Does bleach go bad?

March 13, 2013 12:25 PM

Instead of buying spray disinfectant all the time. . . I usually just mix 1 teaspoon into a spray bottle with water and use that. My wife thinks it goes bad and loses it's disinfecting power. Does it?
posted by patrad to home & garden (19 answers total)

It won't go 'bad', but normal bleach will eventually break down to salt water. I'm not sure how long a dilute mixture will retain its activity.
posted by sevenyearlurk at 12:33 PM on March 13 [1 favorite]

Apparently yes. But for dilution purposes you can simply add a bit more to the water as time goes on.
posted by weapons-grade pandemonium at 12:36 PM on March 13

Favorable storage conditions include: temperature below 70°F, plastic container (not metal or glass), opaque container (to minimize exposure to light), and closed container (to minimize exposure to air). It is common to measure 50% decay within one month under favorable storage conditions. source
posted by ook at 12:37 PM on March 13

Clorax, the manufacturer of a popular brand of household bleach in the US, says in their Bleach Shelf Life article that the active ingredient, sodium hypochlorite, does decompose over time whether or not it is stored in an open or closed container. They state that their 6% active hypochlorite strength required by the EPA for a use as a disinfectant will be maintained at least 6 months under most typical home storage conditions, and suggest that for 12 months it should still be good for general cleaning, laundry, and other household tasks.
posted by RichardP at 12:41 PM on March 13 [1 favorite]

Your wife is correct. And, relevant to the link above, it will decay considerably faster in a spray bottle, assuming the spray bottle is translucent. I learned this first-hand when I bought chlorine bleach for a hot tub that came in milk jugs, and left them out in the sun. By the second weekend I had a couple gallons of ex-bleach. Obviously, that's an extreme example. You're probably not going to leave the spray bottle in the sun, but light definitely speeds bleach decay.
posted by smoq at 12:43 PM on March 13

I'm not sure that's enough bleach to actually work as a disinfectant. Assuming the bottle is 32 fl oz or so, 1 tsp is less than 0.5% bleach. (And the bleach is already dilute hypochlorite.) I mean I wouldn't want to drink that, but it's not exactly strong stuff. You're "disinfecting" with something that's one step up from pool water. That's a bigger problem than the breakdown issue.

I'd go with nothing less than a 1:10 dilution, which if it's normal bleach should give you 5,000 ppm free chlorine, enough to kill c. diff. plus a margin for error in case it starts to break down.

Also from the CDC on the same page: "Hypochlorite solutions in tap water at a pH >8 stored at room temperature (23ºC) in closed, opaque plastic containers can lose up to 40%–50% of their free available chlorine level over 1 month. [...] Sodium hypochlorite solution does not decompose after 30 days when stored in a closed brown bottle." (I presume the "brown bottle" refers to a glass laboratory bottle.)

So, assume a 1-month half-life and do your dilution accordingly.

If you find that chlorine bleach at the required concentration is too harsh or produces too many nasty fumes, then you should use a different disinfectant. There are lots of other products you could use. Homebrew shops will sell you no-rinse disinfectant that you can spray onto just about anything (except maybe aluminum) and will disinfect as well as chlorine if left there for 30s or so. Then you can just wipe them off. Star San is popular.
posted by Kadin2048 at 12:43 PM on March 13 [7 favorites]

Interesting comments. 1. We keep it in an opaque container. 2. I used the dilution recommendations off the side of the gallon jug. I think "for counters & general cleaning" was what was listed. 3. I am a homebrewer and familiar with Star San. I'm not sure, but would you use it to disinfect, say a chefs knife with chicken blood on it? After it was washed with soap of course. . .

Also of note, sometimes out of the spray bottle the liquid is a gray-ish/black color. Still smelled like bleach though. Any guesses?
posted by patrad at 1:02 PM on March 13 [1 favorite]

Bleach should not be anything other than clear. Armchair scientificating says that there is an organism strong enough to grow in that bleach proportion.
posted by kamikazegopher at 2:58 PM on March 13 [1 favorite]

The color is probably the seal in the spray bottle nozzle being eaten by the bleach.
posted by Feantari at 3:03 PM on March 13

The colour is metals falling out of solution in decaying product. It's a pretty good indicator that the chlorine is dissipating. The metals are impurities in the caustic potash used in the manufacturing process.

A rule of thumb for bleach is that it loses 1% of it's strength a month, so 6% domestic chlorine will be approx. 5% after a month, 4% after 2 etc.
posted by Keith Talent at 3:18 PM on March 13

So does the same apply to a jug of full-strength bleach just sitting in a cabinet? (I.e., needs to be replaced every year or so?)
posted by LobsterMitten at 5:57 PM on March 13

Yup. Most bleach will be packed with a vented cap, a cap that allows chlorine gas to be expelled without pressurizing and expanding the jug. As the chlorine gasses off it loses efficacy and should be used asap.
posted by Keith Talent at 6:22 PM on March 13

Yes. Bleach goes bad. I work in restaurant sanitation (ie, dishwashing chemicals). One way food service places sanitize dishes during washing is through chlorine. This is properly done by adding chemically stabilized chlorine to the rinse. Often, customers trying to be thrifty try to substitute bleach for this. As they quickly find, bleach left in open containers starts to break down. In a humid environment, a container of bleach hooked to your dish machine rinse that gives you 50ppm one evening may give you 10ppm by the next day.

I would also point out that quality control on off brand bleaches is abysmal and the active chlorine content isn't always in the advertised 5-6% range. Frequently, it's higher. Off brand bleaches tend to think of that number less as a range to stay within and more as a finish line to cross.
posted by DirtyOldTown at 9:23 PM on March 13

A few other things:

-Kadin2048 is right. 1:10 is a better ratio.
-As far as the color goes... I don't have the scientific background to explain why, but from extensive use, I can tell you that bleach will always appear clear when observed directly, but can often seem sort of light golden hued when in a bottle/storage container. If your bleach solution is turning gray/black, I would suspect corrosion of something the bleach is in contact with, probably the screen in the nozzle as Feantari said.
-The 1% per month guideline is fair, assuming all proper storage guidelines are maintained. Any breakdown in this and you can see a dramatic drop in chlorine concentration much faster. As I said, in humid dish rooms, I have seen the concentration provided by open containers drop by 50-80% in 24-48 hours.

Anyway, if there is a takeaway from this, I think it's that you may save money by mixing your own disinfectants, but they can be kinda volatile and hence, not always reliable. Personally, I prefer using store-bought disinfectants for which chemists and engineers have worked hard to provide stable efficacy.
posted by DirtyOldTown at 9:42 PM on March 13

I'm not sure, but would you use it to disinfect, say a chefs knife with chicken blood on it?

Star San should be okay assuming that the knife is stainless steel. If it's carbon steel I'm not sure; I'd be careful if it's a favorite knife or something.

Although honestly, if you wash the knife thoroughly with soap and water, rinse and allow to dry completely, I'm not sure that either chlorine or Star San is really necessary. Certainly I don't use it on my knives or other small kitchen equipment (aside from brewing stuff that absolutely needs to be sterile or very close). Conventional washing with hot water and soap is pretty effective if you do it right and don't put anything away wet.

Where I use spray disinfectants are on stuff that's hard to wash thoroughly, e.g. countertops, large cutting boards, control pads on the microwave and oven, instant-read thermometers, etc. Basically stuff that you can really only wipe down rather than wash. Or on stuff with internal cavities that can't be washed effectively, like meat injectors.

I'll sometimes use Star San if I have a spray bottle of it sitting around, but most of the time I use Clorox Clean Up, which I'm pretty sure is just a hypochlorite solution with some added stabilizers (so it doesn't go bad) and a little soap.

Looking at the CDC guidelines, you could probably make your chlorine spray more shelf-stable (like Clorox Clean Up) by adding something to alter the pH a bit, since the breakdown of hypochlorite into inactive components depends on pH. And then you could toss a bit of soap in as well. But my feeling is that once you did that, putting some reasonable value on your time and on a bunch of pH test strips, it's easier just to buy Clorox Clean Up in bulk jugs from Costco to refill the spray bottle and be done with it. YMMV of course, and it might be an interesting project to try just for the sake of experimentation.
posted by Kadin2048 at 10:41 AM on March 14

kamikazegopher: Bleach should not be anything other than clear. Armchair scientificating says that there is an organism strong enough to grow in that bleach proportion.

Very, very unlikely. The plastic or rubber components of the spray mechanisms are probably the cause.

I've had spray valves break down completely when used like this - rendering the "spray bottle" essentially a bottle with an open straw sticking out of it when I picked it up to use it weeks later.
posted by IAmBroom at 1:50 PM on March 14

DirtyOldTown: I can tell you that bleach will always appear clear when observed directly, but can often seem sort of light golden hued when in a bottle/storage container.

Oh! That reminded me. American bleach usually has a blue dye agent added (because marketing tests have shown slightly-blue white clothes look "cleaner" to American test subjects than truly white ones). European bleach reportedly has a yellow dye.

The dye is truly slight, but there. Note that, because of the difference between additive and subtractive colors, the blue dye might actually appear yellow under some circumstances.
posted by IAmBroom at 1:57 PM on March 14

I bet the black is in the spray mechanism . . after a couple squirts it runs clear again
posted by patrad at 7:50 PM on March 17

Yes - specifically, I think it's broken-down rubber gasket.
posted by IAmBroom at 9:21 PM on March 18