Am.  Jour.  Pharm. 
May,  1398. 
Hydrogen  Dioxide. 
227 
next  five  minutes,  on  account  of  the  voluminous  precipitate  of  hy- 
drated  manganese  dioxide,  which  prevents  small  bubbles  of  gas  from 
rising  to  the  top.  The  tube  is  then  placed  in  a  vertical  position, 
and,  when  the  volume  of  gas  becomes  constant,  the  liquid  in  the 
equilibrium  tube  is  adjusted  to  the  same  level  as  that  in  the  measur- 
ing tube.  The  volume  of  gas  is  then  read  and  the  temperature 
noted.  Since  the  standard  of  strength  of  hydrogen  dioxide  solu- 
tions is  not  based  on  the  volumes  of  oxygen  yielded  at  ordinary 
room  temperature,  but  on  the  volume  the  gas  would  occupy  at 
o°C.  and  760  millimetres  barometric  pressure,  a  correction  is  re- 
quired for  accurate  determinations.  The  writer  made  correction  for 
variation  in  temperature  only,  as  sufficient  for  ordinary  purposes. 
The  reader  is  referred  to  the  Pharmacopoeia  for  the  formula  and 
tables  required  for  this.  It  should  not  be  omitted,  as  the  error 
would  be  quite  considerable.  A  solution  yielding  10  volumes  of 
oxygen  at  o°C,  would  yield  10-55  volumes  at  150,  1075  volumes  at 
200,  10-9  volumes  at  250,  etc. 
In  the  writer's  hands  this  method  does  not  give  very  encouraging 
results.  Duplicate  estimations  differ  much  more  than  with  other 
methods,  the  precipitated  manganese  dioxide  collects  near  the  top 
of  the  liquid,  and  makes  a  close  reading  of  the  volume  impractica- 
ble, besides  obstructing  the  passage  of  gas  bubbles.  While  not 
entirely  worthless,  it  is  much  less  satisfactory  than  the  following 
method. 
Gasometric  Hypochlorite  Method. — This  method  is  recommended 
by  D.  B.  Dott  (Ph.  J.  Tr.,  1890  p.  833),  who  uses  a  solution  of  ordi- 
nary bleaching  powder,  also  filling  the  apparatus  with  it  instead 
of  sodium  chloride  solution.  Mr.  Dott  obtains  appreciably  higher 
results  than  by  titrimetric  methods,  and  concludes  the  method  to 
be  more  accurate  than  the  latter.  The  reaction  is  analogous  to 
that  of  the  permanganate  methods,  as  follows : 
Ca(C10)2  +  2H202  =  202  4-  CaCl2  +  2H20. 
In  some  of  the  present  writer's  experiments  a  solution  of  chlori- 
nated soda  of  approximately  U.S. P.  strength  was  used,  in  others  a 
solution  of  chlorinated  lime  containing  about  4  per  cent,  of  avail- 
able chlorine.  Preference  is  given  to  the  latter,  as  with  the  other  a 
white,  flocculent  precipitate  (probably  sodium  hydrate  and  carbonate) 
is  formed,  which  is  open  to  the  same  objections  as  that  incidental  to 
