ; 
the  number  of  fluidrachms  of  permanganate  solution  used  divided 
by  2  gives  the  volume  of  oxygen  liberated  by  the  H202. 
Five  cc.  of  a  sample  opened  several  times  during  three  months 
required  38  and  36  cc.  K2Mn2Og,  showing  3-8  and  3-6  volumes;  one 
fluidrachm  of  the  same  sample  required  7,  anc^  7ZA-  fluidrachms 
permanganate,  or  3-5,  3-8  and  3*8  volumes  oxygen  (the  first  assay 
was  made  in  presence  of  only  2  ounces  of  water). 
An  original  y2  lb.  bottle,  purchased  in  a  retail  store,  by  assay, 
using  the  chlorinated  soda  method  (Am.  Journ.  Pharm.,  1892,  126) 
and  measuring  the  evolved  oxygen,  gave  12  5,  12  4  and  12  5  vol- 
umes oxygen  ;  there  was  no  pressure  in  the  bottle  when  opened, 
