Shelf life of bleach

[Samantha_in_AL]

This morning I read an article about saving money. It mostly focused on which items you should and should not buy in bulk and gave the reasons why. A so-called "expert" stated that you should not buy more bleach than you can use in 6 months because it begins to rapidly lose it's effectiveness after that point. Has anyone ever heard that before? I always assumed it had an indefinite shelf life, buy maybe I was wrong. Maybe this explains the confusion I had when I opened my pool this year. I was using the last of the bleach I had left from a pallet I bought over a year ago. I added exactly the amount the pool calculator called for each time, but but when I did the FC test my numbers would be low and I had to keep adding more bleach. At the time I blamed it on my test kit and went out and bought a new one. The new kit gave me the same results.
I never imagined it could be the bleach.
I have now used the old bleach up and bought more. Now then the pool calculator amounts are right on the money.

 

[257WbyMag]

Yes, over time, bleach loses it's strength. Sodium hypochlorite in solution is just unstable from a chemical standpoint. And the higher concentration you have, the quicker it loses it's strength. 6% bleach lasts longer in storage than 12.5%. That's why there isn't anything much higher than 12.5% around to purchase. If the concentration were any higher, it would lose it's strength too quickly to keep that concentration.


There are things you can do to make it last a little longer such as keeping it from being exposed to extreme temperatures and such. But it generally is a good idea to not buy more than what you might use in a couple of months.

 

[chem geek]

As shown in the chart at the bottom of this page, bleach at 6% is going to last quite a while if kept at room temperature with it's half-life extending well beyond 2 years. Even high-quality chlorinating liquid kept at room temperature is going to be reasonable at 3-6 months, but if temps get warmer such as storing the chlorine outside in a shed, then the chlorine will degrade much more rapidly. Basically, 6% bleach lasts around 4 times longer than 12.5% chlorinating liquid, all else equal.

In practice, 10-12.5% chlorinating liquid is something you want to use within 3 months if you can and you don't try and store it for a year unless you keep it quite cool. Bleach can be stored and used even after 1 year if kept at room temperature. This all assumes high-quality product as the rate of degradation becomes much faster than shown in the table if there are metal ion impurities present such as iron, copper, cobalt, zinc, etc.

Of course, in your case, you have a real-life example where your bleach did apparently lose its strength over the year. What brand of bleach was it and what was the temperature where it was stored?

 

[PaulR]

Here's my real-life example; YMMV.

My local pool shop sells 12.5% in 4-gallon crates, and typically has a 2-for-1 deal. So I end up with 8 gallons at a time. I don't recall exactly from last summer, but let's say each gallon lasts me a week; so assuming it is really 12.5% at the start, I'd guess by the end of the last gallon it's probably on the order of 8-9%, because I have to store it outside (although not in the sun). Still, because of the pricing it's a better deal than any other bleach source in town.
--paulr

 

[Samantha_in_AL]

Chem Geek, it was Wal Mart brand, 6% bleach. I kept it out in our storage shed. I have seen the temps in there as high as 118F in the hottest part of summer (we have a thermometer on the wall) and well below freezing in the winter.

I bought 15 of the biggest jugs Wal Mart sells a few weeks ago. I moved them from the shed into my laundry room today. They take up alot of room, but I'm sure we will use them before too long. Next time I wont buy quite as much.

I just converted from Baquacil near the end of last summer so this was my first start up using BBB. It took alot less bleach to do the initial shock than I thought it would. Lesson learned. I went out and bought us some awesome new floats, banana trees and palm trees to plant around the pool with the money I saved by not having not having to buy the Baquacil! I think this start up probably cost me less than $10.00 including the CYA I added!

 

[chem geek]

I have seen the temps in there as high as 118F in the hottest part of summer

Well that would explain the degradation. The half-life of 6% bleach at 118ºF is around 41 days (the initial loss rate is 0.1% per day drop in strength) and even at 100ºF it's 134 days (an initial loss rate of 0.22% per week drop in strength) so a summer with many hot days in that shed could certainly substantially degrade the chlorine strength.

That's great news that your start-up costs have dropped significantly. Manual chlorine addition requires a small amount of time for more frequent chlorine addition, but it pays off being a very low cost solution to pool water maintenance.