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Thread: FC - Concept of "Reserve"

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    FC - Concept of "Reserve"

    From the Pool School:
    Cyanuric acid, often called stabilizer or conditioner, both protects FC from sunlight and lowers the effective strength of the FC (by holding some of the FC in reserve). The higher your CYA level, the more FC you need to use to get the same effect.
    Where can I learn more about the concept of reserve FC as it relates to CYA? (A search brings up too many references to be useful.) Basically, I don't understand why FC has less effect at CYA 100 than it does at CYA 50. Links would be fine - thanks.
    Jaywalker

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    JasonLion's Avatar
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    Re: FC - Concept of "Reserve"

    CYA binds to some percentage of the FC. The percentage of the FC that is bound with CYA depends on the CYA level. More FC is bound up with CYA at higher CYA levels. FC bound to CYA is mostly protected from the effects of sunlight, and it is also much less effective at sanitizing (preventing algae etc).

    Since there is an essentially constant percentage of FC bound to CYA, as the unbound FC gets used up some of the bound FC will be released to the active state (to maintain the percentage). The bound up FC is essentially "held in reserve", available to be used when the unbound/active FC gets used up.

    At higher CYA levels, a higher percentage is bound up, so our CYA/Chlorine table recommends that you raise the total amount of FC. When you follow the table, increasing the FC level as the CYA level increases, a more or less constant amount of active FC is available to prevent algae. If you don't raise the FC level to correspond to the CYA level, less and less FC would be available in the active state and you would be at risk.
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    Re: FC - Concept of "Reserve"

    you might find this thread helpful...chemgeek is a very knowledgable resource on this subject

    http://www.poolforum.com/pf2/showthread.php?t=4236
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    Re: FC - Concept of "Reserve"

    (The Pool Water Chemistry link to The PoolForum above is also available here at TFP.)

    Even though this link is to the 1974 scientific paper that determined the equilibrium constants for the rather complex system of chemical equations concerning varying amounts of chlorine and hydrogen combining with a CYA core ring, the text in the introduction is quite readable. Just keep in mind that what we now call Free Chlorine (FC) is what the paper calls "reservoir chlorine" and what the paper calls free chlorine is just the chlorine unbound to CYA (so hypochlorous acid and hypochlorite ion).

    I go into more detail of the derivation of why the FC/CYA ratio can be used as a rough proxy for the equivalent chlorine concentration with no CYA in this post.

    It comes right down to chemical equilibrium between chlorine and CYA vs. chlorine bound to CYA where the equilibrium is shifted much more to chlorine bound to CYA at normal pool conditions. The chlorine bound to CYA has minimal activity in terms of oxidation and sanitation. Because this is a chemical equilibrium, as chlorine gets used up, more is released from that bound to CYA and this happens relatively quickly and in the time of the FC test which is why the FC test really measures reservoir chlorine (that is, chlorine that is bound to CYA as well as that which is unbound). However, actual chemical reaction rates, including the rate of killing pathogens or algae, is dependent only on the actual chemical species doing that killing which is hypochlorous acid (for oxidation reactions it is sometimes hypochlorite ion, but it is not chlorine bound to CYA). There is an analogy to soldiers fighting that I wrote in this post.

    You might also look at the "Chlorine/CYA Relationship" section in this post where there are links to numerous scientific studies showing how the sanitizing and oxidizing ability of chlorine drops dramatically when there is CYA in the water and in fact it generally follows the predicted relationship using the equilibrium constants from the 1974 paper. You can also look at some of the charts in the Word/PDF file where I suggest improvements to the CPO course.

    Richard
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    Re: FC - Concept of "Reserve"

    Thank you - that's what I needed.
    Jaywalker

    In-ground, 11,000 gallon, plaster, in-line trichlor pucks or BBB bottled chlorine, 1.5 HP pump @ ~100 gallons/minute, cartridge filter

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    Re: FC - Concept of "Reserve"

    Thank you. I now understand the concept that the chlorine bonds to CYA forming a "reservoir," per O'Brien, et al. I also understand that the bound chlorine serves little or no antiseptic purposes.
    chem geek: For example, when the pH is 7.5 and the FC is 3.5 ppm and the CYA is 30 ppm, then 97% of the chlorine is attached to CYA. Nevertheless, the chlorine attached to CYA gets measured in the FC test because the chlorine gets released from the CYA quickly enough to replenish the chlorine that is consumed by the test (by reacting with dye). [EDIT] Free Chlorine (FC) does not measure active chlorine, but rather the chlorine reserve or reservoir that is mostly inactive. [END-EDIT]

    In a very real sense, CYA acts as a hypochlorous acid buffer holding chlorine in reserve, but significantly lowers its concentration which determines the rate of any reaction in which chlorine participates.
    Is the release of chlorine from bound form to active form a time function, or is it a kind of demand function caused by the depletion of active chlorine due to antiseptic processes? IOW, does the consumption of active chlorine, e.g., the dye test, create a kind of vacuum that allows the reservoir to replenish more quickly than it otherwise would absent the dye test or algae?
    Jaywalker

    In-ground, 11,000 gallon, plaster, in-line trichlor pucks or BBB bottled chlorine, 1.5 HP pump @ ~100 gallons/minute, cartridge filter

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    Re: FC - Concept of "Reserve"

    The rate of release of chlorine from being attached to CYA is a fixed rate proportional only to the concentration of the amount of that chemical species (i.e. the chemical species that has chlorine bound to CYA). However, there is a reverse reaction rate of chlorine combining with CYA to form chlorine bound to CYA and that is proportional to the product of concentrations of hypochorous acid and CYA. So for the dye test which rapidly depletes hypochlorous acid, the reverse reaction rate goes close to zero so essentially the forward reaction rate is all that remains.

    Since that forward reaction rate is dependent on the concentration of chlorine bound to CYA and since that goes down as more chlorine is released, the reaction rate is an exponential decay (this comes from the reaction rate being a 1st order differential equation). The half-life for the dominant chemical species where chlorine is bound to CYA is 4 seconds which means that half of this is depleted in 4 seconds. However, there is another chemical species also with chlorine bound to CYA with a half-life of 0.25 seconds and these two chemical species can equilibrate very quickly so in practice the net half-life is probably closer to the 0.25 seconds. So the reaction will be 1-0.54 = 93.75% complete in one second so in practical terms the Free Chlorine (FC) test will be measuring the reservoir chlorine amount though technically speaking it will continue to measure more and more chlorine forever, but the increment gets so small as to be in the noise of everything else that can happen.

    When chlorine isn't consumed as rapidly, as is more typical during pool use, then it is replenished even more quickly since it's not half of it that needs to be replaced. Even so, it doesn't have any major significance to killing or oxidization rates. There is some benefit of having that extra chlorine reserve so that you do not run out locally especially under a heavier bather load, but that's not a reaction rate issue and is instead a capacity/reserve issue.

    Richard
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    Re: FC - Concept of "Reserve"

    chem geek: The rate of release of chlorine from being attached to CYA is a fixed rate proportional only to the concentration of the amount of that chemical species (i.e. the chemical species that has chlorine bound to CYA).
    So, when a large concentration of chlorine is added, as during shocking events, the rate of release from bond is higher and thus more susceptible to UV degradation? I believe I understand the issue now - higher levels of CY will bond more chlorine, but its release will be proportionate to what remains bound - at first, 3% of 3.5ppm in your example (pH is 7.5 and the FC is 3.5 ppm and the CYA is 30 ppm, then 97% of the chlorine is attached to CYA). Higher levels of chlorine in higher levels of CYA will produce lower, and diminishing, levels of active chlorine that will diminish, leaving not enough to process algae, etc.

    Active chlorine doesn't appear to drop to drop to zero, however, over several days normally. Are we, therefore, concerned about it getting too close to zero so that an unfortunate event might absorb the active chlorine before it can be replenished from the reservoir? That's the part I'm having trouble reconciling, given the quarter-second rate of release from bond.
    Jaywalker

    In-ground, 11,000 gallon, plaster, in-line trichlor pucks or BBB bottled chlorine, 1.5 HP pump @ ~100 gallons/minute, cartridge filter

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    Re: FC - Concept of "Reserve"

    You are mixing up two different, though related concepts -- reaction rates and chemical equilibrium -- and also mixing up the reaction rate of release of chlorine from CYA with the reaction rate of chlorine killing algae. Let me describe this in more detail to hopefully clarify it. I'll use the primary chemical reaction ignoring other paths.

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

    At 77şF, the forward reaction rate constant (ignoring ionic strength effects), kfor, (from this paper) is 0.17 sec-1 (t1/2 = ln(2)/0.17 = 4.08 seconds but there is another pathway I don't show that has a faster half-life of 0.25 seconds) while the reverse reaction rate constant, krev, is 7.4x104 M-1sec-1. The equilibrium constant is the ratio of forward to reverse reaction rate constants so K = kfor/krev = 0.17/7.4x104 = 2.4x10-6 (with rounding -- the equilibrium constant was the most accurately determined of these numbers). The formulas are the following (where "M" is concentration in moles/liter):

    Rate of Forward Reaction (M/sec) = kfor*[HClCY-]
    Rate of Reverse Reaction (M/sec) = krev*[H2CY-]*[HOCl]

    A reaction is at equilibrium when the forward and reverse reaction rates are equal which means there is no net change in product or reactant concentrations. Setting the above two equal to each other results in the following definition for the equilibrium constant (both in general in terms of reaction rate constants and specifically for this reaction in terms of reactant and product concentrations).

    Equilibrium Constant (K) = kfor/krev = ([H2CY-]*[HOCl])/[HClCY-]

    Note that none of the above has to do with the rate of killing algae. That is a completely separate sort of reaction (actually, many different reactions) that I over-simplify as follows:

    HOCl + "living algae" ---> "dead algae" + other products (combined chlorine or oxidized organics including nitrogen and carbon dioxide gasses)

    There is also another reaction (of sorts) in terms of algae growth that I generically write as follows:

    "living algae" + "nutrients" ---> "twice as much living algae"

    For bacteria, the time it takes to double in population under ideal conditions is around 15 to 60 minutes. For algae, it's around 3 to 8 hours. So the key factor to killing bacteria or algae is for the rate of killing to be faster than the rate of growth. Specifically, killing more than half of the bacteria or algae in the time that it takes bacteria or algae to double in population. Notice that the rate of killing algae from the above equation (the one starting with HOCl) is dependent on the hypochlorous acid concentration and in practice it's pretty much proportional to that concentration. This is why the amount of chlorine in reserve is irrelevant except to ensure that you don't run out of chlorine during the killing process. The amount in reserve has nothing to do with the rate of killing algae -- only the concentration of hypochlorous acid matters for that. This concentration is roughly proportional to the FC/CYA ratio. Remember the soldier analogy -- it doesn't matter if I've got millions of soldiers in reserve if I've only got a handful on the front lines doing the actual killing. Even if these front-line soldiers were instantly replaced when they got used up killing the enemy, the enemy can increase in size (as bacteria and algae do by growing) faster than they are killed -- the amount of soldiers in reserve does nothing to help in this case. Fighting a war with nearly all of the soldiers in reserve rather than on the front-lines is a prescription for failure.

    So the key is to have an FC/CYA ratio that kills algae faster than it can grow; preferably quite a bit faster to have some margin/cushion for all kinds of factors such as imperfect circulation. The minimum FC for each CYA level, which is pretty much a minimum FC/CYA ratio, ensures that chlorine will kill algae faster than it can grow in almost all conditions -- that is, in spite of having plenty of nutrients. Though the equation above (the one with "nutrients") would seem to imply that increasing the amount of nutrients will have the algae grow faster and faster, this only works up until one reaches some limiting factor for some nutrient and there is ALWAYS such a factor since the amount of sunlight is fixed, for example, and the rate of cell division is also ultimately limited by temperature due to physical processes such as alignment of organelles and DNA in cells. This is why pools even with high phosphates and nitrates can still have algae completely controlled using chlorine alone, though such pools are no question very reactive if you let the chlorine get too low such that algae grows faster than the chlorine kills it. Very reactive is still on the order of 3 hour doubling times, however, so it's not like it's in seconds or anything like that. For bacteria, it's still 15 minutes or so, but can be more noticeable since one bacteria can become 4 billion in 8 hours if there is a doubling every 15 minutes. For algae, even with 3 hour doubling, one algal cell can become 256 after one day. This is why algae growth almost always starts out as being an invisible increased chlorine demand and then only later becomes visible often as dull water, then cloudy, and then green (some algae go pretty much straight to green as even small amounts make their chlorophyll visible).

    So a shocking event increases the FC relative to CYA so that the FC/CYA ratio is higher and that means more of the FC will exist as hypochlorous acid which is the killing agent for bacteria and algae. You can't really say that it forces more of a release of chlorine bound to CYA since normally the chlorine you add for shocking is already unbound -- that is, it's unstabilized chlorine. It would be more accurate to say that adding unstabilized chlorine results in less of it getting bound with more of it remaining unbound, but again the net result is dependent on that FC/CYA ratio and by shocking you are increasing FC without changing CYA.

    If you have higher levels of chlorine with higher levels of CYA, such that the FC/CYA ratio remains constant, then this has roughly the same amount of hypochlorous acid so the same rate of killing power against bacteria and algae. I think what you meant was that increasing levels of CYA without proportionally increasing levels of FC will result in slower killing power possibly to the point of algae growing faster than chlorine can kill it. This is because adding more CYA, say from stabilized chlorine (Trichlor or Dichlor) without raising the FC target results in a lower FC/CYA ratio so lower hypochlorous acid concentration.

    The concern is not about the FC getting to zero, but rather getting too low such that the FC/CYA ratio gets too low thereby allowing algae to grow faster than chlorine can kill it. The amount of chlorine in reserve isn't usually of concern since there is usually plenty of it available. It's the instantaneous hypochlorous acid concentration that is of greater concern and that is roughly proportional to the FC/CYA ratio (at a pH of 7.5, it's roughly half that ratio). The concern for running out of chlorine comes up more frequently when one isn't using any CYA at all since pools may have low FC levels in that case. The German DIN 19643 standard specifies 0.3 to 0.6 ppm FC (or 0.2 to 0.5 ppm if ozone is used) with no CYA and such low levels of chlorine may get used up locally by bather load events though that doesn't seem to be a concern by those using such a system in commercial/public pools in Europe. That system, however uses activated carbon in the filtration system to strip out all chloramines and chlorine itself which then needs to be re-injected after further coagulation/filtration removes organic precursors. This system is supposed to reduce the amount of disinfection by-products and it does seem to help in that regard, though not as much as one might think because the hypochlorous acid concentration is still higher than in most pools that use CYA. In practice, the pools usually don't target much below 0.5 ppm in order to have at least some reasonable amount of chlorine for local bather load. If one instead used 4 ppm FC with 20 ppm CYA, then one could have the equivalent of 0.2 ppm FC with no CYA while still having plenty of reserve, but that isn't practical if you're going to be removing all chlorine in every turnover of water.

    Richard
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    Re: FC - Concept of "Reserve"

    That’s some good info. Understanding what’s going on really helps when deciding what chemicals to pour into something you’re about to jump into.

    Slightly off topic but as a chem minor I’ve never understood why pools deal in ppm and not molarity? I was shocked when I looked at the first test kit I bought and saw ppm. Haven’t put any thought into it but maybe if I was a chem major I would know .

    I also think pools should be measured in liters and not gallons. Maybe this might make it easier for some people. Just my two cents…
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    Re: FC - Concept of "Reserve"

    Deadrabbit, I agree. This whole ppm thing is mildly irritating to me too! Everything's easier in metric.

    Nice nickname, by the way
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    Re: FC - Concept of "Reserve"

    If it makes you feel any better, ppm is essentially a metric measurement since it's really millgrams per liter (mg/L) which is the same as parts-per-million (ppm) since the density of pool water is very close to 1.0 grams per milliliter (g/ml) which is the same as 1,000,000 milligrams per liter (mg/L).

    I think they don't use millimole per liter (mM/L) or micromole per liter (µM/L) because people are usually adding product by weight so would have to use factors to convert using the molecular weight of a product. Of course, those of us using non-metric weights have to do conversions anyway. And except for chlorine gas, one has to factor in the % Available Chlorine. It's really only CYA that measures the concentration in pool water using essentially 100% purity of CYA product that is added.

    Calcium Hardness (CH) and Total Alkalinity (TA) are both measured in units of ppm (or mg/L) calcium carbonate, CaCO3, which gets very confusing, especially for TA since the carbonate counts twice since it can combine with two hydrogen ions. So figuring out sodium bicarbonate or even sodium carbonate amounts directly doesn't work even compensating for molecular weight differences because you still need to divide by 2 to get the effect on TA, assuming the pH ends up in the usual 7-8 range. This is because even sodium carbonate will end up as mostly bicarbonate at normal pool pH and bicarbonate contributes half as much to TA as the same molar amount of carbonate contributes. In other words, you calculate TA by first calculating the normal chemical TA in molar units, multiplying species like bicarbonate by 1 and carbonate by 2, add these all together, and then convert to calcium carbonate molecular weight for mg/L BUT divide the result by 2.
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    Re: FC - Concept of "Reserve"

    Okay, thanks. Lots to absorb there, so to speak.

    Reserve is useful to prevent total depletion of hypochlorous acid over time, but has little effect over instantaneous need if that need exceeds that which is available from a natural conversion to hypochlorous acid from the bonded state. That need must be addressed by either an instantaneous boost of chlorine, or more practicably a higher maintenance level of chlorine at high CYA levels. Bringing the CYA level down to 30 would be better.

    Beside being a poor anti-bacterial/anti-algaeal, are there health drawbacks to FC registering in the 7 - 12 ppm levels for high CYA?
    Jaywalker

    In-ground, 11,000 gallon, plaster, in-line trichlor pucks or BBB bottled chlorine, 1.5 HP pump @ ~100 gallons/minute, cartridge filter

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    Re: FC - Concept of "Reserve"

    Quote Originally Posted by Jaywalker
    Beside being a poor anti-bacterial/anti-algaeal, are there health drawbacks to FC registering in the 7 - 12 ppm levels for high CYA?
    Not that we know of.
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