Hydrogen peroxide, like many household chemicals, can expire.
If you've ever poured hydrogen peroxide solution onto a cut and didn't experience the expected fizz, it's likely your bottle of hydrogen peroxide has become a bottle of plain water. The 3-percent hydrogen peroxide solution you can buy for use as a disinfectant typically has a shelf life of at least a year and up to three years if the bottle is unopened.
Once you break the seal, the hydrogen peroxide will last 30 to 45 days at peak effectiveness, with about six months of useful activity. As soon as you expose the peroxide solution to air, it starts to react to form water. Also, if you contaminate the bottle-such as by dipping a swab or finger into it-you can expect the effectiveness of the remaining liquid to be compromised.
So, if you have a bottle of hydrogen peroxide that has been sitting in your medicine cabinet for a few years, it would be a good idea to replace it. If you've opened the bottle at any point, assume the liquid has outlived its usefulness.
Why Peroxide Bubbles
Whether your bottle of peroxide is open or not, it's always decomposing into water and oxygen. The equation is:
2 H2O2 → 2 H2O + O2(g)
The bubbles that form in the reaction come from oxygen gas. Ordinarily, the reaction proceeds so slowly you can't perceive it. When you pour hydrogen peroxide onto a cut or any of several surfaces, the reaction proceeds much more quickly because a catalyst is present. Catalysts that speed the decomposition reaction include transition metals, such as iron in the blood and the enzyme catalase.
Catalase is found in nearly all living organisms, including humans and bacteria, where it acts to protect cells from peroxide by quickly deactivating it. Peroxide is naturally produced in cells and needs to be neutralized before it can cause oxidative damage. When you pour peroxide on a cut, both healthy tissue and microbes are killed, but the damage to your tissue mends.
Test to See If It Is Still Good
If you're not sure whether that bottle of peroxide is worth using, there's a safe and easy way to test it. Splash a bit into a sink. If it fizzes, it's still good. If you don't get a fizz, it's time to replace the bottle.
Tips to Extend the Peroxide's Life
Don't open the new container until you're ready to use it and don't transfer it to a clear container. In addition to air, light also reacts with peroxide and causes it to change. You can help extend the shelf life of your hydrogen peroxide by storing it in a cool location, since heat accelerates the rate of chemical reactions, including the decomposition of hydrogen peroxide.