Am.  Jour.  Pharm. 
May,  1898. 
Hydrogen  Dioxide. 
229 
working  with  dilute  or  with  concentrated  solutions.  One  possible 
source  of  error  with  the  latter  is  that,  when  standing  in  a  very  warm 
room  prior  to  titration,  slight  losses  of  iodine  might  occur  through 
vaporization.  This  could,  of  course,  be  readily  seen  and  proper 
precautions  taken  to  prevent  loss.  The  following  mode  of  opera- 
tion  is  considered  the  most  practical :  Dilute  the  solution  to  be 
tested  to  exactly  ten  times  its  volume  with  distilled  water,  measure 
5*6  c.c.  of  the  dilution  (=  0  56  c.c.  of  sample)  into  a  beaker  or 
flask,  add  5  c.c.  of  10  per  cent,  sulphuric  acid  and  5  c.c.  of  a  10 
per  cent,  solution  of  potassium  iodide,  and  let  the  mixture  stand  for 
five  minutes.    Then  gradually  add  ^  thiosulphate,  with  constant 
agitation,  until  the  color  of  the  solution  is  discharged.  If  the  solu- 
tion remains  colorless  upon  standing  a  few  minutes  longer,  the  titra- 
tion is  finished.  If  a  color  should  appear,  add  sufficient  thiosulphate 
solution  to  remove  it.  The  total  number  of  cubic  centimeters  of  the 
thiosulphate  solution  required  indicates  the  number  of  volumes 
of  available  oxygen  the  sample  contains.  If  it  is  consider- 
ably stronger  than  ten  volumes,  the  result  will  probably  be  too 
low,  unless  more  time  is  allowed  for  the  reaction.  In  such  a 
case  correspondingly  more  potassium  iodide  and  sulphuric 
acid  should  be  taken.  No  starch  paste  is  necessary,  as 
the  endreaction  is  sufficiently  sharp  without  it,  particularly  when 
the  solution  is  placed  over  a  white  surface.  In  cases  in  which  an 
accurate  dilution  of  the  sample  is  impracticable,  1  c.c.  of  the  undi- 
luted sample  may  be  taken,  with  double  the  quantities  of  reagents 
N 
given  above.   The  number  of  cubic  centimeters  of    thiosulphate 
10 
required  must  then  be  multiplied  by  0-56  to  obtain  the  strength  in 
N 
available  oxygen.    Or,  instead  of  — .  thiosulphate  (24-8  grammes  of 
crystallized  salt  per  liter),  a  solution  containing  44-29  grammes  per 
liter  may  be  used.  In  the  latter  case  each  cubic  centimeter  con- 
sumed indicates  one  volume  of  available  oxygen,  provided  1  c.c.  of 
sample  has  been  taken.  The  per  cent,  of  hydrogen  dioxide  is 
obtained  by  multiplying  the  volume  strength  by  0-3038.  (This  is 
not  strictly  correct,  as  the  specific  gravity  of  the  solution  is  left  out 
of  consideration.)  It  is  important  that  the  reagents  used  are  free 
irom  such  impurities  as  would  act  as  reducing  or  oxidizing  agents. 
