Fantastic resource! Let's compare with the
Odyssey Manufacturing Co. table we've been using in the past. Note that the rate of degradation is very dependent on the quality of the bleach, specifically in not having any metal ions in it since they catalyze degradation. So below is the Odyssey table with the calculator values (days when concentration drops to half initial value) in
blue to the right of each value.
Initial ........................ Half Life in Days ........................
Concentration ........ 90ºF ........ 85ºF ........ 80ºF ........ 75ºF
20 Percent .......... 22.4
24 .... 31.8
33 .. 51.2
48.5 .... 69.2
15 Percent .......... 48 ..
44 .... 68 ..
61 ..... 100 .......... 148
10 Percent ............ 115 ......... 164 .......... 241 .......... 357
. 5 Percent ............ 371 ......... 527 .......... 735 .......... 1147
NOTE: Calculator only goes up to 19% so that was used for the "20 Percent" data above. Also, the calculator does not go beyond 60 days.
It is interesting to note that the calculator shows that higher initial concentrations continue with fairly rapid degradation even as the concentration gets lower over time -- the rate being linear with concentration over time, but to the square of the initial concentration. I believe this is wrong. For the Odyssey chart, the degradation rate is roughly varying on the square of the concentration which makes sense, but that implies a slowdown in degradation rate as the bleach degrades -- something the calculator doesn't do as much. In fact, the calculator seems to show a degradation rate (% loss per time) that depends only on the initial concentration and that does not seem right to me at all.
The temperature dependence seems consistent across all sources with a roughly 3.5 increase in rate for every 10ºC which is a 2x increase in rate for every 10ºF.
This link has another bleach decomposition calculator.
This paper also talks about the rate being proportional to the square of the hypochlorite concentration. Comparing the Miox and Powell calculators at 16% initial concentration and 95ºF (35ºC), one gets the following:
Days ...... Powell ....... Miox
. 0 ......... 16.000% .. 16.00%
. 1 ......... 15.138% .. 15.58%
. 2 ......... 14.364% .. 15.18%
. 3 ......... 13.665% .. 14.78%
. 4 ......... 13.031% .. 14.40%
. 5 ......... 12.454% .. 14.03%
. 6 ......... 11.925% .. 13.66%
. 7 ......... 11.439% .. 13.31%
So clearly the calculations vary a lot by manufacturer though for the above I believe Miox is closer to being accurate since Odyssey would predict a half-life (so getting to 8%) of roughly 28 days. The thing is that the decomposition reaction is 2-stage:
2OCl
^{-} ---> ClO
_{2}^{-} + Cl
^{-}
OCl
^{-} + ClO
_{2}^{-} ---> ClO
_{3}^{-} + Cl
^{-}
where the first step is the rate limiting one so this degradation pathway is proportional to the square of the hypochlorite concentration. There is also a secondary minor pathway:
2OCl
^{-} ---> O
_{2} + 2Cl
^{-}
but this pathway is minimized by removal of metals and suspended solids as well as by maintaining a higher pH. Then we have
this PDF file that shows rate constants as a function of concentration -- making them not really constants after all.
This older paper goes into more details about what is seen where the rate constant is a function of ionic strength -- more accurately, the rate constant is constant but one should use activities and not concentrations. They show the rate constant being roughly proportional to ionic strength above an ionic strength of around 1.7 (ionic strength in 6% bleach is around 1.8, for 12.5% chlorinating liquid it is around 3.6, so ionic strength is proportional to concentration). The net effect is that the rate varies with the cube of the concentration. This means that the half-life would vary as the inverse square of the concentration and this is roughly what one sees in the Odyssey table (varies from 3.2 to 5.1 instead of being 4 for halving the concentration).
This PDF file shows Figure 1 where the decomposition rate is not proportional to the hypochlorite concentration over time, but closer to the square of the concentration over time or greater. It's actually greater due to the increase in ionic strength since as noted in
this link, "The formation of a singly highly charged ionic complex from two less highly charged ions is favored by a high ionic strength because the new ion has a denser ionic atmosphere."
So in summary, my beef with the Miox calculator is that though the overall half-life data appears to be reasonable, the progression of degradation over time does not match other sources. As a simple example, look at the following at 95ºF using the Miox calculator starting with 18.0% vs. starting with 9.0%:
18.0% Day 0 ..........
9.00% Day 18.5 ..... Day 0
4.50% Day 37 ........ Day 60++ (about 77.5 days)
2.25% Day 56 ........
So the above makes no sense. They are claiming that if you start with 18% bleach, it will get go half strength, 9%, over 18.5 days, but will get to half that strength in another 18.5 days. However, if you started out with 9% bleach, then it would get to half strength over more than 60 days. This latter effect makes sense since the degradation rate is roughly proportional to the square of the concentration, but their charts for any given starting concentration make no sense since they degrade proportional to the concentration instead of to the square of the concentration or to something that resembles empirical data.
I would not use their calculator. I think we need to look further for a better calculator.