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Thread: Questions about Chlorine and CYA

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    Questions about Chlorine and CYA

    Perhaps these questions have been answered previously on this forum, but I've been unable to find it.

    I know it's said that when chlorine is added to a pool that contains CYA, some of it combines with CYA and is
    "held in reserve". My question is... Held in reserve for what? When is it ever released and why?

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    Re: Questions about Chlorine and CYA

    Hi and welcome. I think you may be confused about chlorine and CYA and their roles. A great place for you to start is Pool School. Then start reading the various threads in the forum sections, and it will all start to make sense.

    A good hint is put everything you thought you knew about pools aside, and start reading.
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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by ETS
    Perhaps these questions have been answered previously on this forum, but I've been unable to find it.

    I know it's said that when chlorine is added to a pool that contains CYA, some of it combines with CYA and is
    "held in reserve". My question is... Held in reserve for what? When is it ever released and why?
    Chlorine in a pool is in the form of Hypochlorous Acid, which is fairly unstable and ready to oxidize almost any biological it contacts. It also decomposes quickly into hydrochloric acid and oxygen in sunlight. The CYA protects the chlorine from the sunlight, at the expense of making it a little less unstable which slows down its oxidizing action.
    A better way to think about it is that it's wrapped-up. The wrapping protects the chlorine from UV light, and slows down its disinfecting process, but none of it is actually unavailable.
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    Re: Questions about Chlorine and CYA

    Thanks, frustratedpoolmom for your quick response! However, I have already read the pool school thing several times and have been working in the industry for a couple of years. I am quite familiar with the chlorine/CYA relationship. I know that stabilizer protects your chlorine from UV sun rays and that the higher your level the more chlorine that is required to maintain sanitization and prevent algae growth.
    I know that chlorine combines with CYA and is held in reserve as it states in pool school and its effectiveness is therefore reduced. What I don't quite understand is what becomes of those chlorine molecules that are attached to CYA molecules. What do those molecules do when they come in contact with algae or bacteria for example. Is the Chlorine then released to kill it? Or does the chlorine/CYA compound act together as a less effective killer? Or is the combined chlorine/CYA thingy useless unill some later time or what?

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    Re: Questions about Chlorine and CYA

    Oh, ok, I see where you're going now. I thought you were just confused, my bad

    Off to the shallow-end for me!
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    Re: Questions about Chlorine and CYA

    Thanks JohnT, for your reply.
    So is all the chlorine attached to CYA or is some attached and some free?

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    Re: Questions about Chlorine and CYA

    No problem frustratedpoolmom. And thanks again for your reply

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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by ETS
    Thanks JohnT, for your reply.
    So is all the chlorine attached to CYA or is some attached and some free?
    I would assume that that depends on the CYA level and the FC level, with higher CYA levels allowing more chlorine to link up. Hopefully somebody knows the answer and can respond.
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    Re: Questions about Chlorine and CYA

    A percentage of the chlorine is bound to CYA. If some of the unbound chlorine gets used up, chlorine will start unbinding from CYA to reestablish the correct percentage bound.

    Bound chlorine has very little disinfecting ability. But once it is unbound it disinfects again.
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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by frustratedpoolmom
    Off to the shallow-end for me!
    FPM, I know your pain! I once wandered into the deep end and nearly drowned! (And that is some feat for a mermaid!)

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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by The Mermaid Queen
    Quote Originally Posted by frustratedpoolmom
    Off to the shallow-end for me!
    FPM, I know your pain! I once wandered into the deep end and nearly drowned! (And that is some feat for a mermaid!)
    Ditto! I've had to dog paddle out of there on occassion (& that is some feat for a butterfly)
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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by JasonLion
    A percentage of the chlorine is bound to CYA. If some of the unbound chlorine gets used up, chlorine will start unbinding from CYA to reestablish the correct percentage bound.
    Well then, what is the correct percentage? How does it know when some of the chlorine is used up and when it has reached the correct percentage? I'm not sure I buy that explanation yet.
    The reason I'm asking is because I work in a Pool Store as a water test technician and am trying to get a handle on this so I can give my customers a more accurate prescription for their algae problems. You see, our store is very busy ( I sometimes do over 100 tests a day) and don't have time to do a lot of sophisticated tests. So I can only measure FC up to 5.0. The computer program we use calculates gallons of chlorine to use for different levels of algae bloom, but I'm sure it doesn't factor in CYA levels. I have to do a lot of guessing.

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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by ETS
    Well then, what is the correct percentage? How does it know when some of the chlorine is used up and when it has reached the correct percentage? I'm not sure I buy that explanation yet.
    The reason I'm asking is because I work in a Pool Store as a water test technician and am trying to get a handle on this so I can give my customers a more accurate prescription for their algae problems. You see, our store is very busy ( I sometimes do over 100 tests a day) and don't have time to do a lot of sophisticated tests. So I can only measure FC up to 5.0. The computer program we use calculates gallons of chlorine to use for different levels of algae bloom, but I'm sure it doesn't factor in CYA levels. I have to do a lot of guessing.
    Let me jump in here since I just noticed this thread.

    Chemical Equilibrium

    It's a chemical equilibrium between chlorine bound to CYA and hypochlorous acid which is the "active" form of chlorine. This can be described as follows:

    HClCY- + H2O <<<---> HOCl + H2CY-
    "Chlorine bound to CYA" + Water <<<---> Hypochlorous Acid + CYA

    I show the arrows mostly going to the left because at typical CYA levels in a pool the vast majority of chlorine is bound to CYA. When I say "bound", this is technically a separate chemical species -- it's not just some sort of loose association. This chemical species has very little (if any) capability to kill pathogens, prevent algae growth, or oxidize bather waste. It is hypochlorous acid that is the chemical species that does all of these things. With chemical equilibrium, chemical species are colliding into each other frequently in all sorts of combinations, but most don't react. When the combinations shown above (on the left and on the right) collide, they sometimes form the items on the other side. This happens very quickly at normal temperatures, back and forth, back and forth. As for where things settle out in terms of percentages, this is a function of the ratio of reaction rates going from right-to-left vs. left-to-right and this ratio (for simple reactions) is known as the equilibrium constant for the reaction. The equilibrium constant for the above reaction at the concentrations of chlorine and CYA in pools is way toward the left. When there is 3.5 ppm FC with 30 ppm CYA and a pH near 7.5, 97% of the chlorine is bound to CYA, 1.5% is hypochlorous acid, and 1.5% is hypochlorite ion.

    The rate at which the above chemical equation occurs is reasonably fast. If all of the hypochlorous acid on the right-hand-side were used up, then it takes around 4 seconds for half of the amount of "Chlorine bound to CYA" to get converted to hypochlorous acid. In practice, very little gets converted (unless chlorine is rapidly getting used up) and the reaction slows down and then essentially stops and this all occurs in far less than one second. In fact, because this reaction (as well as companion reactions that are only 0.25 seconds that I don't show for simplicity) occurs so quickly, chlorine tests aren't really measuring the hypochlorous acid level, but rather both chlorine species on the left and right above (plus hypochlorite ion which I didn't show). That is, the chlorine tests are measuring both active chlorine and that in reserve because as soon as they use up some active chlorine, the reaction moves to the right releasing more chlorine that then gets used up (reacting with dye, for example). The chemicals don't have to "know" anything -- they are just blindly colliding, but when the chlorine on the right gets taken away, the reaction from left-to-right is all that is occurring (i.e. there isn't any right-to-left reaction because there isn't any hypochlorous acid because it's reacted with dye in the chlorine test).

    Chlorine/CYA Relationship

    It turns out from the chemistry that the same ratios of FC/CYA have roughly the same amount of active chlorine, hypochlorous acid (technical details are in this post). So if the CYA is doubled, then the FC has to be doubled to get the same level of disinfection, algae kill, oxidation, etc. If the CYA is doubled and the FC is kept the same, then the amount of active chlorine is cut in half. If this amount of chlorine kills algae more slowly than algae can reproduce (double in population), then the water will usually turn dull, then cloudy, then cloudy-green as an algae bloom develops. This chart in the Pool School shows the level of chlorine needed to prevent algae growth at various CYA levels and also gives the amount need to shock the pool to kill algae in an algae bloom reasonably quickly.

    You can read the introduction sections in the technical paper written in 1974 in this link that describes in more detail this "reservoir" chlorine concept. Just keep in mind that the terminology in those days was different and the "Free Chlorine" in that paper isn't what is measured in today's chlorine tests and the paper's definition excludes the chlorine bound to CYA which they call reservoir chlorine.

    You can also look at the graphs in this post that show the major chemical species concentrations vs. pH without CYA and with CYA.

    Why Hypochlorous Acid Is Effective While Chlorine Bound to CYA Is Not

    As for why hypochlorous acid is such an effective pathogen and algae killer, take a look at the molecule here and notice how similar it is to water here. This similarity is why this molecule rather easily passes through cell membranes to react with all sorts of chemical species (mostly containing nitrogen, which includes proteins, enzymes, DNA, RNA, etc.) disrupting cell processes that then kill them (see this thread). Now take a look at Dichlor here which is very similar to the main "chlorine bound to CYA" chemical species where the only difference is that one of the two chlorine is removed from being attached to a nitrogen and this makes the overall molecule negatively charged. Notice how much larger this molecule is and that it doesn't look anything like water. The primary chemical species with chlorine bound to CYA are negatively charged and are repelled by most cells since they generally have negatively-charged surfaces (on the outside). It is not only their charge and size that makes these chemicals less reactive, but the "eagerness" (oxidation potential) of the chlorine to release from the CYA ring is somewhat limited -- it does sometimes leave, but it mostly likes to stay where it is (remember the reaction I showed above with water and how mostly it's to the left hand side).

    You might think that with chlorine getting released from the reserve quickly that this should count towards being effective, but that's not correct. You can think of an analogy of people fighting a war where you have a group of "active" soldiers on the front lines fighting hand-to-hand combat with an enemy. You have many more soldiers in the rear that are not directly fighting and are in "reserve". When some soldiers in the front lines get killed or injured, you can replace them with some from the reserve, but the rate at which you will be able to wound or kill the enemy is only dependent on the number of soldiers you have on the front lines doing the hand-to-hand combat. It doesn't matter how many you have in reserve. The amount in reserve only tells you how long you can continue to fight -- not the RATE at which you kill your enemy.

    Does that help?

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    Re: Questions about Chlorine and CYA

    Quote Originally Posted by chem geek
    Does that help?

    Richard
    NO.

    JK. I love ya Richard!
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    Re: Questions about Chlorine and CYA

    That's perfect Richard. Exactly what I was looking for.

    I may have some more related questions as I mull this information over. Right now I have to get ready for work, but I'll go to work with a much better understanding of what I'm doing, thanks to you.

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    Re: Questions about Chlorine and CYA

    So let me see if I have this straight.
    If the hypochlorous acid on the right of the equation is reduced then that upsets the equillibrium and then as a result hypochlorous acid attached to CYA on the left of the equation is released in the amount necessary to reach equillibrium again.
    The release is actually caused by 1. the upset in equillibrium and 2. the collision of the different species?

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    Re: Questions about Chlorine and CYA

    Chlorine and CYA are constantly combining and breaking up at all times. The rate at which they combine depends on both the chlorine level and the CYA level. The rate at which they break up depends only on the level of bound chlorine-CYA. The more chlorine and the more CYA, the more quickly they combine.

    If you start with very little bound, they will be combining much more quickly then they are breaking up. There simply isn't much to break up, so the total amount breaking up is low. Eventually, as the unbound chlorine level falls (because it is combining with CYA), the rate at which they combine will fall. Meanwhile the amount of combined increases and so the rate at which it breaks up increases. Eventually those two rates are equal and everything stays in balance.

    Now say something magically removes all the unbound chlorine. The bound chlorine will still be breaking apart at some rate, but there isn't any (or hardly any) unbound chlorine so it can't combine very quickly. Therefore the amount of unbound chlorine will rise. Eventually the amount of unbound chlorine rises back to the equilibrium level and combining and breaking up will again proceed at equal rates.
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    Re: Questions about Chlorine and CYA

    Thanks Jason.
    What you said confused me until I googled "chemical equillibrium".
    It exists when their is no CHANGE in the rate of activity or the concentration of chemicals in solution.
    Therefore, when hypochlorous acid is reduced drastically (as when it comes in contact with an algae bloom) chemical concentrations are now out of balance, so activity increases until equillibrium is again reached. Right?

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    Re: Questions about Chlorine and CYA

    Right.
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    Re: Questions about Chlorine and CYA

    I came on this thread from a reference ChemGeek made elsewhere...

    In reading this last question - it's important to note that unless more CL without CYA is added to replenish the used CL then a new equilibrium point is reached - but not the same one as before the CL was used. basically I'm restating what was said that as the cya doubles and the CL stays the same then you end up with half the sanitizing agent. If we add just CL (from liquid) then we can reset to the original equilibrium point. But if we use dichlor or trichlor which adds more CYA then over time you approach the point where you no longer have an equilibrium point that maintains enough FC to safely sanitize the pool. How fast you get to this depends on how fast the cya rises from new addition minus dilution due to splash out or rain.

    Did I get this right?
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