Hi guys,
I thought I had some timeframes for H2O2 decomposition in one of my chem textbooks, but its seems I don't.
However, some info for you:
a) H2O2 does decompose over time, the chemical reaction is:
2H2O2 --> 2H2O + O2
That is it produces water and oxygen.
While a slow reaction, I can tell you that a 90% bottle of H2O2 won't be 90% after a year - more like 70% or so.
B) Decomposition is controlled by three main thing - temperature, pH and contaminants (especially metals which cataylise (speed up) the rate of decomposition)
Temperature - for each increase in 10 degrees C, the rate of decomposition increases by 2.2 x - so keep it in the fridge!
pH - if the pH is above 6 or so, the rate of decomposition increases
If the water has metals such as magnesium, iron or copper in it, decomposition rate will also increase.
c) You can tell the concentration of a H2O2 solution made with pure water using the following (correct at 25%):
pH Conc
7 0
5.3 10
4.9 20
4.7 30
4.6 40
4.5 50-70
Hope this helps,
Matt