People typically use chlorine bleach for three different things:
- Cleaning and disinfecting in the kitchen, bathroom or laundry room
- Treating a well to eliminate bacteria as explained here: How do you chlorinate a well?
- Disinfecting drinking water in emergency situations as described here: How to purify drinking water in emergencies (like hurricanes)
Especially in the third situation, this question is important: Does Chlorine Bleach Ever Expire or Go Bad? The answer is Yes, absolutely. If you are keeping chlorine bleach on hand for an emergency situation, you need to be aware of this expiration problem and you need to be refreshing your bleach supply every 3 months or so.
Chlorine dissipates rapidly, and if you have an aquarium you understand this. Tap water will kill fish in aquarium if you use it fresh. But if you boil the water or you simply let tap water sit on the counter for a day or two, the chlorine disappears.
Chlorine from the store is usually 5% to 6% sodium hypochlorite mixed in water. Its chemical formula is NaClO. With time, the NaClO releases the oxygen and a bottle of bleach turns into a bottle of salt water.
How long does it take? Less than a year in many cases - about 3 months if using the bleach as a disinfectant - especially if the bottle of bleach is kept in a garage that gets warm in the summer. This article...
...contains the following quote:
"We recommend storing our bleach at room temperatures. It can be stored for about 6 months at temperatures between 50 and 70 degrees Fahrenheit. After this time, bleach will be begin to degrade at a rate of 20% each year until totally degraded to salt and water. Storing at temperatures much higher than 70 degrees Fahrenheit could cause the bleach to lose its effectiveness and degrade more rapidly. However, if you require 6% sodium hypochlorite, you should change your supply every 3 months."
That article also offers a longer-lasting alternative: dry Calcium Hypochlorite granules available for treating pool water.