Chlorine/CYA and Combined Chlorine

[chem geek]

This is a continuation of a discussion from this post (http://www.poolforum.com/pf2/showthread.php?p=54760&postcount=18).

First, for information about the chlorine/CYA relationship, you can look at this thread (http://www.poolforum.com/pf2/showthread.php?t=4236). The spreadsheet linked to at the end of the first post uses the equilibrium constants for the chlorinated cyanurates definitively determined in 1973. CYA just affects the rate of chlorine reactions, but because chlorine (hypochlorous acid) is released from CYA, it does not affect stochiometry -- that is, the amount of chlorine available to react (even if it's concentration is lower so that it reacts more slowly). FC measures the amount of active plus inactive chlorine; that is, it also measures the amount of chlorine bound to CYA so in effect measures chlorine capacity. CYA is essentially a chlorine (hypochlorous acid) buffer. The FC/CYA ratio is a rough proxy for the hypochlorous acid concentration so determines the rate of disinfection and oxidation. When this rate exceeds the generation (doubling) rate of bacteria and algae, then these are killed faster than they can reproduce.

As for shock level, this is somewhat arbitrary as a higher chlorine level will kill algae faster. The Minimum column in Ben's chart corresponds to killing algae faster than it can reproduce for nearly all pools and seems to work up to phosphate levels of around 3000-4000 ppb above which the algae may reproduce faster and need higher chlorine levels (and may not be worth it), but these are extremely high phosphate levels not normally found in most pools. The shock level is roughly 10 times higher in hypochlorous acid concentration in the table I use while Ben's numbers (in his table here (http://www.poolforum.com/pf2/showthread.php?t=365)) tend to be higher at lower CYA and lower at higher CYA. In theory, Ben's shock levels will take longer to clear a pool of algae at high CYA than at low CYA whereas in my table calculated from the chlorine/CYA relationship shown here (http://www.troublefreepool.com/sticky.php?s=2346) has a more consistent active chlorine level. One uses a higher shock level not only to kill and oxidize algae faster, but also to ensure that the chlorine level inside a bloom is high enough to prevent further growth (circulation can be poor and kill rates slower where algae is clumped or forming biofilms which is why brushing helps).

Second, and not related to the above (i.e. CYA isn't involved except in affecting the rate of reaction, but not the amount of chlorine needed), the breakpoint of ammonia proceeds through a series of reactions, but the net is the following:

3HOCl + 2NH3 --> 2N2(g) + 3H+ + 3Cl- + 3H2O

So the ratio of chlorine to ammonia is 3:2 or 1.5. However, this is a molar (or molecular) ratio whereas chlorine is measured in parts-per-million (ppm) units using a chlorine gas (Cl2) equivalent weight (ppm is a weight proportion, not a mole-to-volume proportion) and ammonia is measured in ppm Nitrogen (atomic, not molecular nitrogen gas). So the ppm ratio of chlorine to ammonia is 1.5 * (35.453*2 / 14.007) = 7.593 (usually quoted as 7.6). In practice, for the reaction to fully complete it takes a little more chlorine due to intermediate steps so usually a full breakpoint needs 8-10 times as much chlorine as ammonia (both in their respective ppm units).

Combined Chlorine (CC) is typically monochloramine (NH2Cl) and this is measured in ppm units similar to chlorine as ppm chlorine gas equivalent, NOT ppm nitrogen as with ammonia. The reaction to break monochloramine is a subset of the one shown above and is as follows:

HOCl + NH2Cl --> NHCl2 + H2O
NHCl2 + NH2Cl --> N2(g) + 3H+ + 3Cl-
----------------------------------------------
HOCl + 2NH2Cl --> N2(g) + 3H+ + 3Cl- + H2O

So the ratio of chlorine to monochloramine is 1:2 or 0.5. Since the ppm are in the same units, it takes half the amount of FC to break CC, not 7.6 times as is the case with ammonia (measured as ppm nitrogen).

Breaking monochloramine is easy. When one has persistent CC, it is usually something else such as a chlorinated organic compound. These may not get more fully oxidized easily, but usually they aren't in large quantities and they might get broken down in sunlight which would explain why they aren't usually seen in outdoor pools (the other reason might be that they form more when the chlorine level is much higher as is the case in most indoor pools that are essentially over-chlorinated due to not having any CYA to slow down all chlorine reactions).

Richard

[Water_man]

Thank you , again, Richard for your comprehensive post.
Here are a few theoretical and practical questions.

A. Theoretical

1. The CYA/HOCL relationship

Let's see if I understood the crux of the matter.
There's a chemical equilibrium between CYA, HOCL ("active chlorine") and HOCL bound to CYA. The latter
is not a "potent" chlorinating agent, however, it protects the bound HOCL from UV degradation. By the simple law of the chemical equilibrium constant, the concentration of HOCL is inversely proportional to the concentration of CYA.
Did I get it right?

2. You mentioned that HOCL acts on organic material by combining with ammonia. While this is easy for simple stoichiometric calculations, wouldn't it be fair to say that the active chlorine actually acts with almost "anything that walks", that is, a lot of organic compounds, especially those that contain amine groups? There’s no free ammonia on fabric stains! How would that affect all the calculations?

B. Practical:

1. I just got my Tailor kit, and read their "Water Chem" brochure.
On one hand they advocate the 10X CC rule for shock, and on the other hand they admit that high CYA level has a negative effect on the affectivity of FC. Nevertheless, they have do not address this issue quantitatively. This is amazing. What's wrong with them? They seem to know their chemistry right.

2. I followed your post regarding shocking of algae-free pool and I'm still not sure if I understood you correctly.
I understood that if the minimum level of FC is maintained, and even if the CC momentarily rises above 0.5 you still don't need to shock. Then when do you?
If CC >0.5 for more than three days?
Suppose, based on a persistent CC > 0.5, you need to shock, why would the FC ppm would be that of an algae shock? In your table, with my 60 ppm CYA, I need 23 ppm FC to shock. According to Ben's table, it's 20 ppm.
Should this (20 - 23) be my shock level for CC >0.5 and no algae problem?

"Traditional" pool procedures recommend shocking every week. From what I understood so far, as far as you are concerned, the only thing that matters is if CC >0.5.
So suppose, my pool goes on CC <0.5 for a long time, and there's no other visible organic contaminant as pollen or suntan lotion film, I don't need to shock?

Apropos pollen: It seems reasonable to shock the pool if there’s pollen on the surface, doesn't it?

[chem geek]

My responses in bold below.


Thank you , again, Richard for your comprehensive post.
Here are a few theoretical and practical questions.

A. Theoretical

1. The CYA/HOCL relationship

Let's see if I understood the crux of the matter.
There's a chemical equilibrium between CYA, HOCL ("active chlorine") and HOCL bound to CYA. The latter
is not a "potent" chlorinating agent, however, it protects the bound HOCL from UV degradation. By the simple law of the chemical equilibrium constant, the concentration of HOCL is inversely proportional to the concentration of CYA.
Did I get it right?

This is pretty close. There is a chemical equilibrium between a collection of chlorinated cyanurate species (1 to 3 chlorine bound to CYA) and CYA or other chlorinated cyanurates with one less chlorine and HOCl. For simplicity, you can just think of it as CYA-Cl (and water) in equilibrium with CYA and HOCl. The simplified chemical equilibrium constant is then K = [HOCl][CYA]/[CYA-Cl] so it's not as simple as an inverse relationship on first appearances. Under normal conditions when the CYA >> FC by a factor of 5 or more, most of the chlorine is bound to CYA so for practical purposes [CYA-Cl] is roughly proportional to FC. So the relationship is K = [HOCl][CYA]/"FC" or solving for [HOCl] = constant x FC / CYA so you can see that the hypochlorous acid concentration is roughly proportional to the ratio of FC to CYA. So for a constant FC you are correct that HOCl is inversely proportional to CYA.

As for the protection of chlorine from sunlight, this comes from two factors. The first is what you mentioned about CYA-Cl being more resistant to breakdown from sunlight. The second factor is that CYA itself absorbs UV light thereby shielding lower depths of water and remember that there is a lot of CYA in the water -- much more than CYA-Cl -- so this second factor is important. Nevertheless, in practical terms, the protection of chlorine from sunlight increases with increasing CYA, but it's not linear -- possibly due to this CYA absorption effect combined with imperfect circulation of chlorine especially near the surface.

2. You mentioned that HOCL acts on organic material by combining with ammonia. While this is easy for simple stoichiometric calculations, wouldn't it be fair to say that the active chlorine actually acts with almost "anything that walks", that is, a lot of organic compounds, especially those that contain amine groups? There’s no free ammonia on fabric stains! How would that affect all the calculations?

Chlorine doesn't react with [EDIT] everything. [END-EDIT] In fact, it mostly reacts with compounds that contain nitrogen and to a lesser extent it reacts with hydrocarbons that have double bonds, but it does not generally react with saturated (single-bond) hydrocarbons. In practice, sweat and urine are composed mostly (after water) of urea and then ammonia and then a much smaller amount of amino acids -- all of these contain nitrogen to which chlorine reacts at various rates. Urea is effectively like having two ammonia attached to a common carbon, but the actual reactions of chlorine with urea are not yet understood (this is currently being investigated via a grant from the NSPF as described here (http://www.innovations-report.com/html/reports/environment_sciences/report-103952.html)). The reaction of chlorine with ammonia, on the other hand, is fairly well understood. Though it is true that general Combined Chlorine may not have a 1:2 relationship to complete oxidation, it isn't going to be far from that. The main point was that it wasn't 10:1 or anything like that because the units are completely different measuring ammonia (as ppm nitrogen) vs. Combined Chlorine (as ppm chlorine gas).

B. Practical:

1. I just got my Tailor kit, and read their "Water Chem" brochure.
On one hand they advocate the 10X CC rule for shock, and on the other hand they admit that high CYA level has a negative effect on the affectivity of FC. Nevertheless, they have do not address this issue quantitatively. This is amazing. What's wrong with them? They seem to know their chemistry right.

There are a lot of people, including chemists, who just take rules on faith and don't question them. They also don't have time to find the original research and do the calculations which, though only requiring 1st year chemistry, are quite tedious (see my spreadsheet, for example). Of course, there are canned programs on the market where you can just plug in the equilibrium constants and ionic strength (salt levels) and out comes an answer, so there really is no excuse. Nevertheless, when there is CYA in the water, the 10x rule isn't bad as it does take a while to get rid of the non-ammonia-based (i.e. not monochloramine) CC so having a higher chlorine level can make that go faster, but sometimes the CC is persistent and no amount of chlorine will get rid of it. We've done bucket tests with some persistent CC from indoor pools and exposed it to very high chlorine levels and very high UV levels (from tanning beds) and long exposure to sunlight and nothing got rid of it. We don't even know if it's real or something that interferes with the test (though we eliminated obvious things like use of MPS).

2. I followed your post regarding shocking of algae-free pool and I'm still not sure if I understood you correctly.
I understood that if the minimum level of FC is maintained, and even if the CC momentarily rises above 0.5 you still don't need to shock. Then when do you?
If CC >0.5 for more than three days?
Suppose, based on a persistent CC > 0.5, you need to shock, why would the FC ppm would be that of an algae shock? In your table, with my 60 ppm CYA, I need 23 ppm FC to shock. According to Ben's table, it's 20 ppm.
Should this (20 - 23) be my shock level for CC >0.5 and no algae problem?

I don't believe there is any need to shock the pool if the pool is clear (not even the dull or cloudy beginnings of algae) and the CC is low (<= 0.5 ppm) and FC holds overnight (<= 1 ppm drop). Even if the CC is higher, if this is soon after a high bather load or organic matter dumped into the pool I'd still wait a day before worrying about it as normal chlorine levels (and sunlight) may take care of it.

"Traditional" pool procedures recommend shocking every week. From what I understood so far, as far as you are concerned, the only thing that matters is if CC >0.5.
So suppose, my pool goes on CC <0.5 for a long time, and there's no other visible organic contaminant as pollen or suntan lotion film, I don't need to shock?

I question everything so don't follow traditional procedures just because they are traditional. Many do make sense, but some don't. The only reason I can think of for periodically raising the FC level higher would be if there is a risk of some pathogens becoming resistant to chlorine as the higher level would wipe out such colonies before they get a foothold. However, most bacteria are very easy to kill and algae doesn't reproduce as quickly so I think the risk is low. In practice, from what I've seen on pool forums I don't think there is any problem with not shocking regularly and instead just maintaining consistent and appropriate FC levels relative to CYA.

Apropos pollen: It seems reasonable to shock the pool if there’s pollen on the surface, doesn't it?

A skimmer sock would probably take care of the problem more quickly. Shock levels of chlorine will break down the pollen, but generally it's a bit slow as pollen is 1) larger than bacteria and possibly some algae and 2) often have a hearty shell that is slow to oxidize. You generally notice a chlorine demand from a lot of pollen, but physical removal is usually preferred when possible.

[waterbear]

"Traditional" pool procedures recommend shocking every week. From what I understood so far, as far as you are concerned, the only thing that matters is if CC >0.5.
So suppose, my pool goes on CC <0.5 for a long time, and there's no other visible organic contaminant as pollen or suntan lotion film, I don't need to shock?


Pool chemical companies (Arch Chemical is a prime example) are the ones that recommened shocking weekly ( and with a product labeled as 'shock') This just helps increase their bottom line (BTW, Arch Chemical is one of the largest cal hyp manufacturers and the vast majority of it they sell as 'shock')

Shock when the CC is above .5 ppm, the reason for this is that the threshold level for the majority of people to detect CC is .4 ppm but , unless you are doing a high resolution FAS-DPD test or a liquid color standard DPD test it is impossible to detect this small a change. Most DPD color comparators in home test kits can only measure .5 or 1 ppm of CC accurately (and only if you have good color recognition for shades of red, which many men have difficulty with. It is much easier for women).

If your FC is holding, Your CC is .5 ppm or lower, and your water is not cloudy then there is no need to shock. However, if you just had a big pool party and the pool had a much bigger bather load than normal, then shock. If it just rained heavily, then shock. If the pool looks cloudy, then shock. It's really just common sense.

[Water_man]

Alright, guys, you made it clearer and clearer, however one sentence by chem geek threw a monkey wrench into my newly acquired pool chem understanding.

Chem geek is saying:

Nevertheless, when there is CYA in the water, the 10x rule isn't bad as it does take a while to get rid of the non-ammonia-based (i.e. not monochloramine) CC so having a higher chlorine level can make that go faster, but sometimes the CC is persistent and no amount of chlorine will get rid of it.

Well, in most cases there is CYA in the water, isn’t there? So after challenging the validity of the 10x rule persistently, are you saying that it isn’t bad? Is this a typo? Please explain.

From both of your posts, chem geek and Waterbear, I understand better when I need to shock
an algae-free pool. I am still at odds at the amount of FC needed.
Ben’s table for my 60 ppm CYA reads 20 ppm FC. Chem geek’s number is 23.7. If the previous chem geek’s statement of the 10x rule is true, isn’t the 20 ppm FC an overkill?
So how much do I really need?
Suppose I do shock at 20 ppm at night, would it be safe to swim in the following morning?

I really appreciate the time, effort and knowledge that you guys invest in this forum,
which leads me to the following suggestion. I hope you won’t be offended and that this would not be a violation of the forum rules, and if either of the above is affirmative, I offer my apologies in advance.

I don’t think that it’s fair that pros like you (and others pros in this forum) offer their hard earned professional expertise and time for free. This is your career, not just a hobby. I suggest that you put together a detailed and comprehensive manual dealing with all the theoretical and practical issues of pool chem and maintenance routines which you scattered all over this forum and elsewhere and offer it for sale. This would be a great service for the pool owners community and a fair compensation for your dedicated service to this community.
Again, please accept my apologies if this was inappropriate.

[chem geek]

(I was out of town overseas and unable to access this site which is why I took so long to respond.)

The FC numbers in the shock table are not at a level so much for getting rid of CC as for killing algae quite a bit faster than it can grow after an algae bloom has formed. Ben's original numbers were based on a combination of experience and theory. My numbers were based on Ben's as a starting point, but made more consistent based on theory. It's roughly an FC that is 40% of the CYA level and that results roughly in 0.3 ppm hypochlorous acid concentration (where "ppm" is in standard chlorine Cl2 units). So I wouldn't think about the shock column numbers in terms of CC or the 10x traditional industry rule. The shock table is essentially "rate-based", not based on stoichiometry (reaction completion).

As for CC and the 10x, the stoichiometry which says how much chemical is needed for the reaction to complete is an FC that is 0.5x the CC level. The 10x clearly came from the breakpoint chlorination of ammonia (which is measured in different units of ppm Nitrogen), not of monochloramine (which is measured in ppm Cl2). My comment with regard to 10x not being bad is not based on stoichiometry, which is still 0.5x (so perhaps with some amount of excess to ensure completion it's still less than 1x), but rather based on reaction rates since CYA reduces the effective "instantaneous" chlorine concentration while holding a lot more chlorine in reserve that is released reasonably quickly. Stoichiometry is based on total amount of chlorine available including that in reserve while reaction rates are based on the "instantaneous" concentrations of relevant chemicals (in this case, hypochlorous acid). These are two separate concepts -- the 10x rule is incorrect for CC for stoichiometry (i.e. reaction completion), but I was just saying that for reasonable reaction rates a 10x rule is OK to use when CYA is present -- not necessary, but it makes the reaction go faster. I was just trying to put some "reason" into an industry rule.

A properly maintained outdoor residential pool usually measures little or no CC so the need for shocking the pool is rare. Higher bather load can temporarily lead to an increase i CC, but this should drop on its own if the normal FC level is maintained and the bather load returns to a normal lower level.

By the way, I've been asked about more detail about the chemistry behind the chlorine/CYA relationship in a couple of posts so I created a new post with that info here (http://www.troublefreepool.com/viewtopic.php?p=50548#50548).

Regarding the time and effort that many have put into this and other pool forums, many of us are not professionals in this industry. Speaking for myself, I do not work in the pool/spa industry (or anything related to it) and am just a pool owner like most who post here. It is true that we don't get paid for this, but I'd much rather see the industry shift towards full disclosure of basic information (the chemistry of the chlorine/CYA relationship has been known since at least 1974, for example). These forums can only help those who look at them and are a great "workaround" until the integrity of the pool/spa industry improves. It's the CPO and TECH courses that need to get updated (I've written to the NSPF and APSP organizations about that) as well as training materials for pool store employees (except for waterbear/Evan who already knows this stuff because he's the smartest most honest pool store employee you'll likely ever meet!).

Richard

[Water_man]

Welcome back, Richard. I hope you had a good trip.
At the beginning this idea was new to me:

"A properly maintained outdoor residential pool usually measures little or no CC so the need for shocking the pool is rare"

I've started to witness its validity. It's been two weeks now that I've maintained my FC between 4.5 and 6.8 as per the tables (CYA=60) and my CC has been < 0.5 , although it's non-zero. You can say that I keep maintaining the 10X rule without actually "shocking" whether it's a valid rule or not, because my average FC is at least 5.

On the other hand, you can also say that a properly maintained pool (FC according to charts, etc) shouldn't have an algae bloom, and still people get it.

So suppose CC is >0.5 for a few days. What should be the level of FC needed in shocking, regardless of what the underlying arithmetics is based on - kinetics or stoichiometry?


You said:

"I'd much rather see the industry shift towards full disclosure of basic information.... until the integrity of the pool/spa industry improves".

I did a little reading on the "official" view of a few Health Depts in the country and I was shocked (but not superchlorinated :D ) at what I read. Their covering of the obvious disadvantage of using "stabilized chlorine" can be explained by either sheer stupidity or corruption (I prefer to call a spade a spade, and misinforming the public in a way that benefits a particular industry smells of corruption.)
I'd be glad to share my findings. I wonder what would be the right forum. "Using Chlorine" may not be the right one because I wouldn't post anything that we don't know about proper chlorine usage. OTOH, the "China Shop" is supposedly for debating or clarifying topics in more depth, and again, I'm not debating you, but on the contrary. Please advise.

[chem geek]

On the other hand, you can also say that a properly maintained pool (FC according to charts, etc) shouldn't have an algae bloom, and still people get it.

That's not really true if you look at the large number of posts on multiple forums. For green algae, we have hardly ever seen such algae bloom in the presence of sufficient FC relative to CYA with the cutoff being an FC around 7.5% of the CYA level (unless the CYA is really low in which case a minimum FC of around 2 ppm is needed so that chlorine does not run out locally). With very high phosphate levels > 3000 ppb, the algae could grow faster than the chlorine can kill it, but that's a very high phosphate level and higher chlorine levels would work, but may not be worth it. Mustard/Yellow algae is the one that requires almost double the amount of chlorine to keep away -- about an FC that is 15% of the CYA level -- but it's less common and is something that seems one can completely get rid of, though it isn't easy since it hangs around in light niches, etc.

A lot of people let their pools go over the winter so I don't count opening a pool to green algae when you don't have chlorine in it as being maintained with chlorine.

Why do you say that pools get algae even with sufficient chlorine?



So suppose CC is >0.5 for a few days. What should be the level of FC needed in shocking, regardless of what the underlying arithmetics is based on - kinetics or stoichiometry?

Generally if you raise the FC to the shock levels in the table then that's what we normally recommend, but I've seen cases where the CC won't go away no matter what you throw at it -- clearly such CC isn't traditional chloramine. Fortunately, this is a rare occurrence and is most frequently seen in indoor pools.

However, in my own pool, if I have a bunch of people over and the CC goes up, I just wait a day since it will invariably go back down. In other words, I usually don't worry about it and it's almost always <= 0.2 ppm in my pool. Basically, if the CC is just monochloramine, it will break down over time so long as there is FC in the pool. Of course, high bather load pools such as commercial and public pools are a different matter and may require higher chlorine levels or periodic shocking to stay ahead of such higher demand (i.e. introduction of urea, ammonia and some amino acids from sweat and urine).

The China Shop isn't just about debate -- it's also where we put things that are too technical for most pool owners. We've talked about renaming this or having another section, but never got around to it. So go ahead and post such items in The China Shop (it's probably better to link to the info and just refer to short excerpts). I'm pretty aware of the lack of awareness of the chlorine/CYA relationship and am trying to change that. I don't think it's corruption, but rather deceit since it's not lying but rather intentionally withholding information that might produce a less than desirable economic result. The irony is, IMHO, that this deceit (for 34 years) has led to more pool owners having algae and other problems with their pools with the pool stores not being very effective. This more than anything else has driven pool owners to SWG pools, even when such pools were not as economical (i.e. when SWG cells were more expensive in the past). So the stabilized chlorine industry has shot itself in the foot where now 85% or more of new pools have an SWG. The trend towards SWG pools would have happened anyway just based on convenience alone, but probably not as quickly and perhaps to not as high a percentage.

The lack of detailed information about the chlorine/CYA relationship has had other side effects that I believe were unintentional. With an industry mantra that "CYA doesn't matter; only FC matters" and that "CYA is only needed to protect chlorine from breakdown from sunlight", this leads one to the conclusion that there is no reason to use CYA in indoor pools. However, the chemistry says that most indoor pools are over-chlorinated because even 1-2 ppm FC with no CYA is 10-20 times the level of disinfecting chlorine (hypochlorous acid) as is found in outdoor pools where the FC is around 10% of the CYA level. My wife experiences this difference every year when her swimsuits degrade after one winter of use in an indoor pool with no CYA while duplicate swimsuits in our own pool during the summer do not degrade. There are similar differences in flaky skin and frizzy hair as well. Furthermore, the most recent breakpoint chlorination model from Jafvert & Valentine (1992) shows that the higher disinfecting chlorine level in indoor pools leads to 10-20 times faster production of disinfection by-products and 10-20 times greater end-point concentration of nitrogen trichloride. What is not known is if this is also true of urea which is the primary component of sweat and urine (ammonia is present in smaller amounts). Interestingly, this patent (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=5591692.PN.&OS=PN/5591692&RS=PN/5591692) assigned to one of the largest companies in the stabilized chlorine industry describes how CYA as well as a CYA-like compound called glycoluril reduces the rate of production of chlorine disinfection by-products (DBPs). I just don't think they put 2-and-2 together. I couldn't imagine how anyone could live with themselves given the number of incidents of asthma, respiratory problems and ocular issues there are with many indoor pools. Though some people blame lower air exchange due to higher energy efficient buildings (and I don't dispute that change), I think it's a lot easier to use a small amount of CYA in indoor pools (say, 20 ppm) to cut down DBP production by a factor of 10 or more -- that seems a lot easier than trying to exchange the air 10 times faster!

Richard

[Water_man]

Why do you say that pools get algae even with sufficient chlorine?

This was an assumption. I'm accepting your convincing rebuttal of this assumption.

By the way, today I measured CC with the highest sensitivity (25 ml sample in Taylor's k-2006 kit) and found CC <0.2 for a FC 4.5 water sample. It's amazing that the pink color for CC <0.2 is detectable.

I still don't understand why the same shocking level needed to kill algae is also needed to destroy CC because I'm sure that one algae cell has more than one or two Cl receptors, so the high level of FC in the case of algae makes sense from a stoichiometric point of view, not only just for kinetics, but if you say that this is a well established empirical fact, I'll accept it by the same way I'm accepting your calculated values of FC needed for preventive maintenance and sanitizing.

As to the other issue of deceit etc. - please wait until I post it and see what I mean.

[chem geek]

I still don't understand why the same shocking level needed to kill algae is also needed to destroy CC because I'm sure that one algae cell has more than one or two Cl receptors, so the high level of FC in the case of algae makes sense from a stoichiometric point of view, not only just for kinetics, but if you say that this is a well established empirical fact, I'll accept it by the same way I'm accepting your calculated values of FC needed for preventive maintenance and sanitizing.

It's not a fact -- you do not need to use the shock table values to get rid of CC. It's just easier and convenient to refer to those values since anything above the minimum stochiometric amount will get rid of the CC -- it will just take longer at lower FC/CYA ratios. So how does one determine how quickly one wants to get rid of their CC? It's arbitrary (when the CC isn't increasing), hence the simple rule of just follow the shock table (if you don't want to wait for normal FC levels to handle it in most cases). Also, CC is often measured when fighting an algae bloom so tying the two concepts together is just easier to explain as one rule, especially in that case where the shock table sets a reasonable rate for clearing a pool of algae (chlorine kill rate faster than algae reproduction rate by an amount that clears the pool in a matter of days).

As for the stochiometric amount of FC needed to completely kill and oxidize algae, we don't list that anywhere since it varies considerably depending on the extent of the bloom. So instead, the shock table just determines a rate of kill and we tell people to MAINTAIN that shock FC level by adding more chlorine as frequently as you can -- several times a day. You keep that FC level up until three things occur: 1) the pool is crystal clear, 2) there is minimal (<= 1 ppm FC) drop in FC level overnight and 3) there is minimal (<= 0.5 ppm) CC. The total amount of chlorine to get to this point is usually fairly large and you are right that it is much larger than the amount of FC needed to get rid of a small amount of CC.

Nevertheless, there are some situations where an extraordinary amount of FC is needed even when there is no algae. This usually happens when opening the pool if the CYA in the pool has been converted over the winter to ammonia by soil bacteria (that got into the pool) and the ammonia hasn't dissipated. In this case, it takes a total FC that is 10x the amount of ammonia (measured as ppm Nitrogen) to get rid of it. I just saw a report on one forum of an 8 ppm ammonia measurement. That would take 80 ppm FC to get rid of! A bucket test is being done to verify that this is indeed the case as one doesn't want to over shoot by too much. Technically, you don't have to add all the chlorine at once and can do it in steps. Initially, chlorine added to ammonia will produce monochloramine which will register as CC. Once all the ammonia is converted, any additional chlorine will lead to breakpoint. I've read about problems with not using enough chlorine and getting "stuck" at dichloramine, but I've run through the models and this simply isn't true (though obviously without enough chlorine you can have leftover monochloramine). In fact, by adding too much of a concentration of chlorine to monochloramine, one can produce more di and trichloramine -- going slower is actually more effective with less by-products though obviously takes longer.

I look forward to your new post or thread regarding industry or health department info.

Richard

[Water_man]

This was a great thread. I feel that all my questions and speculations were properly answered and dealt with. Thank you for your patience and the keen desire and ability to share your valuable knowledge in such a thorough and time consuming way.