What happens to hydrogen peroxide when it is exposed to light? How fast does this reaction occur (i.e., on the order of seconds, hours, months, etc.)? Can this reaction occur at any humanly significant rate without the aid of light, also?
Thanks.
Hydrogen peroxide (H[sub]2[/sub]o[sub]2[/sub]) is always sold in dark bottles, because when it’s exposed to light it breaks down more quickly into water and oxygen gas. This reaction isn’t particulary rapid, and it would take on the order of days for a bottle of the stuff to break down even in direct sunlight. Shielded from light, hydrogen peroxide is fairly stable, with a shelf life of a few years.
Different grades of hydrogen peroxide have different amounts of stabiliser added.
At one extreme, dilute home consumer grade has a lot of stabiliser to give it a long shelf life. Semiconductor grades can be obtained with no stabiliser. Not sure what the half lifes of various grades are (do a search) but less stabilised grades should be kep in the fridge.
There are things that can break down peroxide in the absence of light – solid manganese dioxide, MnO2, will do it, IIRC. There is an enzyme called catalase that catalyses the decomposition of peroxide to water and oxygen and does it extremely quickly. A fond high-school biology memory of mine is of adding chunks of beef liver (which contains a lot of catalase) to solutions of hydrogen peroxide and watching the reaction mixture go crazy (foaming and froth as lots of oxygen is generated).
MnO2 is conveniently available inside alkaline batteries, if anyone wants to play. 
I have a brown glass bottle of hydrogen peroxide that is at least 25 years old, but has been kept on a shelf in a closet for most of its years. Should I keep it or throw it out? Is there a simple way to test it?
Sure, put some on an open wound. If it fizzes, it’s still good. In all probability, it’s long since broken down into water and O[sub]2[/sub], though.
Throw it out. Is it a small bottle of disinfectant? Just buy another. Better to have something you know works. Temperature and stabiliser content are the biggest determinants of rate of decomposition.
To test, you can add a crystal of potassium permanganate and drop in to a drop of the H2O2 soln and see if it still fizzes. Or if it is medical grade put a drop on a cut to see if it fizzes. Compare this to the new stuff.
Accurate quantitative determination of H2O2 is via oxidation of iodide (KI) to iodine then titration of the iodine with sodium thiosulfate. Can also titrate H2O2 with potassium permanganate.
Realted question:
Why does the HP fizz when poured on a wound, and what is it doing exactly?
As mentioned earlier, catalase is an enzyme which act to catalyse the decomposition to water and oxygen.
My mom is a professional hair stylist and used Peroxide of 20,30,and 40 volume, unrefrigerated on the shelf.
The only ingredients are Hydrogen Peroxide, and water…no stabilizer (phosphoric acid, IIRC).
IIRC Hydrogen peroxide use as a disinfectant is based on the fact that it is a HIGHLY reactive oxygen species. The reason for catalase is to destroy the peroxide before it damages your cells and it’s contents, but as a disinfectant, the bet is that you are killing/damaging bacteria long before any significant cell damage occurs on your wound.
Peroxides can break down into free radicals, such as *OH, which bind very strongly to things like membrane lipids (the “bag” that is your cell), proteins and DNA.
There’s another enzyme, called superoxide dismutase, that exists to disproportionate the superoxide anion, which is chemically related to hydrogen peroxide. These are super-reactive species, as mnemosyne mentions, and will rapidly initiate radical reactions that trash biomolecules real quick, which is why nature has evolved efficient enzymes to take care of them.
Curiously, there aren’t many examples in synthetic organic chemistry of hydrogen peroxide being used for simple organic oxidations. Metal-based catalysts are usually required. A big area of modern organic chemistry research involves looking for ways to get cheap and common oxidizers – hydrogen peroxide and hypochlorite (chlorine bleach) – to do useful organic oxidations, replacing nasties like manganese and “hexavalent chromium”.