Breakdown of Bleach Over Time by Storage Temp

Predicting the loss of strength of stored bleach over time based on the storage temperature

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 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

TIME	100ºF	95ºF	90ºF	85ºF	80ºF	75ºF	70ºF
Initial	12.50%	12.50%	12.50%	12.50%	12.50%	12.50%	12.50%
1 day	11.92%	12.10%	12.22%	12.31%	12.37%	12.41%	12.44%
2 days	11.42%	11.74%	11.97%	12.13%	12.25%	12.33%	12.39%
3 days	10.99%	11.41%	11.73%	11.96%	12.13%	12.25%	12.33%
4 days	10.61%	11.12%	11.51%	11.80%	12.02%	12.17%	12.28%
5 days	10.28%	10.85%	11.30%	11.65%	11.91%	12.09%	12.22%
6 days	9.98%	10.60%	11.10%	11.50%	11.80%	12.02%	12.17%
1 week	9.71%	10.37%	10.92%	11.36%	11.69%	11.94%	12.12%
2 weeks	8.33%	9.13%	9.87%	10.51%	11.04%	11.46%	11.78%
3 weeks	7.45%	8.29%	9.10%	9.85%	10.50%	11.04%	11.47%
1 month	6.68%	7.51%	8.36%	9.18%	9.93%	10.57%	11.11%
2 months	5.14%	5.90%	6.73%	7.60%	8.49%	9.33%	10.10%
3 months	4.27%	4.95%	5.72%	6.57%	7.48%	8.40%	9.28%
4 months	3.79%	4.42%	5.13%	5.93%	6.81%	7.74%	8.67%
5 months	3.47%	4.05%	4.72%	5.48%	6.33%	7.24%	8.18%
6 months	3.23%	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

TIME	100ºF	95ºF	90ºF	85ºF	80ºF	75ºF	70ºF
Initial	8.25%	8.25%	8.25%	8.25%	8.25%	8.25%	8.25%
1 day	8.10%	8.15%	8.18%	8.20%	8.22%	8.23%	8.24%
2 days	7.96%	8.05%	8.11%	8.15%	8.19%	8.21%	8.22%
3 days	7.83%	7.95%	8.04%	8.11%	8.15%	8.18%	8.21%
4 days	7.70%	7.86%	7.98%	8.06%	8.12%	8.16%	8.19%
5 days	7.58%	7.77%	7.91%	8.02%	8.09%	8.14%	8.18%
6 days	7.47%	7.69%	7.85%	7.97%	8.06%	8.12%	8.16%
1 week	7.36%	7.61%	7.79%	7.93%	8.03%	8.10%	8.15%
2 weeks	6.75	7.11%	7.42%	7.65%	7.83%	7.96%	8.05%
3 weeks	6.28%	6.72%	7.10%	7.41%	7.65%	7.83%	7.96%
1 month	5.82%	6.31%	6.75%	7.13%	7.43%	7.67%	7.85%
2 months	4.77%	5.32%	5.86%	6.37%	6.82%	7.20%	7.50%
3 months	4.10%	4.64%	5.22%	5.78%	6.32%	6.79%	7.19%
4 months	3.70%	4.22%	4.79%	5.37%	5.94%	6.46%	6.92%
5 months	3.41%	3.92%	4.46%	5.04%	5.63%	6.19%	6.69%
6 months	3.20%	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

In the Handbook of Chlorination by Geo. C. White, 1999 it states that sodium hypochlorite degradation is as follows:

Temp 77 degrees

Days	12%	10%
10	11.5	9.7
20	11.0	9.0
30	9.7	8.7
40	8.8	8.5
60	6.2	8.0
90	4.7	7.2

Looks like the 10% is more stable for longer periods of time at 77 degrees.

How To Test

Taylor sells the K-1579 test kit for measuring bleach concentrations in % value. 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.