Breakdown of bleach over time by storage temperature


Active member
Sep 3, 2013
Thanks for the welcome!

I've always closed my pool right after labor day using an all black bubble type cover beneath my mesh Loop Loc to keep the sun out. Next one will be solid with center drain! I'll add some chlorine before and after the freeze with very little problems when opening. The pool just doesn't get used much when the kids go back to school and the cold nights here in PA really lower the water temps.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
I finally got around to making a spreadsheet using the rate constants from this link (same as this link) plus some fitting and fudge factors to be consistent with Odyssey Manufacturing half-life and this paper so I'm showing decomposition rates for 12.5% (trade %) chlorinating liquid below over time vs. temperature.

Degradation of 12.5% Chlorinating Liquid
1 day11.92%12.10%12.22%12.31%12.37%12.41%12.44%
2 days11.42%11.74%11.97%12.13%12.25%12.33%12.39%
3 days10.99%11.41%11.73%11.96%12.13%12.25%12.33%
4 days10.61%11.12%11.51%11.80%12.02%12.17%12.28%
5 days10.28%10.85%11.30%11.65%11.91%12.09%12.22%
6 days9.98%10.60%11.10%11.50%11.80%12.02%12.17%
1 week9.71%10.37%10.92%11.36%11.69%11.94%12.12%
2 weeks8.33%9.13%9.87%10.51%11.04%11.46%11.78%
3 weeks7.45%8.29%9.10%9.85%10.50%11.04%11.47%
1 month6.68%7.51%8.36%9.18%9.93%10.57%11.11%
2 months5.14%5.90%6.73%7.60%8.49%9.33%10.10%
3 months4.27%4.95%5.72%6.57%7.48%8.40%9.28%
4 months3.79%4.42%5.13%5.93%6.81%7.74%8.67%
5 months3.47%4.05%4.72%5.48%6.33%7.24%8.18%
6 months3.23%3.78%4.41%5.13%5.95%6.84%7.78%

The following shows the decomposition rates for 8.25% (weight % sodium hypochlorite) bleach below over time vs. temperature.

Degradation of 8.25% Bleach
1 day8.10%8.15%8.18%8.20%8.22%8.23%8.24%
2 days7.96%8.05%8.11%8.15%8.19%8.21%8.22%
3 days7.83%7.95%8.04%8.11%8.15%8.18%8.21%
4 days7.70%7.86%7.98%8.06%8.12%8.16%8.19%
5 days7.58%7.77%7.91%8.02%8.09%8.14%8.18%
6 days7.47%7.69%7.85%7.97%8.06%8.12%8.16%
1 week7.36%7.61%7.79%7.93%8.03%8.10%8.15%
2 weeks6.75%7.11%7.42%7.65%7.83%7.96%8.05%
3 weeks6.28%6.72%7.10%7.41%7.65%7.83%7.96%
1 month5.82%6.31%6.75%7.13%7.43%7.67%7.85%
2 months4.77%5.32%5.86%6.37%6.82%7.20%7.50%
3 months4.10%4.64%5.22%5.78%6.32%6.79%7.19%
4 months3.70%4.22%4.79%5.37%5.94%6.46%6.92%
5 months3.41%3.92%4.46%5.04%5.63%6.19%6.69%
6 months3.20%3.68%4.21%4.78%5.37%5.95%6.48%

Since day and night temperatures are not the same, one must use a special average that accounts for the non-linearity of degradation as a function of temperature. As derived in this post, the following table shows how you can take a known temperature difference between day and night and add an effective temperature adder to the nighttime temperature to get the effective temperature to look up in the above tables.

TempDiff . EffAvgTempAdder
..... 5 ............ 2.7
... 10 ............ 5.8
... 15 ............ 9.4
... 20 .......... 13.2
... 25 .......... 17.3
... 30 .......... 21.7
... 35 .......... 26.2
... 40 .......... 30.9


LifeTime Supporter
Nov 26, 2013
Wow. I see that the 10% sitting outside at Lowe's with average temperatures around 85 degrees here in Austin falls below the 8.5% I get at the grocery store in about two weeks. I didn't realize it was that fast and that's ignoring the heat and UV from the sunlight hitting it.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
The degradation rate drops rapidly at lower concentration so starting with 10% then using 85ºF temperature after two weeks this is 8.93%. Basically, the chlorine should not be sitting in the sun anywhere and in hot locations it should ideally be in air conditioning. Where I live the chlorine is outside next to the pool store where the sun hits it only during part of the day but our temps aren't as hot as yours. It's not great, but their turnover is fast enough that it's not a problem and I always make sure to get a crate under another (i.e. not the one on top).


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
Is there a way to test the liquid if you don't know the age to see how much it has degraded?
Taylor sells test kits for measuring bleach concentrations in % value. I forget the test kit number but you can find it on their website. It's marketed to the industrial cleaning industry to be used to test the effectiveness of bleach stored in large volume tanks. A pool owner could use it just as easily but being something one would use very infrequently, it may not be worth the cost.

Sent from my iPhone using Tapatalk

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
125,000. So you basically want to do a 1:10,000 dilution and then the ppm you measure is the Trade % for the chlorinating liquid so that's two 1:100 dilutions (1.28 fluid ounces out of one gallon or 0.48 teaspoons out of one cup). For 8.25% bleach where the 8.25% is the weight % of sodium hypochlorite, the Trade % is 8.64%. Trade % is the volume % of Available Chlorine so relates more directly to the ppm FC result when added to water -- where 1 gallon added to 10,000 gallons will raise the FC by the Trade % (i.e. 12.5 ppm FC for 12.5% chlorinating liquid).


Well-known member
Mar 29, 2012
While in the pressure washing business we used, and stored, many thousands of gallons of 12% bleach. And our mix really needs 0.5% accuracy to be effective. Your chart appears to correlate to our rules of thumb. That said, there are few points worth making.

- Never store bleach in direct sunlight. The UV and the heat generated by the sunlight are bleach killers. For this reason we buy, and store, bleach in thick, dark plastic drums.

- The temps above are liquid temps, not air temps. A bleach drum in 90° daytime air and 70° nighttime air may never reach 80° internally.

- Retail gallons may be very old. Pool stores often buy thousands and sell in relatively small quantities, so watch their tanks and inquire about turnover.

- Pressure washing suppliers sell cheaply and sell many thousands of gallons a week. This is a pro and con. Your stock will be fresh, but it may also be 15% or higher concentration. This happens because 15+% is unstable and decays to 12% rapidly. Manufacturers produce product with the expectation that it will degrade to 12% by sale time. With a high volume mover, it can get some HOT bleach.

If you're buying in volume, consider buying 15 gallon drums from a pressure washing supplier. They will sell you a container which you get refilled. Unless you live in Arizona, garage storage should be fine, and basement storage will be excellent. I kept mine in a outside storage shed under a shade tree with no issues.
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