Ask A Scientist© Chemistry Archive

name         Winona S.
status       student
age          16

Question -   Hello, I have been trying to find this information on the
Internet but have been unable to do so. How can the concentration of
hydrogen peroxide in solution be analyzed. Say, if I had 30% hydrogen
peroxide, how long would it take (if left exposed to air) to become 3%
hydrogen peroxide, or just pure water? Thanks.
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There are a number of analytical procedures for the analysis of H2O2. One
common way (actually there are several procedures of this type) is to react
the H2O2 with an excess of
standardized potassium iodide solution (it is available in high purity).
Then back titrate the iodine produced by the oxidation of potassium iodide
with standardized sodium thiosulfate. If the titration is done by weight
rather than the conventional volume titration accuracies of between 1/10000
and 1/100,000 are possible. There are other procedures based on the
oxidation of something by H2O2 but few can match this method for ease,
accuracy, and reliability.

How long it takes H2O2 to decompose will depend upon the temperature, the
presence of light
(photochemical decomposition), other impurities present (especially
transition metal ions which greatly catalyze the decomposition).
Vince Calder
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Hi Winona!
You asked 2 different questions:
a) How to analyse a solution of hydrogen peroxide?
  You can titulate it with a solution of  Potassium Permanganate.
  In the reaction, the permanganate  acts as a strong oxidizing agent and the
  hydrogen peroxide accepts the role  of reducing agent and is oxidized.:
  6 H+ + 2 (MnO4) -    + 5 H2O2  = 5 O2   + 2 (Mn) 2+  + 8 H2O
b) Now for the second question,  how long it takes to degrade
  from a 30% solution to a 3% one?
  It will depend upon various factors: volume considered,  temperature,
  surface of exposition, agitation or not...The oxygen in a 30% solution
  goes away fairly easily, so...do not let the flask open...
 And thanks for asking NEWTON! Tell your friends about it!
 Mabel
(Dr. Mabel Rodrigues)
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