Re: How can I test the amount of hydrogen peroxide in a solution?

This is a sophisticated problem for an elementary school 
science project!  I do not know of a simple way for a 
9-11 year old to do this.  The help & supervision of a high
school chemistry teacher would be needed to carry out the 
analysis as I outline below. If you wish you could get more
information on H2O2 analytical methods from a college text on 
quantitative analytical chemistry.  I don't think that high 
school or lower level texts discuss the topic.   The techniques 
are complicated for someone who does not have some chemistry
background.

I do not think that visible light
by itself affects hydrogen peroxide (H2O2).  The heat that light
would generate in the H2O2 would cause it to more quickly 
decompose into oxygen (O2) and water (H2O).  This is why it is
packaged in brown bottles.  I think that it 
would be difficult to differentiate H2O & H2O2 by spectroscopy
in the UV/visible & infrared ranges.

I'll outline a chemical method used to determine how much H2O2
is in a sample, to give an idea of what is involved.  This method
uses the fact that H2O2 reacts with (reduces) a purple potassium permanganate
(KMnO4) solution to produce colorless products.  The chemical
equation is: 
5 H2O2 + 2 MnO4(-) + 6H(+)-----> 5 O2 (gas) + 2 Mn (+2)  +  8 H2O

The procedure involves diluting the H2O2  solution with 
water (H2O) and adding some dilute sulfuric acid (H2SO4) to make
the H2O2 solution acidic.  This acidic H2O2 solution is reacted
with a dilute KMnO4 sloution of known concentration.  The purple
KMnO4 solution is measured into the acidic H2O2 solution using
a volume measuring device (called a buret) in a procedure called
a titration.  As long as H2O2 is present the added purple KMnO4
turns clear.  When the H2O2 is used up, the next few drops of 
the KMnO4 turn the solution pink or light purple.  This color
signals that the H2O2 is used up.  We stop adding the KMnO4
at this point and read the volume delivered from the buret.
Because the concentration and
volume (from the buret) of the KMnO4 are known we can calculate 
the amount of
KMnO4 chemically used up.  From the equation, this amount of
KMnO4 can be used to calculate the amount of H2O2 that was
present in the original sample. 

This is a summary of the procedure.  I've left out details that
would only complicate things.  All of the chemicals I've mentioned
are dangerous for an untrained person to use.