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#59124 - 01/29/06 08:54 AM Re: Bleach Potency
countrymouse Offline

Registered: 01/23/06
Posts: 24
That's French Bleach - American Bleach never breaks down!

Seriously, thanks for the info. <img src="/images/graemlins/smile.gif" alt="" />

#59125 - 01/29/06 06:49 PM Re: Bleach Potency
Arney Offline

Registered: 09/15/05
Posts: 2485
Loc: California
I've been doing more research into the topic (finding the right Google terms make all the difference!) and what I found confirms my previous claim--even an unopened bottle of bleach does have a limited shelf life.

First of all, to my surprise, Google turned up this hit on the ETS website itself. It says, "Bleach has a limited shelf life and for best results you should use fresh bleach no more that a few months old." Sounds a lot like that Clorox statement on the Cornell link I referenced in an earlier post. Speaking of Clorox, here's a Clorox webpage that has the "3-6 months for optimum performance" statement that I previously referenced on that Cornell University webpage. We should ask Doug where he got his information from.

Here's another link, and here's another one about the limited shelf life of bleach. They don't specify whether the bottle needs to be opened or not, but it doesn't seem to matter.

As for the reason why bleach degrades (or "where does it go?"), this webpage from the CDC describes the particular chemical reaction that is the source of the degradation. When bleach is just sitting in its bottle, there's a chemical reaction that I mentioned previously where underchloric acid (HOCl) breaks down into hydrochloric acid(HCl). There is sodium ion floating around in bleach and the hydrochloric acid reacts with the sodium to form sodium chloride, aka "table salt". Over time, you lose the disinfecting HOCl as dissolved salt. Salt is very stable so the process is basically irreversible under storage conditions.

The manufacturer makes "straight" bleach strongly alkaline (pH 11) by adding sodium hydroxide (NaOH), to minimizes the amount of HOCl around, so this degradation is relatively slow in the original container. However, once you dilute the bleach with water, the pH drops to around neutral, which highly favors HOCl. That is a good thing because you want lot's of HOCl around to zap the bad stuff, but it's also bad because with so much HOCl around, this degradation reaction occurs much more quickly. That's one (the main?) reason why once you mix up some disinfecting solution, you can't keep it long regardless of how carefully you store it. There is some offgassing of chlorine gas, but I have yet to read anything that attributes offgassing as a major reason for bleach's limited shelf life. Logically, you would think so (seems logical to me, too), but I guess it doesn't. Anyway, regarding how long your disinfecting solution can last, most hospital/lab disinfection protocols I read instruct you to make up a fresh batch every 24 hours. Others say you can keep the solution for up to a month before throwing it out.

Anyway, based on what I read (much more than what I reference here), for optimal drinking water disinfection, it seems that you should rotate the bleach stock out at six month intervals if you want to be reasonably certain of using bleach that is strong enough to conform to the typical water treatment guidelines (e.g. X drops for clear water, Y drops for cloudy). As the bleach ages and weakens, you could adjust the dilution to get the proper disinfecting concentration, but it's hard to know how much to adjust it unless you actually tested the chlorine concentration.

We are always instructed to utilize the "smell test" after treating water with bleach, and I guess that is supposed to be a crude way of seeing if there was enough HOCl around to properly treat the water. If the bleach is old and much of the chlorine is bound up as salt, not enough will be left to turn into enough chlorine gas to smell once added to the proper volume of water. That would also mean that there might not be enough to kill all the microorganisms properly. So, add more bleach and try it again.

The loss of concentration figures I have uncovered range from 20-50% per year. Alain's figure of weakening to 50% after three years is consistent with a 20% weakening/year. But at 50%/year, it would only be a fourth as strong after three years, which is a significant difference.

So, whether you're storing French eau de javel or American Chlorox, bleach does not have an indefinite shelf life.

I must sound like the Alton Brown of bleach (for those of you familiar with the science-oriented Food Network cooking show). <img src="/images/graemlins/grin.gif" alt="" />

#59126 - 01/30/06 06:54 AM Thanks Everyone
Susan Offline

Registered: 01/21/04
Posts: 5163
Loc: W. WA
Okay, all that sort of seems to make sense... chemistry wasn't my forte in school. I'll date it, then use the old stuff to wash the moss off the deck.

Sue (39th continuous day of rain here in WA--- blech!)

#59127 - 01/30/06 08:20 PM Re: Thanks Everyone
JohnN Offline
Old Hand

Registered: 10/10/01
Posts: 966
Loc: Seattle, WA

Sue (39th continuous day of rain here in WA--- blech!)

Yah, quite a drag, eh? My yard turned into a swamp last night...


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