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.
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:
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:
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- ---> O2 + 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.
that´s bad news and much faster than i´d always assumed. i´ve just taken delivery of 80 containers of liquid chlorine which is sitting in my garage. i was hoping to use between now and the end of august.
I don't think the Miox calculator is a marketing ploy. It does appear that they used some actual data to come up with rates to get to roughly the correct half-life, but the curve in between is wrong and should degrade more quickly at first and slow down later -- in percentage terms. They show a constant percentage reduction as the strength gets lower so are probably under-estimating the rate of loss before the point where half has degraded and over-estimating the loss after that halfway point.
As for 15% chlorinating liquid in 110ºF day but lower nighttime temperatures, that would be expected to lose half its strength in around 1-2 months so after 2 months it would still be usable, though at lower strength -- perhaps in the 4-6% range. The key is temperature. Keeping the chlorine cooler will have it last much longer.
Based on the chart at the bottom of this link, the half-life of 15% chlorine at 75ºF is 148 days while for 10% chlorine it is 357 days. White's chart shows 12% degrading to half (6%) in just over 60 days. His data is old and likely using bleach that contained small amounts of metal contaminants. Those are what make the degradation go much faster. I know from my own experience with 12.5% chlorinating liquid kept outside at day/night temps of 81ºF/54ºF (roughly 69ºF average for longer day temps) that it barely degrades over a month -- perhaps dropping to 12.0% or 11.5% at the lowest and most certainly is not dropping to around 10% as White would indicate.
I am converting to all LC next season and I'm a little concerned about the temperature degrading the concentration. I believe I'll need about 18 gallons per month for my 27K gallon pool. I can get 12.5% in 15 gallon drums at $47 each or 5 gallon at $17 each.
So I was thinking about digging a hole and dropping my plastic 15 gallon drum in it so the ground would keep it cooler. I would keep the top just above ground level to keep any water out.
Do you guys think this will work out okay or am I just wasting my time? Or should just go with the 5 gallon jugs which would keep them fresher.
At 2 ppm FC per day, you would need around 13 gallons per month of 12.5% chlorinating liquid for your 27,000 gallon pool.
As for bleach or chlorinating liquid stability, it depends a lot on the quality of product since it should be free of metals that cause the degradation. Assuming such a product, you can look at the table at the bottom of this link to get an idea of the stability of chlorine vs. concentration and temperature. It varies roughly with the square of the concentration and roughly has half the half-life for every 10ºF temperature increase. If you can keep the average temperature at 75ºF or below, you should be fine for a one month supply. If the temp is 90ºF, then that's when you'd have significant degradation, but still after just one month it would still be usable and in practice your temps probably aren't 90ºF at night.
I think I'll try my idea of keeping the chlorine cooler by putting my drum in the ground.
I was thinking closer to 3 ppm FC but I like the sound of 2 ppm better. Pool is in full south central PA sun and right now the cya is around 100.
I dumped about 16" before closing and with my mesh cover this winter I'm hoping with a lot of rain it will get down to at least 70 and eventually my goal is 40. And I want to see how the LC injection works before jumping into the BBB method.
I purchased a Stenner45MPHP22 Single Head Fixed Output High Pressure Pump with EH40 Intermatic timer and backup relays for timer failure. Those tabs in the chlorinator sure were easy but the cya keeps getting away from me and it's time for a change. Don't want to go the swg either because of cost and other issues.
The information on here is very enlightening to say the least. When I go into a pool store and start telling them some things I've learned on here, they say they've never heard of such things and that I worry too much and I should be running FC around 1-3 ppm with 100 cya. Now I know why I keep getting a small amount of algae and shocking doesn't keep it away.
Thanks Again everyone for all your great pool knowledge!
With a CYA of 100 ppm, you should have low chlorine loss so could have less than 2 ppm FC per day. However, if you've already got nascent algae growth then you need to kill that off by SLAMing the pool and that takes a lot of chlorine with your high CYA level. That's why we usually don't recommend going above 80 ppm.
As for pool stores, they don't know about the chlorine/CYA relationship because the manufacturers and their reps selling stabilized chlorine products don't tell them about it because they believe it would heard sales. They instead claim "only FC matters; CYA doesn't matter". The science behind chlorine and CYA has been known definitively since at least 1974. You will only frustrate yourself if you try and teach the pool store anything unless they express a true desire to know the truth.
By the way, we recommend not closing a pool until the temp is consistently below 60F (I usually do it even cooler) and opening it up again before the temp gets consistently above 60F. This helps minimize (for me it eliminates) the amount of clean up you will have to do in the spring.