Pool Water Chemistry

chem geek

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Mar 28, 2007
San Rafael, CA USA
This thread presents my findings so far on pool water chemistry including the following:
  1. More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI)
  2. Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)
  3. Determination of pH and Alkalinity changes when adding chemicals to the pool
  4. Various reaction rates including chlorine breakdown by UV

Disinfecting Chlorine (HOCl) vs. Total Free Chlorine (FC) and Cyanuric Acid (CYA)

The most important finding was how little disinfecting chlorine (HOCl) is left after chlorine combines with Cyanuric Acid (CYA) to get "stored" as chlorinated cyanurates (aka cloramides). The chart at the following link shows this relationship. (I recently discovered that all forms of chlorine are measured as ppm equivalents of chlorine gas, so all charts, graphs and the spreadsheet have now been updated to reflect this.)

HOCl Chart

Note that the red in the linked chart above represents a cutoff of 0.011 ppm HOCl which roughly corresponds to the 650 mV ORP level that the U.S. and WHO set as the minimum required for disinfection. The green color is a guess at 0.05 ppm HOCl of the minimum level of chlorine needed to prevent algae. The actual number may be quite different, from 0.02 or less to 0.1 or more, but based on Ben's "Best Guess CYA Chart" which is based on real-world experience, I suspect the actual number will be somewhere in this range. So, red means bacterial growth while green means possible algae growth. Blue is the safe area.

The following shows this same data in graphical form with lines showing the same two (probably correct) "bacteria" and (totally a guess) "algae" levels.


The following is an approximate formula you can use so long as your CYA ppm is at least 5 times your FC (the formula really falls apart terribly below a ratio of CYA/FC of 3).

(HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

and if you are interested in the FC for a given HOCl (to construct the equivalent of Ben's table, for example), you can use the following which just solves for ppm FC from the above.

(FC as ppm Cl2) = ( 2.7*(ppm CYA) + 5 ) / ( 4.9 + 1/(ppm HOCl) )

The constants in the above formulas are for a pH of 7.5 (which is the only parameter that significantly affects these constants). With the spreadsheet I can easily calculate the constants for other pH, but remember that the above formulas are approximate. For example, with FC of 3 and CYA of 15 the formula gives HOCl as 0.098 when the correct answer is 0.095. That's not terrible (about an 3% error). However, with FC of 5 and CYA of 15 the formula gives HOCl as 0.239 while the correct answer is 0.199 (about an 20% error) which isn't as good.

A rough rule of thumb that applies at a pH of 7.5 is that the effective chlorine level is reduced by a factor about equal to the ppm of the CYA. So, a CYA of 30 ppm reduces the disinfecting chlorine (HOCl) level to about 1/30th of what it would be with no CYA.

The inverse of the above chart may be seen at this link:

FC Chart

The chart columns from 0.02 to 0.1 ppm HOCl roughly correspond to "Ben's Best Guess CYA Chart". Ben's chart converted to show HOCl may be found here where you can see that the rough Min FC corresponds to 0.03 ppm, the rough Max FC corresponds to 0.07 ppm (implying an ideal target of 0.05 ppm) and the shock table is not consistent, but probably implies a minimum of 0.3 ppm, at least for green algae. User experience indicates that hard-to-kill yellow or mustard algae (and maybe black algae) may need 1.0 ppm HOCl for shock. User experience with black algae indicates that keeping active black algae from growing requires around 0.07 ppm HOCl.

A comparison of the "traditional" HOCl/OCl- graph with the same graph in the presence of CYA may be found at this post. This also shows how CYA is a "chlorine (specifically HOCl) buffer" that makes HOCl concentration about half as sensitive to changes in pH.

The original source for the equilibrium constants was done in 1973 (and published in 1974) where the recommended maximum CYA level was 25 ppm:

J. O'Brien, J. Morris and J. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurateâ€, Chapter 14 in A. Rubin, ed., Chemistry of Water Supply, Treatment and Distribution, 1973 Symposium, (published 1974), Ann Arbor Science, Ann Arbor, MI, pp. 333-358.

[EDIT] A searchable PDF of this paper may be found on a link on this web page. [END-EDIT]

A Little CYA Goes A Long Way

NOTE: The mechanism of protection of chlorine from sunlight by CYA is currently under review in this post. Higher CYA levels may protect even proportionately higher levels of chlorine more, especially in deeper pools.

The following is a graph showing that a large amount of the benefit of CYA protection of chlorine from UV (sunlight) is already there at around 20 ppm. This data is approximate, not only because it is dependent on the amount of sun exposure, but because the rate constants themselves change with FC level (because there is a mix of two different rates of destruction -- one from HOCl and the other from the chlorinated cyanurates which are more stable, but still breakdown from sunlight). The limiting half-life for HOCl/OCl- is 35 minutes which is consistent with pool studies, but some experimental studies give 11.6 minutes. The limiting half-life of the chlorinated cyanurates is 8.4 hours though some other data shows it could be 6 hours.


The following graph combines the two concepts of needing more chlorine at higher CYA vs. the greater protection of chlorine by CYA. The graph shows the total chlorine (FC) loss rate in ppm/hour vs. CYA at different HOCl levels. Remember that this rate of loss will slow down as chlorine gets used up. Nevertheless, [EDIT] while in theory [END-EDIT] the absolute loss of chlorine is greater at higher CYA levels (keeping HOCl constant) and is the downside to a "high CYA & high Chlorine" approach, [EDIT] in practice there is some sort of CYA shielding effect such that higher FC and CYA levels at the same FC/CYA ratio lose less absolute amounts of chlorine (see this post later in this thread and see Mark's experiments in this post and this post [END-EDIT]. However, the primary reason to have higher CYA and Chlorine is to have a sufficient buffer of chlorine to prevent it from dropping to dangerous levels. There is obviously a tradeoff here. Though using no CYA results in the least amount of chlorine loss, the fact is that you simply can't maintain a pool with only 0.05 ppm chlorine everywhere in it -- hence a minimum level is needed as a buffer.


Salt Water chlorine Generation (SWG) pools seem to require a higher level of CYA, about 70-80 ppm, to operate efficiently. The theory is that the CYA is slow to "store" the chlorine as it is being generated so without enough CYA there is a build-up of chlorine that degrades the performance of the salt cell. I would prefer that the SWG manufacturers offer a larger lower-power (per length) cell that would work efficiently at lower CYA concentrations.

pH Rising

If you find that your pH wants to keep rising, this may be due to your pool outgassing CO2 to the air. The rate of outgassing increases with lower pH, higher alkalinity, and aeration of water (splashing, water fountains or slides, high wind, jets pointed up, etc.). The aeration of pool water is a physical process that will vary greatly from pool to pool, but the following chart shows the relative outgassing rate as a function of pH and Total Alkalinity. It is possible that the hydrogen gas bubble production from SWG systems contributes to significant aeration and is a source of rising pH in such SWG pools. The rate is actually a function of Carbonate Alkalinity so this chart is for a CYA of 30, but the variation with different amounts of CYA is not large. Note that there is a large variation with pH (the Y-axis is logarithmic). I have drawn a somewhat arbitrary "Limit" line at a relative rate of 15 that I have found is roughly the tolerance limit where many people start complaining about rising pH, but again aeration is a factor I cannot predict.

CO2 Chart


Spreadsheet For Detailed Calculations

The link to the spreadsheet (in a ZIP file) that calculates all of the above data is PoolEquations.zip and was last updated 14-Mar-2009. It also does some of the things that BleachCalc does, but is not for novice users.

Also see Equations for Chlorine Chemistry.

Also see Oxidation-Reduction Potential (ORP) vs. HOCl

(I will continue to edit this post to add more detail and discussion.)[/URL]
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chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Chlorine/pH/CYA Relationships

OK everyone. Here are some graphs I put together so now you can give me feedback as to whether this is what you are looking for. First, I show the traditional HOCl/OCl- relationship on the left including a total line (for HOCl + OCl-) that is always at 100%. It should be noted that the chart on the left is valid for any Total Free Chlorine (FC) level.

The chart on the right shows the same situation when there is 30 ppm CYA and in this case the Total Free Chlorine (FC) level matters and is 3 ppm for this chart though when CYA >> FC it is roughly the ratio of CYA to FC that determines HOCl and OCl- levels. Also note that the percentage of disinfecting chlorine (HOCl) at a pH of 7.5 fell from about 50% on the chart on the left to around 1.5% on the chart on the right. Note that the total HOCl+OCl- level is not 100% when CYA is present. The difference from 100% (the Cl-CYA curve) is the amount of chlorine "bound" to CYA and though it is better protected from degradation from sunlight, it is also not immediately available for disinfection or oxidation (but is available "in reserve" as HOCl gets used up). Finally, notice how much "flatter" the HOCl curve is in the graph on the right indicating that the presence of CYA has made the amount of HOCl less sensitive to changes in pH (though we really need to look at a log scale for relative changes -- more on that next).


If we want to see changes in disinfecting chlorine (HOCl) in percentage terms, then a logarithmic scale is more appropriate so that equal distances on the chart represent the same relative amount of change. That is, it answers the question of how much improvement there is in the relative amount of chlorine when you lower pH. It is not quite as obvious in this graph, but the HOCl curve is a bit flatter on the right with CYA present, though at lower pH at around 7.0 the pH sensitivity of HOCl is about the same at 30 ppm CYA as it is with no CYA (and below 7.0 the pH sensitivity of HOCl is actually greater with 30 ppm CYA than with no CYA, but this is mostly due to the fact that with no CYA and at low pH most of the chlorine is already HOCl so there's no room for relative "growth"). Also note that at higher pH above 7.5 that the presence of CYA allows one to operate at higher pH without losing that much chlorine effectiveness (without CYA the effectiveness of chlorine drops rapidly above pH 7.5). The graph on the left cannot show the 0% flat line for Cl-CYA since it is off the chart (the 0% is at negative infinity since this is a logarithmic scale).


How's that?

It is interesting to note that the traditional HOCl/OCl- graph with no CYA showed the large variation in HOCl percentage vs. pH, but that this was rather pointless (for pools; not for drinking water disinfection) because the absolute concentration of HOCl was typically so large that it didn't really matter if only 10% of the total was HOCl. The minimum HOCl concentration for preventing algae is on the order of 0.05 ppm (disinfection minimum is around 0.01 ppm) whereas even a pool with no CYA and a pH of 8.4 (which is only 10% HOCl) with even a low total FC of 1 ppm still gives 0.10 ppm HOCl which is double where we normally run our pools today when we use CYA!


chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Equations for Chlorine Chemistry

I'll do this in words and in symbols. Adding bleach is a basic process; it is the using up of bleach (chlorine) that is an acidic process so the net result is almost neutral. When I said "chlorine usage" I didn't mean your using chlorine (i.e. adding it) -- I meant when chlorine gets used up by "doing its thing" or "breaking down". Sorry for the confusion I caused.

Adding Chlorine
NaOCl + H2O --> Na+ + HOCl + OH- (+ extra base Na+ + OH-)
HOCl --> H+ + OCl-
Sodium Hypochlorite (liquid chlorine or bleach) combines with water to produce sodium ions (part of regular table salt) plus disinfecting chlorine plus hydroxyl ion. The hydroxyl ion makes this a basic reaction that raises pH, but because the disinfecting chlorine is a weak acid this overall reaction raises the pH by less than a strong base would. Note that there is a small amount of extra base in the form of Sodium Hydroxide (lye or caustic soda) that comes with Sodium Hypochlorite and is there to help preserve it, but this amount is rather small.

Using Up Of Chlorine
Breakdown of Chlorine by Sunlight (UV)
2HOCl --> O2(g) + 2H+ + 2Cl-
2OCl- --> O2(g) + 2Cl-
Chlorine breaks down in the presence of ultraviolet radiation, such as found in sunlight, and forms oxygen gas and chloride ion (and hydrogen ion, if starting with HOCl hypochlorite). Because a hydrogen ion is produced, this is an acidic process, but since disinfecting chlorine is a weak acid, only some of it breaks down in a way that lowers pH as shown above (i.e. only HOCl produces H+; OCl- does not). During the process of chlorine breakdown by sunlight, there are hydroxyl (OH•), oxygen anion (O-•) and chlorine (Cl•) radicals that are also produced as short-lived intermediates (technical details in this post). This can help oxidize organics in the pool.

Net Chlorine To Breakpoint (Ammonia "Oxidation")
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
OCl- + H+ --> HOCl
The disinfecting form of chlorine (HOCl) combines with ammonia through a series of reactions (that I have not shown) with the net result being the production of nitrogen gas (which is why it is important to keep your cover off and have good circulation when shocking) plus hydrogen ion and chloride ion. Though by itself this would be a strong acid reaction, there is also OCl- present that will combine with hydrogen ion to form more HOCl since the ratio of HOCl to OCl- will remain constant (and is about 50/50 at pH 7.5). So the net reaction is acidic, but not strongly so. Further technical details are in this post.

Overall combination of adding chlorine and having it used up
The net reactions are as follows if you combine the ones I showed above.
2NaOCl --> 2Na+ + 2Cl- + O2(g)
3NaOCl + 2NH3 --> 3Na+ + 3Cl- + N2(g) + 3H2O
So the overall net reaction of adding sodium hypochlorite to your pool and having it used up in its most typical ways is simply to produce salt (yes, sodium chloride or table salt, dissolved in water, of course) and either oxygen or nitrogen gas (and water).

Other things that could happen
If you do not have enough chlorine in your pool relative to your bather load (ammonia demand), then the chlorine may not completely oxidize ammonia and instead you will get chloramines (first, monochloramine). This reaction is basic. However, sunlight may break down monochloramine which will result in the rest of the breakpoint process which overall is acidic (so it's the same as I showed above overall).

It is also possible for chlorine to combine with organic compounds to form chlorinated organics that are hard to breakdown. When people talk about the health problems with chlorine, it is usually about some of these chlorinated organics (Disinfection By-Products, DBPs) known as Tri-Halo-Methanes(THMs) such as chloroform. Also, some chloramines such as nitrogen trichloride (NCl3) not only smell bad, but can cause health problems (especially in indoor pools with poor air circulation). In an outdoor pool exposed to sunlight and with a good residual of chlorine you typically don't get these "bad" compounds. If you are really concerned and have money to burn, you can use a constant maintenance level of non-chlorine shock (monopersulfate, MPS) to oxidize organics before chlorine gets a chance, but this is probably overkill for an outdoor pool (though may be a good idea for an indoor pool).

Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reactions:

At the anode (positive plate):
2Cl- --> Cl2(g) + 2e-

At the cathode (negative plate):
2H2O + 2e- --> H2(g) + 2OH-

which nets out to the following where the chlorine gas dissolves in water:

2H2O + 2Cl- --> Cl2(g) + H2(g) + 2OH-
Cl2(g) + H2O --> HOCl + H+ + Cl-
H+ + OH- --> H2O
2H2O + Cl- --> HOCl + OH- + H2(g)

or equivalently

H2O + Cl- --> OCl- + H2(g)

Note that the products of HOCl and OH- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is "used up".

The net reactions in an SWG pool for chlorine addition from the SWG and then breakdown from sunlight and oxidation of ammonia are as follows:

2H2O --> O2(g) + 2H2(g)
2NH3 --> N2(g) + 3H2(g)

The chlorine is not "seen" in the above net reactions because the chloride that became chlorine goes back to being chloride again. The oxygen gas comes from water when chlorine gas dissolved in it (i.e. from hypochlorite ion or hypochlorous acid) while the nitrogen gas comes from the ammonia (the oxygen or hydroxyl in the chlorine reverts back into water in this case, using the hydrogen from the ammonia to do so).

If you have a salt pool and don't use CYA (this isn't normal) then you could also outgas chlorine in the same way that CO2 is outgassed. This is more likely if you are aerating the water (e.g. have water features, slides, fountains, jets pointed up, lots of splashing, ...). This process is strongly basic and greatly increases the pH (HOCl + Cl- --> Cl2(g) + OH-). The reason this would tend to only happen in a salt pool without CYA is that a high concentration of both chloride ion (Cl-) and disinfecting chlorine (HOCl) are needed and it occurs more readily at lower pH. [EDIT] If chlorine gas produced by the SWG did not fully dissolve and instead was outgassed, then this would result in a net pH rise and could be one factor for the pH rise seen in SWG pools (the other factor being carbon dioxide outgassing from slightly increased aeration from the SWG, but that is not enough to fully explain most pH rise in SWG pools by itself). [END-EDIT]

I know, I know...more than you wanted to know. I hope it helps and that you made it this far...:)


chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Oxidation-Reduction Potential (ORP) and HOCl

Oxidation-Reduction Potential (ORP) and HOCl

On page 5 of the following link is a summary of results from the "Commercial Spas Study, Portland, Oregon".


The following linked table shows this same data, but with an extra column I added where I calculated the ppm of HOCl (using a temperature of 104ºF for spas). I also resorted the table in order of decreasing HOCl (the table in the link is sorted by ORP).


It appears that HOCl is about as good if not a little better than ORP except for one case where the FC was low (0.83), the CC was high (1.04) and there was no CYA (0). Perhaps the chlorine demand was used up at some point or the high CC had some adverse effect, but the low ORP (564) with chlorine present without CYA is certainly strange. Having a rule of a minimum of 0.011 ppm HOCl and a minimum FC or 1.0 (or perhaps 2.0) would work slightly better than the ORP rule of having a minimum of 650 mV. Of course, this is just one study, but I can't seem to find any other data as comprehensive as this.

[EDIT] HOWEVER, note the following sort of the same table by FC:


So perhaps using the HOCl concentration for predicting disinfection rates isn't as good as using FC and FC may even beat ORP. [END-EDIT]

When I first started looking at ORP, I was shocked to find how inconsistent it was even from the same source. The rather definitive work by Clifford White called "The Handbook of Chlorination" had inconsistencies as did several different manufacturers. Before looking at the absolute ORP to ppm Chlorine readings, I first looked at something that shouldn't vary that much between ORP sensors and that was the amount of mV change for each doubling of ppm of Chlorine (everything else constant, including pH, TDS, etc.). The following table shows the results. I have put the sources for each as a link where appropriate.

Product or Source       ORP mV per 2x Chlorine (near pH 7.5)
----------------------  ----------------------
Sensorex                        83.0
Aquarius Technologies           45.7
Chemtrol (SBControl)            22.7 (at pH 7.5)
Uniloc/Stranco              not logarithmic
Siemens Strantrol               20.0 (same as “Petaluma” water from Fig. 4-19 Pools below)
CYA Paper                       28.4
Theoretical 1 electron          17.8
Theoretical 2 electron           8.9
Clifford White’s Handbook of Chlorination:
Fig. 4-19 Distilled Water       10.0
Fig. 4-19 Pools              12.0 – 20.0
Fig. 9-93a (0-1 ppm)            35.0
Fig. 9-102 (0-0.035 ppm)        30-40
Fig. 9-103 (0-110 ppm)          15-20
Aquarius Technology
Chemtrol (SBControl)
Unilock/Stranco (link is dead; may need subscriber access)
Siemens Strancol (link is dead)

I also developed formulas for each of the manufacturers ORP vs. ppm Chlorine relationships and they also vary in absolute ORP readings by large amounts. It appears that the most accurate and consistent and carefully controlled and measured readings were from the Chemtrol (SBControl) paper so that is what I have used in my spreadsheet though I no longer prominently use ORP and have relegated it to a minor section of the spreadsheet. This also does not mean that I believe they make better sensors than the others (I simply do not know). The following shows the variation in absolute ORP for these same sources.

Product or Source              0.2 ppm   1.0 ppm   2.0 ppm
---------------------------    -------   -------   -------
Aquarius Technologies            635       730       775
Chemtrol (SBControl)             720       770       792
Uniloc/Stranco                   635       794       822  (at 7.4: 650, 800, 826)
Siemens Strantrol                655       697       715
CYA Paper                        702       767       796  (at 7.4: 716, 780, 810)
Clifford White’s Handbook of Chlorination:
Fig. 4-19 Distilled Water         -        755       765
Fig. 4-19 Petaluma                -        700       715
Fig. 4-19 San Rafael              -        675       690
Fig. 4-19 Santa Clara             -        665       685
Fig. 4-19 Gilroy                  -        645       670
Unfortunately, Clifford White’s Handbook of Chlorination is inconsistent even when measuring the same waters under the same conditions. The chart in Figure 4-18 that shows the relationship between ORP and pH shows at a pH of 7.5 (the same as Figure 4-19) ORP readings that are 35 mV lower for all cities (i.e. Petaluma at 2.1 ppm is 680 mV) and 165 mV lower for distilled water (i.e. at 1.8 ppm is 610 mV).

So what's the bottom line? I agree with Ben that though ORP is measuring something, namely the oxidation potential of the water, that this is not necessarily the important thing to measure or that it is not always measured accurately or consistently, etc. It should be noted that this is a rather controversial assertion to make as an entire ORP industry was created on the basis of ORP being the superior measurement to use, but when people looked at studies, such as the Commercial Spas Study mentioned earlier in this post, they only compared ORP to variables such as Free Chlorine (FC), CYA and pH as individual variables (possibly through correlation analysis) and it does not appear that anyone looked at theoretically calculated HOCl levels from those same variables (as I did, again results are earlier in this post). It is possible that some combination of HOCl concentration with H+ (pH) concentration determines the rate of disinfection and oxidation, but that is something I will have to look at later, probably using the Commercial Spas Study data since that's all I've got right now.

[EDIT] I should also add that there is a difference between a dynamic (i.e. real-time) measurement of a parameter such as ORP or HOCl concentration vs. a static (i.e. test kit) measurement since the former can result in a lower value due to the continual introduction of organic load. This is especially true when CYA is present since the half-life of converting a chlorinated cyanurate into CYA and HOCl is relatively slow at 0.25 seconds for one pathway and 4 seconds for another (but remember, this is the time it takes to convert half of the total chlorine and that's generally quite a lot). Another place this effect shows up is if you take a UV lamp and wave it over water being measured for ORP where you can see the ORP drop and then remove the lamp and the ORP rises again. So there is clearly some dynamics going on and only a real-time measurement is going to pick that up. Of course, if one could measure HOCl directly, then that might be even better than ORP, but as "bad" as ORP sensors have been, HOCl sensors appear to be worse (in Ben's experience from seeing such products come and go in the market). It does seem that once an ORP sensor is calibrated then it does roughly track the HOCl concentration so is at least something to use in automated chlorine control systems, especially in a commercial pool environment where the organic load from bathers (and the breakdown of chlorine from sunlight) varies greatly throughout the day. [END-EDIT]


chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
CYA and Lifetime of Chlorine

After accumulating multiple pieces of conflicting evidence, I think it's about time to discuss and investigate the mechanism of how CYA protects chlorine from sunlight. The starting point for the theory, that I'm starting to think is only partially correct and needs to be enhanced, is that the chlorine that is in the form of hypochlorous acid or hypochlorite ion breaks down in direct noontime sunlight with a half-life of around 35 minutes while chlorine that is attached to Cyanuric Acid (CYA), also known as chlorinated isocyanurates, breaks down from the sun with a half-life of around 8.4 hours.

This graph shows the net result. The conclusion from this graph is that a little CYA provides a lot of protection of chlorine and that there are diminishing returns for using high CYA levels. There are two pieces of evidence that are in conflict with this theory:

1) Some users, most notably Janet (user name Aylad), report that in their non-SWG pools using high levels of CYA shows dramatic improvement in chlorine's staying power. In Janet's case, with a CYA below 60 she found that the FC would go from 7-8 to 2-3 in one day (5 ppm FC per day) while with a CYA of 80-90 the FC would go from 8-9 and take 3 days to go below 5 (about 1.2 ppm FC per day). That is a huge improvement that is wholly inconsistent with the graph.

2) Several users of SWG pools have found that raising the CYA to higher levels, especially approaching 70-80 that some manufacturers recommend, has a dramatic increase in FC levels at the same SWG output. Though one theory is that the CYA makes the SWG cell more efficient by combining with the generated chlorine in the cell "hiding" it from the plates in terms of equilibrium (thus making the generation proceed more quickly), an alternative explanation proposed by some is that the higher CYA levels simply protect the chlorine from destruction from sunlight at a rate faster than the baseline theory outlined at the start of this post.

I've been thinking of mechanisms that might explain the above data and that could be added to the theory to make it predict more accurately. One possibility is that CYA itself is able to absorb UV radiation and possibly re-radiate it as non-UV radiation at lower energy, with the rest of the energy becoming kinetic (i.e. heat or temperature increase). This link shows that indeed CYA does absorb UV at the pH found in pools, though it absorbs even more in more basic/alkaline solutions.

If one adds direct CYA absorption and essentially shielding of UV from lower depths of the pool, then the "CYA shielding chlorine" description would in fact be accurate for this mechanism (while it is not accurate to describe the chlorinated isocyanurates which do not "shield" chlorine but are distinctly different molecules with different absorption rates and affect disinfecting chlorine levels). The net effect of this new mechanism would be to have higher CYA levels reduce chlorine loss at a greater rate than shown in the graph I linked to at the top of the post.

So how can we prove that this new mechanism exists (or is likely) and explains what is being reported in (1) and (2) above? Let's start with the easier of the two, namely the second item of whether CYA improves SWG cell efficiency. This can readily be determined by comparing SWG FC output at different CYA levels, BUT with no sunlight shining on the pool (i.e. either at night or with an opaque cover or with an indoor pool). To the degree that CYA increases the SWG cell output to generate higher FC levels, then this leads credence to the efficiency theory; if not, then the protection from degradation from sunlight is more likely.

As for whether CYA "shields" chlorine through absorption of UV (clearly it does absorb some UV, but the question is more one of whether this is a significant mechanism in quantity), this should be a function of the depth of the pool. The presence of higher concentrations of CYA essentially lower the density of UV radiation reaching lower depths in a pool. So this protective effect of CYA should show up more in deeper pools where a significant fraction of the water is at greater depths and should be less effective in shallower pools. The chlorinated isocyanurates, on the other hand, do not have this same effect since they do in fact degrade (the chlorine attached to them degrades to chloride ion) and are in fact less likely to interact with sunlight in this way so light is more likely to continue to lower depths (i.e. it doesn't act as a shield and even if it does, it's a much smaller concentration than unbound CYA itself).

The experiment would be harder and would require measuring the difference in the destruction of chlorine in waters of different depths at varying CYA levels. The half-life of the chlorinated isocyanurate would be the dominant factor in the shallowest basin of water while CYA's "shielding" effect would be a greater factor in the deepest basin of water.

The formula for determining the light intensity passing through a solution is as follows:

I/Io = e-µl

where µ (mu) is the absorption coefficient and is a function of wavelength (so the above formula is for a specific wavelength). "l" is the path length which for the units in the links I referred to is in centimeters.

There is also a formula for absorbance defined as follows:

A = -log10(I/Io)

and there is a molar extinction coefficient defined by the following equation:

A = µ*c*l

where ε (epsilon) is the molar extinction coefficient, c is the molar concentration in moles/liter, and l is the path length in centimeters.

so µ = µ / (log10(e) * c) = µ / (2.303 * c)

3 feet is about 91 centimeters and the molar concentration of 50 ppm CYA is 0.00039 moles/liter. So,

µ = 2.303 * c * µ = 0.00090 * µ

So to get any reasonable absorption from CYA (so that µ is near 1/91 so at 91 cm we have µ*l = 1) we need µ to be over 10. This source gives an extinction coefficient (for gaseous HOCl) of 123 M-1cm-1 with an absorption peak at 220 nm. This source gives an extinction coefficient for hypochlorite (OCl-) of 350 M-1cm-1 at 290 nm. This link has graphs that show the molar absorption coefficient for both HOCl and OCl-. It looks like CYA absorption will protect HOCl from breakdown more than OCl- though both are protected (so in theory that implies that lower pH has less chlorine loss which is also true due to the greater photolysis of OCl- and correspondingly shorter half-life).

This link gives an extinction coefficient for Cyanuric Acid (CYA) of 6.283 OD220 units · ml · µmol-1 which implies 6283 M-1cm-1 at 220 nm. The link given earlier that showed graphs of CYA absorption are consistent with this measurement, but the absorption drops rapidly at higher wavelengths and it is not clear at which wavelengths HOCl dissociates (breaks down from UV).

The rate constant (k) for a first-order reaction is related to the half-life (t1/2) as follows:

C/Co = 0.5 = e-kt
t1/2 = -ln(0.5)/k = 0.693/k
k = -ln(0.5)/t1/2 = 0.693/t1/2

and the rate constant (k) is presumed to be proportional to the intensity of light.

This link provides interesting detailed information about chlorine (and bromine and chlorine dioxide) in terms of half-life at various depths (no CYA present). Interestingly, there is quite a difference in half-life by depth at higher concentrations of Dissolved Organic Carbon (DOC). Chlorine itself can shield lower depths, but it is unclear what the overall net molar extinction coefficient is independent of wavelength. The numbers in the chart with the lowest DOC show an absorption coefficient of 0.010 at 1 meter, 0.0081 at 2 meters, 0.0067 at 3 meters and 0.0057 at 4 meters. These numbers are not far of from the absorption of water itself as seen in this link though the DOC may certainly be a contributor. Thus, the chlorine is most depleted from water near the surface so having good circulation is essential in order to keep chlorine levels more uniform throughout the pool. It also appears, from the pH dependence, that perhaps hypochlorous acid (HOCl) is less susceptible to breakdown from sunlight than hypochlorite ion (OCl-). This implies that having a pool at lower pH results not only in more disinfecting chlorine, but has the chlorine last longer (though the effect may not be very strong from, say, 7.8 to 7.2).

Also, note that there is a non-linear effect from the concentration of whatever protective agent is present at the shallower depths (be it hypochlorous acid itself or CYA). So if I use a molar extinction coefficient of 10 and 50, then I would get the following for I/Io at 3 foot depth:

CYA (ppm) ... I/Io (10) .. I/Io (20) .. I/Io (50)
0 ................. 1.00 ........ 1.00 ........ 1.00
10 ................ 0.85 ........ 0.72 ........ 0.44
20 ................ 0.72 ........ 0.52 ........ 0.20
30 ................ 0.61 ........ 0.38 ........ 0.09
40 ................ 0.52 ........ 0.27 ........ 0.039
50 ................ 0.44 ........ 0.20 ........ 0.017
60 ................ 0.38 ........ 0.14 ........ 0.0077
70 ................ 0.32 ........ 0.10 ........ 0.0034
80 ................ 0.27 ........ 0.074 ........ 0.0015
90 ................ 0.23 ........ 0.054 ........ 0.00067
100 .............. 0.20 ........ 0.039 ........ 0.00030

So to see the dramatic change seen from higher CYA levels, the CYA shielding effect has to be strong enough to be the predominant effect. The shielding effect would "shield" not only unbound chlorine, but also chlorine bound to CYA. Note that using an extinction coefficient of 20 in the above table one finds the difference between 50 and 90 ppm CYA being a factor of 3.7 which is not far off from the factor of 4.2 that Janet was seeing. So perhaps adding an additional protection factor similar to the "20" column in the above table might be the thing to do. This link indicates that the chlorinated isocyanurates are unstable in sunlight, but it is unclear how much of that is due to breakdown from the equilibrium hypochlorous acid vs. direct breakdown itself. The study just shows that CYA is itself stable in sunlight. If the CYA absorption effect is really this strong, then deeper pools should be more protected at the same CYA level since more of their water volume will be at deeper depths "shielded" from the UV.

An experiment using shallow depth water with different levels of CYA will help isolate the two effects. If the CYA "shielding" or absorption is the main effect, then there should be little protection of chlorine in shallow water. If instead the chlorine combined with CYA has a longer half-life and that is the main effect, then higher CYA levels even in shallow depths should show significant protection and should roughly follow the curve in this graph. I suspect that there will be a some of both processes going on.

The original CYA patent by Fuchs may be seen at this link. There were interesting laboratory tests that appear to have been made at shallow depths and only show a small amount of the "depth" variation one sees with higher chlorine levels. The UV lamp they used appeared to have 1 ppm FC drop to 0.5 ppm FC in 1.7 hours so was not as strong as sunlight. The rate of chlorine loss seemed to track the amount of unbound chlorine, but with diminishing returns starting at a rate of 0.29 per hour at no CYA, 0.16 per hour with 1 ppm CYA, 0.13 per hour with 2 ppm CYA, 0.092 per hour with 5 ppm CYA, 0.071 per hour with 50 ppm CYA and an actual increased loss of 0.088 per hour at 100 ppm CYA. This is somewhat consistent with the original theory of a 35 minute half-life in direct sunlight with no CYA and an 8.4 hour half-hour limit when bound with CYA. This is probably where the industry got its original data for its tables. Note that CYA also has a protective effect on chlorine loss from oxidation of iron and copper. Though the patent speculates CYA may coat metals, it appears that the effect is explained by the reduction in disinfecting chlorine and therefore the rate of corrosion based on its concentration. It should be noted that in the patent "real pools" showed the greater protection effect of higher CYA levels by about a factor of 2 at 10 ppm CYA and over a factor of 3 at 50 ppm CYA. Thus there does appear to be a "shielding" depth factor for CYA protection separate from that explained solely by Cl and Cl-CYA breakdown. The fact that the chlorine levels were the same and only the CYA level increased, yet had a greater effect in a real pool with "depth" is very strong evidence.

The good news with this new information is that at sufficiently high CYA levels using a higher FC (to compensate for disinfection and prevention of algae) should not result in larger losses. Going from 30 ppm to 90 ppm requires about triple the FC level, but the loss rate may be cut down by a factor of 7 for a net overall savings of over a factor of 2. If we can validate this, then it should be possible to run a high CYA pool with high FC levels economically, especially in deeper pools.

To calculate the average intensity of light in the pool overall, one needs to integrate it over depth as follows:

I = (Integral over 0 to D of Ioe-µl dl) / D = (Io/(µD))*(1 - e-µD)
which with small µ expands to
I = Io*(1 - µD/2 + (µD)2/6 - ...)
so that as µ approaches 0, "I" approaches "Io" as expected.

In many real pools, there is less volume below around 3 feet as the pool bottom drops only in the deep end such that the pool can be seen as the sum of 3 pools, one with a depth equal to the shallow end, one with a depth equal to the deep end, and one with a depth that varies from the shallow depth to the deep depth. If Ds is the depth of the shallow end and Dd is the depth of the deep end, then the overall average intensity (that can be used to calculate an average breakdown rate or whose inverse can calculate an average half-life) is the following assuming each section is one-third of the pool area:

I = ( (Io/µDs)*(1 - e-µDs) + (Io/(µ*(Dd-Ds)))*(Dd + e-µDd/µ - Ds - e-µDs/µ) + (Io/µDd)*(1 - e-µDd) ) / 3

[EDIT] See this post and this post for experiments Mark did that pretty much conclusively proves that the improved salt cell efficiency at higher CYA does not come from any internal chemistry in the salt cell (since there was no such change seen at night) but rather is from reduced chlorine loss from sunlight that more than makes up for the higher FC needed at higher CYA to maintain disinfection and prevention of algae. [END-EDIT]



Well-known member
Mar 28, 2007
A couple of comments on the above.

There is 2 ways to control pool chemistry, reactive or predictive.

When we measure the water parameters then and add agents to correct these parameter
we are being reactive.

When we try to predict what the pool parameters are then then add our agents we are being

If we try to automate the pool correction parameters with the various meters(or probes) we continously
try to correct to these parameters. How succesful we are is determined by the accuracy of the continuous
measurements. As we have found ORP measurements are difficult to maintain with godd accuracy, pH measurements
have proven somewhat more accurate.

Lets take another look from a different angle on pool parameter corrections. We know that the biggest user of
FC is the Sun and its UV rays. So Chlorine use is mostly dictated by the intensity and the length of time the
sun is exposed to the pool water. We have a good measure of this by the sunrise and sunset times. We cannot
predict the bather load, but there is another way around this I will mention later. Another parameter of Chlorine use is
the water temperature. The water temperature in a non-heated pool will be a direct relation to the amount of sun
that strikes the pool, in a heated pool this of course won't apply.

With a SWG we know the amount of time the SWG is running gives a set amount of Chlorine generation. I am only considering
running the SWG at 100% ouput. This is the most efficient for the SWG and the shortest pump run time, because the pump
must be running for the SWG to work.

Now, we know the pool water temperature and the Chlorine demands are directly related to the amount of sunshine
and water temperature, so if we time the operation of the SWG to coordinate with the sunrise and sunset we can closely
match the Chlorine needs. I have found that actually as the water temperature decreases the SWG will have to be run a shorter
time because the Chlorine demands as less than just the function of the Sunrise and Sunset. I have confirmed this over the
last 2 years. The next step is to add the water temperature into the pump time run calculations. As a practical problem I have to find
a way to run a wire to get my pool water temperature.

Since a pool is a large volume of water the parameters change rather slowly and it not necessary to try to correct them
minute by minute. The system I am testing is, to use a predictive set of parameters to control Chlorine addition(generation).

I have installed a CO2 system for pH control using the same type parameters, because the pH control is needed as a function
of SWG run time. In the summer I have to make a correction for pool water addition.

I find that these predictions are quite accurate over time. I might add it does take some kind of computer controlled system
to do all these calculations. I do it with my home automation a Elk M1G.

In summation, if by varying the SWG(pump) run time and the time the CO2 is added I can control the pool water parameters
that correct for the various seasons and water temperature, then I have almost a hands off system. Even with one of the
ORP-pH sytems you still have to periodically measure the water parameters. The difficulty is that the probes are really
ment for laboratory usage and are not dependable enough for unattended use. Their calibration is hard to maintain and
require periodic replacement.

As I mentioned earlier if you run the pool pump and SWG while the pool is in use you will add enough extra Chlorine to
offset the bather load(in a residential pool only).

The bottom line is that are is more than one way to maintain pool water chemistry.

Cliff s

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA

I understand your points, but I don't think we're really in that much disagreement. Basically, though we start off by telling people to measure and adjust water parameters, that's mostly so people can get a "sense" of their pools and what they need. Then, over time, people don't have to measure as often (at least for some parameters) if they "know" their pools better. That is, one starts off with measurements and adjustments to keep things in line and then transitions more towards what you are describing as practical ways of essentially maintaining the water chemistry the same as would be done with measurements. In my own pool, I don't measure chlorine every day, but rather twice a week before I normally add chlorine since I know its usage rate and never get too low (I have an opaque electric safety cover, so loss from sunlight is minimized).

The problem we have found is that one can't really skip to the second stage as you have without first doing the first stage of measurements. The reason is that the factors in every pool are complex and not easily calculated. Yes, one knows that if sun hits the pool then there will be loss due to sunlight, but the exact loss depends on many factors including sun angle (time of year), pool depth, trees and other obstructions, CYA level, etc. Bather load is also a difficult variable since people sweat different amounts and this depends a lot on water temperature and level of activity. Chlorine demand from other organics depends on how much junk (leaves, pollen, etc.) gets into the pool and that can change over time depending on what gets caught in the filter, how often it is cleaned, etc.

So I don't disagree with what you are saying and I'm glad you've found a timing approach that works well for you. I'm not so sure that the 100% SWG ontime only when the pump runs is necessarily more efficient. My understanding of SWG percentage is that when it's on, it's fully on, so the percentage is just one of the amount of on time during the pump run time. I don't think the SWG cell necessarily lasts longer trying to have it run 100% of the time that the pump is running. The pump run time should be based on circulation needs while the SWG percentage time (and when this occurs) should be based on chlorine demand and those are separate factors that may not coincide. I do agree with your "when to run the SWG" analysis such that it runs more frequently during expected higher demand.

still digesting all this. quite a bit here and some interesting implications but no big surprises. We've always been pretty much on the same page, Richard!


LifeTime Supporter
May 4, 2007
I accidently wandered in here myself and I think I got a Brian aneurysm.

Hey Sean, you need to put a warning on this advance chemistry section.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
waterbear said:
still digesting all this. quite a bit here and some interesting implications but no big surprises. We've always been pretty much on the same page, Richard!

Just FYI -- this is not a new thread. Only the post from cliff_s was new and triggered this thread to show up in "View Posts Since Last Visit". The information on pool water chemistry is pretty old, much of it copied from the Pool Forum China Shop where I originally posted it here.

JCJR and Grace, aren't the graphs pretty? I thought my choice of color scheme was particularly well planned. Next time you see the word "Advanced" in a forum name, run, do not walk, to your nearest exit. I hope the aneurysm clears up soon so the dyslexic typing gets cured (unless your name is Brian). :lol:

chem geek said:
Just FYI -- this is not a new thread.
They say when you get old the mind is the second thing to go! (I was 54 on Feb. 24) Can't remember what the first thing is! Been too long! :shock: :-D :-D


LifeTime Supporter
May 4, 2007
I am sure glad that others are able to understand this stuff and debate/add to the findings because it benefits us all.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
I am adding a post that explains the chlorine/CYA relationship in chemical terms at varying levels of detail as some people ask for a more detailed explanation or derivation and the original 1974 O'Brien paper is hard to find [EDIT] (you can see a copy of the paper here). [END-EDIT]


Though Cyanuric Acid (CYA) absorbs ultraviolet (UV) radiation directly thus shielding the lower depths of water and protecting chlorine in those depths from breakdown, the primary result of having CYA in the water with chlorine (hypochlorous acid) is that it combines with chlorine to form a set of chemical species collectively called chlorinated isocyanurates (and these compounds also absorb UV without breaking down significantly). The full chemistry is complicated (well, tedious) because there are 6 different species of chlorinated isocyanurates (that is, chlorine attached to CYA) and 4 different species of Cyanuric Acid and its dissociated ions. There are 13 simultaneous chemical equilibrium equations of the CYA, chorinated isocyanurates, hypochlorous acid and their combinations though only 10 of these are independent from each other.

Look at the chemical structure for CYA here and that of Trichlor here and of Dichlor here and notice that essentially the Nitrogen can have either hydrogen or chlorine attached to it and that there are three such sites. Qualitatively, chlorine combines with CYA to form new chemicals that are essentially not disinfectants nor oxidizers (at least not even close to hypochlorous acid; more like hypochlorite ion at best). CYA has a moderately strong affinity for chlorine such that when CYA >> FC (when both are measured in their respective ppm), then most of the chlorine is attached to CYA. 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. You can see from the structure of Hypochlorous Acid here that it looks similar to water with a chlorine atom substituting for a hydrogen atom. When chlorine combines with CYA, this is a chlorine substitution for a hydrogen atom or essentially an exchange of the chlorine atom to the CYA and the hydrogen atom from the CYA to make water. When chlorine is released from CYA, then the opposite exchange occurs.


To simplify the description, I will only talk about the most dominant chemical species found at the pH of pool water. For Cyanuric Acid (which I designate as H3CY), the species at highest concentration is the one that has dissociated one hydrogen ion which I will designate as H2CY-. For the chlorinated isocyanurate species, it is CYA with one hydrogen, one chlorine, and one open slot so is negatively charged which I will designate as HClCY-. The following is the primary relevant chemical equation to focus on:

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

Hypochlorous Acid is the strongly disinfecting and oxidizing form of chlorine so is all I will talk about (as opposed to hypochlorite ion). The chlorinated isocyanurates show little if any disinfecting capability and minimal oxidation power. The above equation is described by a chemical equilibrium constant as shown by the following:

[H2CY-] * [HOCl] / [HClCY-] = 10-5.62 = 2.4x10-6

At 3.5 ppm Free Chlorine (FC), this is equivalent to 4.9x10-5 moles/liter concentration while 30 ppm CYA is 2.3x10-4 concentration. Since the CYA concentration is much higher than the FC concentration, even if all the chlorine could attach to CYA via the above equation, the net effect is that the total amount of "chlorine bound to CYA" can't be more than the amount of FC and the H2CY- does not drop very much. Rearranging, we have:

[HOCl] = 2.4x10-6 * [HClCY-] / [H2CY-]

Hypochlorous acid (HOCl) is also in equilibrium with hypochlorite ion (OCl-) where at a pH of 7.5 this is roughly split 50/50 between these two species. So we can rewrite the above in terms of measured concentrations as follows where CYA and FC are total concentrations:

FC = [HOCl] + [OCl-] + [HClCY-]
CYA = [H2CY-] + [HClCY-]

[HOCl] = 2.4x10-6 * ([FC] - [HOCl] - [OCl-]) / ([CYA] - [HClCY-])
and at a pH near 7.5,
[HOCl] = 2.4x10-6 * ([FC] - 2*[HOCl]) / ([CYA] - [FC] + 2*[HOCl])

For practical purposes, because CYA is much larger than FC, the HClCY- can be initially ignored in the above. The above equation implies that the HOCl concentration must be very small and that most of the chlorine is bound to CYA. The following is an approximation we can test:

[HOCl] is approximately 2.4x10-6 * [FC] / [CYA]

The chlorine values of HOCl and FC can be measured in the same units (as they are on both sides of the equation so any factors cancel), but we can convert the [CYA] concentration into ppm by multiplying the right hand side (numerator) by the molecular weight of CYA, 129.075 g/mole, and 1000 mg/g (multiplying the denominator by this number converts CYA into ppm) resulting in:

HOCl is approximately 0.3 * FC / CYA

The above approximation isn't terribly far off from the accurate calculation. At an FC of 3.5 ppm and a CYA of 30 ppm, the actual HOCl is 0.051 ppm while the above approximation gives 0.035 ppm. You can see where the FC/CYA ratio comes from -- it is a direct result of the chemical equilibrium between chlorine attached to CYA vs. separate chlorine and CYA. A more accurate approximation is given by modification of the formula not removing the [FC] term in the denominator (which results in a factor that is the ratio of CYA and Cl2 molecular weights):

HOCl is approximately 0.31 * FC / (CYA - (1.8 * FC))

which with the FC of 3.5 ppm and CYA of 30 ppm results in 0.046 which is within 10% of the correct result. However, the above approximation falls apart rather quickly when the CYA/FC ratio is less than 5 and it is still pH dependent (the assumptions were at a pH of 7.5 for the dominant species which determines the equilibrium constant). We can rewrite the equation to look at the FC/CYA ratio explicitly as follows:

HOCl is approximately (FC/CYA) * (0.31 / (1 - 1.8 * (FC/CYA))

where for an FC/CYA ratio of 0.1 the factor is 0.38 while at a ratio of 0.2 the factor is 0.48. A very rough rule of thumb uses 0.5 and if one looks at the equivalent FC with no CYA then at pH 7.5 this is double the HOCl so the factor becomes 1.


So how can one conclude what the dominant species are since that is the assumption I started with above? Let's look at the detailed equations and go through a process of elimination based on the pH. We'll start with the easier case to analyze, namely CYA and its dissociated species. Some of the following equations use an adjusted equilibrium constant for the ionic strength in typical pool water at 300 ppm CH, 100 ppm TA, 30 ppm CYA and 525 ppm TDS. All of the equilibrium constants come from the original 1974 O'Brien paper I refer to in the first post in this thread, but you can also see these constants (with some minor errors due to using slightly different sources) in this link on document page 12 ( PDF page 18 ).

H3CY <--> H2CY- + H+ ..... pK = -log10(K) = 6.83
H2CY- <--> HCY2- + H+ ..... pK = 11.26
HCY2- <--> CY3- + H+ ..... pK = 13.32

Let's take a look at the first reaction's equilibrium expression:

[H+] * [H2CY-] / [H3CY] = 10-6.83

Taking the negative log10 of both sides gives:

pH - log10([H2CY-] / [H3CY]) = pK
log10([H2CY-] / [H3CY]) = pH - pK

So from the above, and generalizing, one can see that when pH < pK then the ratio in the log10 is less than 1 while when pH > pK the ratio in the log10 is greater than 1. So this means that at a pH of 7.5, the following are true:

[CY3-] << [HCY2-]
[HCY2-] << [H2CY-]
[H2CY- ] > [H3CY]

So this is where we get our initial assumption of H2CY- being the dominant cyanurate species where we can see that the next most dominant cyanurate species is H3CY.

For the chlorinated isocyanurates, we have the following (the pK are adjusted for ionic strength):

H2ClCY <--> H+ + HClCY- ..... pK = 5.28
HClCY- <--> H+ + ClCY2- ..... pK = 9.98
HCl2CY <--> H+ + Cl2CY- ..... pK = 3.70

where we can conclude the following at a pH near 7.5:

HClCY- >> H2ClCY
ClCY2- << HClCY-
Cl2CY- >> HCl2CY

So of the above species, HClCY- and Cl2CY- are dominant, but we cannot yet tell which is more dominant between these two. There are additional chemical equations relating to the interaction of chlorine with the chlorinated isocyanurates as follows:

Cl2CY- + H2O <--> HClCY- + HOCl ..... pK = 4.51
HCl2CY + H2O <--> H2ClCY + HOCl ..... pK = 2.93
Cl3CY + H2O <--> HCl2CY + HOCl ..... pK = 1.80

Because the HOCl concentration is relatively small (pHOCl > 4.6), this implies the following:

HClCY- > Cl2CY-
H2ClCY >> HCl2CY
HCl2CY >> Cl3CY

So the assumption that HClCY- is the dominant chlorinated isocyanruate species is reasonable and the next most dominant chlorinated isocyanurate species is Cl2CY-.

In spite of the above equilibrium, the rate of release of chlorine from CYA is rather fast so all of the chlorine attached to CYA measures as FC in the FC test because the HOCl gets used up reacting with the dye in the test and more HOCl is released from that attached to CYA (or from hypochlorite ion) in the time of the test.

All of the chemical equations are solved for explicitly through iteration (due to changes in ionic strength) in this spreadsheet.


chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA

It's copyrighted material so I can't post it (at least not in its entirety). I have a copy of the out-of-print book that it is in: Chemistry of Water Supply, Treatment and Distribution that I refer to in my first post in this thread. I pretty much bought up the world's supply of that book sending it to the most important people in the world who should have that book -- the people on the committee defining the APSP-11 standards as well as the CDC and others who should be aware of the chlorine/CYA relationship and that this isn't new. The only copy I can still see available for purchase is here and it's much more expensive because, well, it's now more rare.

Nevertheless, many university library systems have this book which is where I originally found it (in an off-site archive facility, since it's not a frequently accessed book) after I saw it referenced in this EPA document. The most relevant information from the paper are the equilibrium constants for the chlorinated cyanurates (and cyanuric acid) in Table 14.IV of that paper. I will quote a few paragraphs or partial paragraphs (as an excerpt) from that paper below:

Moreover, chlorinated cyanurates, in addition to acting as stabilizers, exhibit a limited degree of hydrolysis to yield a relatively constant level of germicidally potent, free chlorine. In other words, chlorinated cyanurates may be considered as analagous to a protected reservoir which liberates a small but relatively constant level of free chlorine in accordance with clearly defined principles of chemical equilibrium.
Ordinarily, most of the reservoir chlorine consists of chlorinated cyanurates. Although these provide a readily available source of active chlorine, Andersen1 has submitted evidence that chlorinated cyanurates, as such, are not particularly germicidal. As a result, the germicidal activity must be borne by the relatively small fraction of free chlorine present at any given time. Since, in general, increase in cyanurate concentration results in decreased free chlorine, the use of large cyanurate concentrations to achieve maximum stability will tend to give inadequate germicidal activity. For the same reason the continual addition of chlorinated cyanurates as a source of chlorine is not recommended since this will lead to build up of cyanurate concentration and consequent repression of the concentration of free chlorine below that necessary for effective germicidal activity.
Although chlorinated cyanurates serve as a reservoir of free chorine, bactericidal efficacy is more closely related to the relatively small fraction of free chlorine present at equilibrium. Therefore, the use of excessive cyanurate in an overly zealous attempt to reduce photolysis may repress free chlorine to the point of suppressing germicidal activity. For the same reason the continual addition of chlorinated cyanurates as a source of chlorine is not recommended because this will lead to build up of cyanurate concentration.
1. Anderson, J.B. "The Influence of Cyanuric Acid on the Bactericidal Effectiveness of Chlorine," Ph.D. Thesis, University of Wisconsin, Madison, Wisconsin, 1963.

The paper defines "free chlorine" as being the sum of hypochlorous acid and hypochlorite ion while "reservoir chlorine" is free chlorine plus all chlorinated cyanurate species. In our modern terminology based on what is measured in the Free Chlorine (FC) test, FC is actually reservoir chlorine. So when the paper says that higher CYA levels lower Free Chlorine, they are referring to hypochlorous acid and hypochlorite ion, not to what we now call FC.

Over the subsequent years, especially in the 1980's, a series of papers demonstrated chlorine's dramatic reduction in effectiveness in the presence of Cyanuric Acid. The most careful of such studies (this PDF file) shows that the germicidal effect of chlorine (against a species of protozoan cyst) is based on the hypochlorous acid concentration. Other studies show the effects against bacteria, viruses, algae and oxidative power (against amino acids).



LifeTime Supporter
Jun 26, 2007
SF Bay Area

Thanks for your excellent reply.

sending it to the most important people in the world who should have that book -- the people on the committee defining the APSP-11 standards as well as the CDC and others who should be aware of the chlorine/CYA relationship and that this isn't new.
Are you making any headway with changing hearts and minds yet? What kind of reception are you receiving? I would think that the CDC - being scientists with no profit ax to grind - would be more receptive as compared to the APSP committee?

In a couple of years, perhaps TFP will have a large enough following (and corresponding political power) in order to either influence the APSP committee. Or perhaps we will put our own people on the APSP committee. :twisted:

[Side note to Sean B: Is TFP able to become an APSP member? I assume that TFP would have to be an APSP member in order to be eligible for APSP committee positions.]

It's copyrighted material so I can't post it (at least not in its entirety).
Have you considered contacting the publisher (Ann Arbor Science ? ) directly and asking them for permission to reproduce the paper for specific purposes? It is hard to imagine that Ann Arbor Science, if they even exist any more, really cares that much about an obscure 35 year old paper. If you wanted, I would volunteer to do some legwork on contacting the publisher for permission to reproduce this paper.


chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA

Yes, if you could find out if the publisher still exists or got bought out and can get permission for me, that would be great. I don't have a lot of time to add something like that to my plate so you could certainly help me out.

As for the APSP-11 developing standard, I gave rather extensive comments on it. I was very encouraged that it contained some information on the chlorine/CYA relationship though not to the level of detail I would have preferred so I sent some graphs and "rules of thumb" info in the comments. I have no idea what will come of it since there is no feedback process back to those who make comments. I proposed using an "FC as % of CYA" with a minimum FC as a standard and said that this wasn't that different than the LSI that is already a multi-parameter index and is referred to in the standard. I also made the radical proposal of allowing (at least not recommending against) a small amount of CYA for indoor pools.

As for the CDC, they are very busy and generally underfunded and besides, they don't set pool standards. They are mostly just focussed on disease prevention. Nevertheless, I have written to them about possible ways of handling Crypto, but I'm probably just a PITA to them -- I don't have any relevant credentials and don't work in the industry. They've always been cordial, but I just don't think they have much time to devote to these issues.