Temperature and Chlorine consumption.

[TreeFiter]

Over the years, I have heard all kinds of things about swimming pool chemistry that I would classify as mythology. Many of these things are rooted in some sort of misunderstood truth. One thing that I suspect walks the line between fact and myth is that increased temperature would result in higher chlorine consumption. I did a search and read through a few posts on the topic, and I see that warmer water will lose more FC due to sunlight than cooler water. I also saw that algae will grow faster in warmer water, which will result in more rapid consumption of FC.

So I'm curious to learn more about what water temperature does. With regard to the combination of sunlight and temperature, how might Cyanuric acid make a difference? Are there other situations in which increased temperature will result in increased FC consumption?

What about a situation such as a heated pool with a solar cover. Here we would have warm water, but limited exposure to sunlight. Should it be expected that there would be increased FC consumption?

 

[chem geek]

I don't know where you saw that warmer water will lose more FC due to sunlight than colder water. That simply is not true. Please link to such posts. Chlorine loss from the UV in sunlight does not depend on temperature because it only depends on the number of photons per area entering into the pool and on the concentration of chlorine. The photons of light are traveling much faster (at the speed of light) than the molecules containing chlorine so the temperature which relates to the speed of those chlorine molecules is irrelevant. From the point of view of the photons, the molecules of chlorine are essentially standing still so the cross-section of those molecules which is the area with which the photon has a quantum probability of reacting with the molecule is independent of the temperature and only related to the concentration of such molecules in the water.

Now as noted in this paper there can be a temperature dependence on the subsequent chemical reactions that occur after the photochemical reaction occurs, but for the breakdown of chlorine by UV, it's pretty much all over with such breakdown since the probability of having the OH• and Cl• reform compared to other reactions that lead forming chloride and oxygen gas is low and not temperature dependent. At most, the intermediate concentrations of some intermediate species such as hydrogen peroxide may be higher at lower temperatures, but by that time it's too late and the chlorine is already on it's way to becoming chloride. Also as noted in this paper, there can be a small dependence on temperature in terms of the rate of relaxation from excited states so there is a small but negligible temperature dependence on having the HO-Cl vibrational state be excited and not break apart as often because of low temperature, but it takes a very low temperature before that effect would be seen in practice. As noted in this paper:

When the concentration of free chlorine is low (3.5 mg Cl/L) to moderate (70 mg Cl/L), the quantum yields of HOCl and OCl^- are 1.0 ± 0.1 and 0.9 ± 0.1, respectively.

The quantum yield of 1.0 means that every photon that reacts with HOCl results in its breakup into OH• and Cl• radicals while the quantum yield of 0.9 means that 90% of photons that react with OCl^- result in its breakup into O^-• and Cl• radicals. You can read more about photochemistry in this link.

Chlorine consumption that is related to temperature is for chemical reactions with chlorine such as oxidizing pool covers, bather waste, pollen, leaves, algae, etc. And yes, algae grows faster in warmer water but such consumption won't matter if there is sufficient FC/CYA since algae will get killed before it can reproduce so the rate will be based solely on the rate of blown-in algae spores which is usually not measurable (pollen, on the other hand, can be voluminous as can pods and other material dropped from trees).

A pool with a solar cover that is opaque to UV would lower the loss of chlorine from sunlight that is not temperature dependent but would increase the loss of chlorine from oxidizing the cover which is temperature dependent.

 

[TreeFiter]

OK, I'm a little bit confused. Perhaps I misunderstood some things along the way, but isn't the reason that Bromine is preferred in Hot Tubs that it will last longer than chlorine? In the case of a Hot Tub, what is causing the accelerated chlorine loss? Is this where the reaction rate is increased with chlorine as it oxidizes bather waste and organics? I'm not sure I understand why faster reaction times would be a bad thing in a hot tub.

So as far as pools go, if the pool has been properly maintained, and there is no algae living in it, the temperature shouldn't have any effect on the rate of chlorine consumption unless there is a significant amount of debris being introduced? Its typical for some debris to end up in a typical pool on a regular basis. Based on my experience, I don't think a normal amount of debris should have a major impact with regard to chlorine consumption at higher temperatures. The only time I see a problem is usually after a new deck is poured, and dirt, gravel, and concrete dust get in the pool. Other than that, I don't really see a noticeable change with regard to the amount of chlorine in the pool.

 

[chem geek]

That is also not true. When the (ignorant) pool and spa industry compares bromine loss to chlorine loss, they compare bromine against chlorine with no CYA in the water. That is of course completely unfair. Chlorine with no CYA does break down faster in sunlight than bromine, BUT with CYA in the water chlorine actually lasts longer than bromine.

As for a hot tub, that isn't about breakdown from sunlight, but about reacting with chemicals in the water and outgassing. Here again, chlorine with no CYA outgases and reacts with more chemicals so will get used up faster than bromine, BUT chlorine with CYA in the water will not and will be roughly comparable in its usage rate compared to chlorine though may be somewhat faster because chlorine oxidizes some bather waste that bromine does not (so water turns dull/cloudy sooner with bromine and needs chlorine to fix it on occasion).

If you have a pool with no debris and no algae, then if there is no sunlight there will be no noticeable chlorine loss. That isn't the situation for any real pool -- there is always some loss. If the water temperature gets much hotter, closer to spa temperatures, then the outgassing rate can become a small factor for loss even with CYA in the water though the loss rate is still somewhat slow, especially compared to bather load in spas. Remember that we have people doing an Overnight Chlorine Loss Test at SLAM levels and they report <= 1 ppm FC loss. The loss rate at normal chlorine levels will be much, much lower -- less than 0.2 ppm FC over 12 hours. The vast majority of chlorine loss is from sunlight. If a pool cover is used, then that is the next largest loss. So what you see is consistent with what I'm talking about -- the chlorine loss that is a function of temperature is pretty negligible in clean pools.

 

[TreeFiter]

Thanks for clearing that up for me. Little by little, I'm chipping away at the mythology I keep hearing on a daily basis. The other day, one of my coworkers was claiming that the water in a pool looks blue because of the salt in it. Today another coworker told me that there were probably phosphates in a pool based on how blue it looked. Things like temperature eating up chlorine come up all the time. I've also heard that high pH will eat up chlorine quite a few times. These things drive me nuts because I know they aren't true, and they are usually used to justify why a pool is not holding chlorine, or turning green. I just keep hoping that if I can eliminate the excuses, maybe someday I'll be able to get through to some of these guys the real reason their pools are turning.

 

[chem geek]

So high pH has some indirect truth to it because at higher pH there is more hypochlorite ion and that breaks down from the UV in sunlight faster than hypochlorous acid. However, at normal FC/CYA levels, most chlorine loss seems to be from chlorine bound to CYA because even though its rate is slower there is more of it. At SLAM levels, though, there's 10 times more chlorine unbound to CYA so the effects on that unbound chlorine from sunlight are more prominent so higher pH which has there be quite a lot more hypochlorite ion becomes much more of a factor.

To be specific, at a minimum FC/CYA ratio of 7.5% at a pH of 7.5 there is 0.031 ppm HOCl and 0.033 ppm OCl^- and the half-life of HOCl is 2 hours and 10 minutes (32% per hour) while for OCl^- it is 35 minutes (118% per hour). So the overall loss rate is 0.049 ppm FC per hour so over 8 hours of equivalent noontime sun that would be 0.39 ppm FC so as I wrote most chlorine loss isn't from the unbound chlorine in this situation. At a pH of 8.0, there is 0.026 ppm HOCl and 0.089 ppm OCl^- so the overall loss rate is 0.11 ppm FC per hour so over 8 hours that would be 0.91 ppm FC so noticeably more significant though still probably not the majority of loss (again that's from chlorine bound to CYA). So the pH change from 7.5 to 8.0 at normal FC/CYA levels increased chlorine loss by around 0.5 ppm FC.

At SLAM levels where the FC/CYA ratio is 40% at a pH of 7.5 (which is rare since the pH rises when adding chlorine) there is 0.301 ppm HOCl and 0.320 ppm OCl^-. So the overall loss rate is 0.47 ppm FC per hour so over 8 hours of equivalent noontime sun that would be 3.8 ppm FC so is now more significant. At a pH of 8.0, there is 0.256 ppm HOCl and 0.860 ppm OCl^- so the overall loss rate is 1.10 ppm FC per hour so over 8 hours that would be 8.8 ppm FC so noticeably more significant. So the pH change from 7.5 to 8.0 at SLAM levels increased chlorine loss by around 5 ppm FC. This is yet another reason to lower the pH ahead of doing a SLAM.

Note that the above computations are based on losses near the surface and do not account for any CYA shielding effect so actual losses may be lower.