Pool Water Chemistry

8/10/2019 Pool Water Chemistry

1/34

Forum Swimming Pool Care The Deep End... Pool Water Chemistry

Welcome to our new server and newforum software.

If this is your first visit, be sure t o che ck out the FAQ by clicking the link above. You may have to register beforeyou can post: click the register link above to proceed. To start viewing messages, select the forum that you wantto visit from the selection below.

Thread: Pool Water Chemistry

User Name Password

Log in

Remember Me?

Register HelpForgot your password?

Home Pool School PoolMath Rules Store What's New?

FA Q Calendar For um A ctions Quic k Links Adv anced Search

Results 1 to 20 of 73 Page 1 of 4 1 2 3 4 Last

Thread Tools Display

05-25-2007, 06:56 PM

Po ol Water Chemistry

This thread presents my findings so far on pool water chemistry

including the following:1. More Accurate Calcite Satur ation Index (CSI) toreplace Langelier Sat uration Ind ex (LSI)

2. Calculation of ppm HOCl (disinfecting chlorine) atvarious levels of Total Free Chlorine (FC) andCyanuric Acid (CYA)

3. Determination of pH and Alkalinity changes whenadding chemicals to the pool

4. Various reaction rates including chlorine breakdownby 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). Thechart at the following link shows this relationship. (I recentlydiscovered that all forms of chlorine are measured as ppmequivalents of chlorine gas, so all charts, graphs and thespreadsheet have now been updated to reflect this.)

HOCl Chart

Note that the red in the linked chart above represents a cutoff

#1

Join Date:Location:Posts:

Mar 2007Sa n Rafael, CA USA 8, 657

chem geek

Senior Member

Forum

8/10/2019 Pool Water Chemistry

2/34

of 0.011 ppm HOCl which roughly corresponds to the 650 mVORP level that the U.S. and WHO set as the minimum required fordisinfection. The green color is a guess at 0.05 ppm HOCl of theminimum level of chlorine needed to prevent algae. The actualnumber 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 onreal-world experience, I suspect the actual number will besomewhere in this range. So, red means bacterial growth whilegreen 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 (totallya guess) "algae" levels.

The following is an approximate formula you can use so long asyour CYA ppm is at least 5 times your FC (the formula really fallsapart 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 (toconstruct the equivalent of Ben's table, for example), you canuse the following which just solves for ppm FC from the above.

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

The constants in the above formulas are for a pH of 7.5 (whichis the only parameter that significantly affects these constants).With the spreadsheet I can easily calculate the constants forother pH, but remember that the above formulas areapproximate. For example, with FC of 3 and CYA of 15 theformula gives HOCl as 0.098 when the correct answer is 0.095.That's not terrible (about an 3% error). However, with FC of 5and CYA of 15 the formula gives HOCl as 0.239 while the correctanswer 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 theeffective chlorine level is reduced by a factor about equal to theppm of the CYA. So, a CYA of 30 ppm reduces the disinfectingchlorine (HOCl) level to about 1/30th of what it would be with noCYA.

8/10/2019 Pool Water Chemistry

3/34

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 roughlycorrespond to "Ben's Best Guess CYA Chart". Ben's chartconverted to show HOCl may be found here where you can seethat the rough Min FC corresponds to 0.03 ppm, the rough MaxFC corresponds to 0.07 ppm (implying an ideal target of 0.05ppm) and the shock table is not consistent, but probably impliesa minimum of 0.3 ppm, at least for green algae. User experience

indicates that hard-to-kill yellow or mustard algae (and maybeblack algae) may need 1.0 ppm HOCl for shock. User experiencewith black algae indicates that keeping active black algae fromgrowing requires around 0.07 ppm HOCl.

A comparison of the "traditional" HOCl/OCl- graph with the samegraph in the presence of CYA may be found at this post . Thisalso shows how CYA is a "chlorine (specifically HOCl) buffer" thatmakes HOCl concentration about half as sensitive to changes inpH.

The original source for the equilibrium constants was done in

1973 (and published in 1974) where the recommended maximumCYA level was 25 ppm:

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

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

A Little CYA Goes A Long Way

NOTE: The mec hanism of protec tion of c hlorine from sunlight by CYA is currently under review in this post . Higher CYA levelsmay protect even proportionately higher levels of chlorinemore, especially in deeper pools.

The following is a graph showing that a large amount of thebenefit of CYA protection of chlorine from UV (sunlight) isalready there at around 20 ppm. This data is approximate, notonly 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 cyanurateswhich are more stable, but still breakdown from sunlight). Thelimiting half-life for HOCl/OCl- is 35 minutes which is consistentwith pool studies, but some experimental studies give 11.6minutes. The limiting half-life of the chlorinated cyanurates is 8.4hours though some other data shows it could be 6 hours.

8/10/2019 Pool Water Chemistry

4/34

The following graph combines the two concepts of needing morechlorine at higher CYA vs. the greater protection of chlorine byCYA. The graph shows the total chlorine (FC) loss rate inppm/hour vs. CYA at different HOCl levels. Remember that thisrate of loss will slow down as chlorine gets used up.Nevertheless, the absolute loss of chlorine is greater at higherCYA levels (keeping HOCl constant) and is the downside to a"high CYA & high Chlorine" approach. However, the primaryreason to have higher CYA and Chlorine is to have a sufficientbuffer of chlorine to prevent it from dropping to dangerouslevels. There is obviously a tradeoff here. Though using no CYAresults in the least amount of chlorine loss, the fact is that yousimply can't maintain a pool with only 0.05 ppm chlorineeverywhere in it -- hence a minimum level is needed as a buffer.

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

pH Rising

If you find that your pH wants to keep rising, this may be due to

8/10/2019 Pool Water Chemistry

5/34

your pool outgassing CO2 to the air. The rate of outgassingincreases 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 willvary greatly from pool to pool, but the following chart shows therelative outgassing rate as a function of pH and Total Alkalinity.It is possible that the hydrogen gas bubble production from SWGsystems 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 thevariation with different amounts of CYA is not large. Note that

there is a large variation with pH (the Y-axis is logarithmic). Ihave drawn a somewhat arbitrary "Limit" line at a relative rate of 15 that I have found is roughly the tolerance limit where manypeople start complaining about rising pH, but again aeration is afactor 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 anddiscussion.)

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

8/10/2019 Pool Water Chemistry

6/34

05-25-2007, 06:58 PM

Chlorine/pH/CYA Relationships

OK everyone. Here are some graphs I put together so now youcan give me feedback as to whether this is what you are lookingfor. First, I show the traditional HOCl/OCl- relationship on theleft including a total line (for HOCl + OCl-) that is always at100%. It should be noted that the chart on the left is valid forany Total Free Chlorine (FC) level.

The chart on the right shows the same situation when there is30 ppm CYA and in this case the Total Free Chlorine (FC) levelmatters and is 3 ppm for this chart though when CYA >> FC it isroughly 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 theleft to around 1.5% on the c hart on the right. Note that thetotal HOCl+OCl- level is not 100% when CYA is present. Thedifference from 100% (the Cl-CYA curve) is the amount of chlorine "bound" to CYA and though it is better protected fromdegradation from sunlight, it is also not immediately available fordisinfection or oxidation (but is available "in reserve" as HOClgets used up). Finally, notice how much "flatter" the HOCl curveis in the graph on the right indicating that the presence of CYAhas made the amount of HOCl less sensitive to changes in pH(though we really need to look at a log scale for relativechanges -- more on that next).

#2

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

7/34

If we want to see changes in disinfecting chlorine (HOCl) inpercentage terms, then a logarithmic scale is more appropriateso that equal distances on the chart represent the same relativeamount of change. That is, it answers the question of how muchimprovement there is in the relative amount of chlorine when youlower pH. It is not quite as obvious in this graph, but the HOClcurve is a bit flatter on the right with CYA present, though atlower pH at around 7.0 the pH sensitivity of HOCl is about thesame at 30 ppm CYA as it is with no CYA (and below 7.0 the pHsensitivity of HOCl is actually greater with 30 ppm CYA than withno CYA, but this is mostly due to the fact that with no CYA andat low pH most of the chlorine is already HOCl so there's no roomfor relative "growth"). Also note that at higher pH above 7.5 thatthe presence of CYA allows one to operate at higher pH withoutlosing that much chlorine effectiveness (without CYA theeffectiveness of chlorine drops rapidly above pH 7.5). The graphon 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 alogarithmic scale).

8/10/2019 Pool Water Chemistry

8/34

8/10/2019 Pool Water Chemistry

9/34

How's that?

P.S.It is interesting to note that the traditional HOCl/OCl - graph withno CYA showed the large variation in HOCl percentage vs. pH,but that this was rather pointless (for pools; not for drinkingwater disinfect ion) because the absolute concentration of HOClwas typically so large that it didn't really matter if only 10% of the tot al was HOCl. The minimum HOCl concentration forpreventing algae is on the order of 0.05 ppm (disinfection

minimum is around 0.01 ppm) whereas even a pool with no CYAand a pH of 8.4 (which is only 10% HOCl) with even a low totalFC of 1 ppm still gives 0.10 ppm HOCl which is double where wenormally run our pools today when we use CYA!

Richard

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF

pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

05-25-2007, 07:00 PM

Equations for Chlorine Chemistry

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

Adding ChlorineNaOCl + H2O --> Na + + HOCl + OH- (+ extra base Na + + OH-)HOCl --> H+ + OCl-Sodium Hypochlorite (liquid chlorine or bleach) combines withwater to produce sodium ions (part of regular table salt) plusdisinfecting chlorine plus hydroxyl ion. The hydroxyl ion makesthis a basic reaction that raises pH, but because the disinfectingchlorine is a weak acid this overall reaction raises the pH by lessthan 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 helppreserve it, but this amount is rather small.

Using Up Of ChlorineBreakdown of Chlorine by Sunlight (UV)2HOCl --> O2(g) + 2H + + 2Cl-

2OCl- --> O 2(g) + 2Cl

-

Chlorine breaks down in the presence of ultraviolet radiation,

#3

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

10/34

such as found in sunlight, and forms oxygen gas and chloride ion(and hydrogen ion, if starting with HOCl hypochlorite). Because ahydrogen ion is produced, this is an acidic process, but sincedisinfecting chlorine is a weak acid, only some of it breaks downin a way that lowers pH as shown above (i.e. only HOClproduces H+ ; OCl- does not). During the process of chlorinebreakdown 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 helpoxidize organics in the pool.

Net Chlorine To Breakpoint (Ammonia "Oxidation")2NH3 + 3HOCl --> N2(g) + 3H + + 3Cl- + 3H2OOCl- + H+ --> HOClThe disinfecting form of chlorine (HOCl) combines with ammoniathrough a series of reactions (that I have not shown) with thenet result being the production of nitrogen gas (which is why itis important to keep your cover off and have good circulationwhen shocking) plus hydrogen ion and chloride ion. Though byitself this would be a strong acid reaction, there is also OCl-present that will combine with hydrogen ion to form more HOClsince the ratio of HOCl to OCl- will remain constant (and is about50/50 at pH 7.5). So the net reaction is acidic, but not stronglyso.

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

Other things that could happenIf you do not have enough chlorine in your pool relative to yourbather load (ammonia demand), then the chlorine may notcompletely oxidize ammonia and instead you will get chloramines(first, monochloramine). This reac tion is basic. However, sunlightmay break down monochloramine which will result in the rest of the breakpoint process which overall is acidic (so it's the sameas I showed above overall).

It is also possible for chlorine to combine with organiccompounds to form chlorinated organics that are hard tobreakdown. When people talk about the health problems withchlorine, it is usually about some of these chlorinated organics(Disinfection By-Products, DBPs) known as Tri-Halo-Methanes(THMs) such as chloroform. Also, some chloraminessuch as nitrogen trichloride (NCl 3) not only smell bad, but cancause health problems (especially in indoor pools with poor aircirculation). In an outdoor pool exposed to sunlight and with agood residual of chlorine you typically don't get these "bad"compounds. If you are really concerned and have money toburn, you can use a constant maintenance level of non-chlorineshock (monopersulfate, MPS) to oxidize organics before chlorinegets a chance, but this is probably overkill for an outdoor pool(though may be a good idea for an indoor pool).

8/10/2019 Pool Water Chemistry

11/34

Salt (SWG) PoolIn a salt water pool you produce chlorine through the followingreactions:

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

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

which nets out to the following where the chlorine gas dissolvesin water:

2H2O + 2Cl- --> Cl 2(g) + H 2(g) + 2OH -

Cl2(g) + H 2O--> HOCl + H+ + Cl-

H+ + OH- --> H 2O----------------------------------------------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 asyou get when you add liquid chlorine or bleach (ignoring sodiumion). This process is partly basic, but not strongly so due to theHOCl weak acid. So the overall net result in a salt pool is simplythe production of oxygen or nitrogen gasses. The disinfectingchlorine that was created from chloride ion gets converted backto chloride ion as it is "used up".

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

2H2O --> O 2(g) + 2H 2(g)2NH3 --> N 2(g) + 3H 2(g)

The chlorine is not "seen" in the above net reactions becausethe chloride that became chlorine goes back to being chlorideagain. The oxygen gas comes from water when chlorine gasdissolved in it (i.e. from hypochlorite ion or hypochlorous acid)while the nitrogen gas comes from the ammonia (the oxygen orhydroxyl in the chlorine reverts back into water in this case,using the hydrogen from the ammonia to do so).[END-EDIT]

If you have a salt pool and don't use CYA (this isn't normal) thenyou could also outgas chlorine in the same way that CO 2 isoutgassed. 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 greatlyincreases the pH (HOCl + Cl - --> Cl 2(g) + OH -). The reason thiswould tend to only happen in a salt pool without CYA is that a

high concentration of both chloride ion (Cl -) and disinfectingchlorine (HOCl) are needed and it occurs more readily at lower

8/10/2019 Pool Water Chemistry

12/34

pH. [EDIT] If chlorine gas produced by the SWG did not fullydissolve and instead was outgassed, then this would result in anet pH rise and could be one factor for the pH rise seen in SWGpools (the other factor being carbon dioxide outgassing fromslightly increased aeration from the SWG, but that is not enoughto 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 helpsand that you made it this far...

Richard

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

05-25-2007, 07:25 PM

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".

http://www.sbcontrol.com/ppmorp.pdf

The following linked table shows this same data, but with anextra column I added where I calculated the ppm of HOCl (usinga temperature of 104F for spas). I also resorted the table inorder of decreasing HOCl (the table in the link is sorted by ORP).

ORP.htm

It appears that HOCl is about as good if not a little better thanORP except for one case where the FC was low (0.83), the CCwas high (1.04) and there was no CYA (0). Perhaps the chlorinedemand was used up at some point or the high CC had someadverse effect, but the low ORP (564) with chlorine presentwithout 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) wouldwork 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 tofind any other data as comprehensive as this.

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

ORP-FCsort.htm

So perhaps using the HOCl concentration for predictingdisinfection rates isn't as good as using FC and FC may evenbeat ORP. [END-EDIT]

#4

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

13/34

When I first started looking at ORP, I was shocked to find howinconsistent it was even from the same source. The ratherdefinitive work by Clifford White called "The Handbook of Chlorination" had inconsistencies as did several differentmanufacturers. Before looking at the absolute ORP to ppmChlorine readings, I first looked at something that shouldn't varythat much between ORP sensors and that was the amount of mVchange for each doubling of ppm of Chlorine (everything elseconstant, including pH, TDS, etc.). The following table showsthe results. I have put the sources for each as a link where

appropriate.Code:

Product or Source ORP mV per 2x Chlorine (near pH 7.5)---------------------- ----------------------Sensorex 83.0Aquarius Technologies 45.7Chemtrol (SBControl) 22.7 (at pH 7.5)Uniloc/Stranco not logarithmicSiemens Strantrol 20.0 (same as Petaluma water from Fig. 4-19 Pools below)CYA Paper 28.4Theoretical 1 electron 17.8Theoretical 2 electron 8.9Clifford Whites Handbook of Chlorination:Fig. 4-19 Distilled Water 10.0Fig. 4-19 Pools 12.0 20.0Fig. 9-93a (0-1 ppm) 35.0Fig. 9-102 (0-0.035 ppm) 30-40

SensorexAquarius TechnologyChemtrol (SBControl)Unilock/Stranco (link is dead; may need subscriber access)Siemens St rancol (link is dead)

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

Code: Product or Source 0.2 ppm 1.0 ppm2.0 ppm--------------------------- ------- --------------Aquarius Technologies 635 730

775Chemtrol (SBControl) 720 770

792Uniloc/Stranco 635 794

822 (at 7.4: 650, 800, 826)Siemens Strantrol 655 697

715CYA Paper 702 767

796 (at 7.4: 716, 780, 810)

8/10/2019 Pool Water Chemistry

14/34

8/10/2019 Pool Water Chemistry

15/34

cartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

05-26-2007, 08:18 PM

CYA and Lifetime of Chlorine

After accumulating multiple pieces of conflicting evidence, I thinkit's about time to discuss and investigate the mechanism of howCYA protects chlorine from sunlight. The starting point for thetheory, that I'm starting to think is only partially correct andneeds to be enhanced, is that the chlorine that is in the form of hypochlorous acid or hypochlorite ion breaks down in directnoontime sunlight with a half-life of around 35 minutes whilechlorine that is attac hed to Cyanuric Ac id (CYA), also known aschlorinated 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 andthat there are diminishing returns for using high CYA levels.There are two pieces of evidence that are in conflict with thistheory:

1) Some users, most notably Janet (user name Aylad), reportthat in their non-SWG pools using high levels of CYA showsdramatic improvement in chlorine's staying power. In Janet'scase, with a CYA below 60 she found that the FC would go from7-8 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 CYAto higher levels, especially approaching 70-80 that somemanufacturers recommend, has a dramatic increase in FC levelsat the same SWG output. Though one theory is that the CYAmakes the SWG cell more efficient by combining with thegenerated chlorine in the cell "hiding" it from the plates in termsof equilibrium (thus making the generation proceed more quickly),an alternative explanation proposed by some is that the higherCYA levels simply protect the chlorine from destruction fromsunlight 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 abovedata and that c ould be added to the theory to make it predictmore accurately. One possibility is that CYA itself is able toabsorb UV radiation and possibly re-radiate it as non-UVradiation at lower energy, with the rest of the energy becomingkinetic (i.e. heat or temperature increase). This link shows thatindeed CYA does absorb UV at the pH found in pools, though itabsorbs 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

#5

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

16/34

it is not accurate to describe the chlorinated isocyanurateswhich do not "shield" chlorine but are distinctly differentmolecules with different absorption rates and affect disinfectingchlorine levels). The net effect of this new mechanism would beto have higher CYA levels reduce chlorine loss at a greater ratethan shown in the graph I linked to at the top of the post.

So how can we prove that this new mechanism exists (or islikely) 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 orwith an opaque cover or with an indoor pool). To the degreethat CYA increases the SWG cell output to generate higher FClevels, 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 shouldbe a funct ion of the depth of the pool. The presence of higherconcentrations of CYA essentially lower the density of UV

radiation reaching lower depths in a pool. So this protectiveeffect of CYA should show up more in deeper pools where asignificant fract ion of t he water is at greater depths and shouldbe less effective in shallower pools. The chlorinatedisocyanurates, on the other hand, do not have this same effectsince they do in fact degrade (the chlorine attached to themdegrades to chloride ion) and are in fact less likely to interactwith sunlight in this way so light is more likely to continue tolower 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 differentdepths at varying CYA levels. The half-life of the chlorinatedisocyanurate would be the dominant factor in the shallowestbasin of water while CYA's "shielding" effect would be a greaterfactor in the deepest basin of water.

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

I/Io = e -l

where (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 tois in centimeters.

There is also a formula for absorbance defined as follows:

A = -log10(I/I o)

and there is a molar extinction coefficient defined by thefollowing equation:

A = *c*l

where (epsilon) is the molar extinction coefficient, c is the

8/10/2019 Pool Water Chemistry

17/34

8/10/2019 Pool Water Chemistry

18/34

7.2).

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

CYA (ppm) ... I/I o (10) .. I/I o (20) .. I/I o (50)0 ................. 1.00 ........ 1.00 ........ 1.0010 ................ 0.85 ........ 0.72 ........ 0.44

20 ................ 0.72 ........ 0.52 ........ 0.2030 ................ 0.61 ........ 0.38 ........ 0.0940 ................ 0.52 ........ 0.27 ........ 0.03950 ................ 0.44 ........ 0.20 ........ 0.01760 ................ 0.38 ........ 0.14 ........ 0.007770 ................ 0.32 ........ 0.10 ........ 0.003480 ................ 0.27 ........ 0.074 ........ 0.001590 ................ 0.23 ........ 0.054 ........ 0.00067100 .............. 0.20 ........ 0.039 ........ 0.00030

So to see the dramatic change seen from higher CYA levels, theCYA shielding effect has to be strong enough to be the

predominant effect. The shielding effect would "shield" not onlyunbound chlorine, but also chlorine bound to CYA. Note thatusing an extinction coefficient of 20 in the above table one findsthe difference between 50 and 90 ppm CYA being a factor of 3.7which 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 linkindicates that the chlorinated isocyanurates are unstable insunlight, but it is unclear how much of that is due to breakdownfrom the equilibrium hypochlorous acid vs. direct breakdownitself. The study just shows that CYA is itself stable in sunlight.If the CYA absorption effect is really this strong, then deeperpools should be more protected at the same CYA level sincemore 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" orabsorption is the main effect, then there should be littleprotection of chlorine in shallow water. If instead the chlorinecombined with CYA has a longer half-life and that is the maineffect, then higher CYA levels even in shallow depths shouldshow significant protection and should roughly follow the curvein this graph . I suspect that there will be a some of bothprocesses going on.The original CYA patent by Fuchs may be seen at this link. Therewere interesting laboratory tests that appear to have been madeat shallow depths and only show a small amount of the "depth"variation one sees with higher chlorine levels. The UV lamp theyused appeared to have 1 ppm FC drop to 0.5 ppm FC in 1.7hours so was not as strong as sunlight. The rate of chlorine lossseemed to track the amount of unbound chlorine, but withdiminishing 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 CYAand an actual increased loss of 0.088 per hour at 100 ppm CYA.This is somewhat consistent with the original theory of a 35

8/10/2019 Pool Water Chemistry

19/34

minute half-life in direct sunlight with no CYA and an 8.4 hourhalf-hour limit when bound with CYA. This is probably where theindustry got its original data for its tables. Note that CYA alsohas a protective effect on chlorine loss from oxidation of ironand copper. Though the patent speculates CYA may coatmetals, it appears that the effect is explained by the reduct ionin disinfecting chlorine and therefore the rate of corrosion basedon its concentration. It should be noted that in the patent "realpools" showed the greater protection effect of higher CYA levelsby about a factor of 2 at 10 ppm CYA and over a factor of 3 at50 ppm CYA. Thus there does appear to be a "shielding" depth

factor for CYA protection separate from that explained solely byCl and Cl-CYA breakdown. The fact that the chlorine levels werethe same and only the CYA level increased, yet had a greatereffect in a real pool with "depth" is very strong evidence.

The good news with this new information is that at sufficientlyhigh CYA levels using a higher FC (to compensate for disinfectionand prevention of algae) should not result in larger losses. Goingfrom 30 ppm to 90 ppm requires about triple the FC level, butthe loss rate may be cut down by a factor of 7 for a net overallsavings of over a factor of 2. If we can validate this, then itshould 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 I oe-l dl) / D = (I o /(D))*(1 - e -D)which with small expands toI = I o*(1 - D/2 + (D) 2 /6 - ...)so that as approaches 0, "I" approaches "I o" as expected.

In many real pools, there is less volume below around 3 feet asthe pool bottom drops only in the deep end such that the poolcan be seen as the sum of 3 pools, one with a depth equal tothe shallow end, one with a depth equal to the deep end, andone with a depth that varies from the shallow depth to the deepdepth. If D s is the depth of the shallow end and D d is the depthof the deep end, then the overall average intensity (that can beused to calculate an average breakdown rate or whose inversecan calculate an average half-life) is the following assumingeach section is one-third of the pool area:

I = ( (I o /Ds)*(1 - e -Ds) + (I o /(*(Dd-Ds)))*(Dd + e -Dd / - Ds- e -Ds /) + (I o /Dd)*(1 - e -Dd) ) / 3

[EDIT] See this post for an experiment Mark did that prettymuch conclusively proves that the improved salt cell efficiencyat higher CYA does not come from any internal chemistry in thesalt cell (since there was no such change seen at night) butrather is from reduced chlorine loss from sunlight that more thanmakes up for the higher FC needed at higher CYA to maintaindisinfection and prevention of algae. [END-EDIT]

Richard

8/10/2019 Pool Water Chemistry

20/34

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

03-17-2008, 09:59 PM

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 tocorrect these parameterwe are being reactive.

When we try to predict what the pool parameters are then thenadd our agents we are beingpredictive.

If we try to automate the pool correction parameters with thevarious meters(or probes) we continouslytry to correct to these parameters. How succesful we are isdetermined by the accuracy of the continuousmeasurements. As we have found ORP measurements aredifficult to maintain with godd accuracy, pH measurementshave proven somewhat more accurate.

Lets take another look from a different angle on pool parametercorrections. We know that the biggest user of FC is the Sun and its UV rays. So Chlorine use is mostly dictatedby 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 cannotpredict the bather load, but there is another way around this Iwill mention later. Another parameter of Chlorine use isthe water temperature. The water temperature in a non-heatedpool will be a direct relation to the amount of sunthat 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 runninggives a set amount of Chlorine generation. I am only consideringrunning the SWG at 100% ouput. This is the most efficient forthe 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 Chlorinedemands are directly related to the amount of sunshineand water temperature, so if we time the operation of the SWGto coordinate with the sunrise and sunset we can closelymatch the Chlorine needs. I have found that actually as thewater temperature decreases the SWG will have to be run ashortertime because the Chlorine demands as less than just thefunction of the Sunrise and Sunset. I have confirmed this overthe

last 2 years. The next step is to add the water temperature intothe pump time run calculations. As a practical problem I have to

#6

Join Date:Posts:

Mar 200792

cliff_s Member

8/10/2019 Pool Water Chemistry

21/34

finda way to run a wire to get my pool water temperature.

Since a pool is a large volume of water the parameters changerather slowly and it not necessary to try to correct themminute by minute. The system I am testing is, to use apredictive set of parameters to control Chlorineaddition(generation).

I have installed a CO2 system for pH control using the same typeparameters, because the pH control is needed as a function

of SWG run time. In the summer I have to make a correction forpool water addition.

I find that these predictions are quite accurate over time. Imight add it does take some kind of computer controlled systemto do all these calculations. I do it with my home automation aElk M1G.

In summation, if by varying the SWG(pump) run time and thetime the CO2 is added I can control the pool water parametersthat 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 waterparameters. The difficulty is that the probes are reallyment for laboratory usage and are not dependable enough forunattended use. Their calibration is hard to maintain andrequire periodic replacement.

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

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

Cliff s

Reply With Quote

03-18-2008, 11:08 AM

Ack!! I just accidentally wandered into the 'deep end'!!

just ignore me as I make my way to the more easily navigablewaters...

#7

Join Date:Location:Posts:

Mar 2007Northern KY 2,532

The Mermaid Queen

Senior Member

18x32 grecian IG vinyl; ~23000 gallons; 250# sand filter, Haywardsuperpump with AO Smith motor 1HP SF1.0; booster pump for

8/10/2019 Pool Water Chemistry

22/34

polaris. Handy Links: Jason's Calculator , TF-100 Test Kit , PoolSchool , CYA-Chlorine Chart"Shock" is a process, not a product!

Reply With Quote

03-18-2008, 11:49 AM

#8

Join Date:Location:Posts:

Apr 2007Raleigh, NC21,392

duraleigh

Senior MemberIn the Industry

Ack!! I just accidentally wandered into the 'deep end'!!

just ignore me as I make my way to the more easily

navigable waters...

Dave S.Site Owner 42k vinyl and concrete pool, 1.5hp pump, 140gpm filter, NoSWGTFTestkits ownerTFTestkits , PoolMath , Pool School

Reply With Quote

03-18-2008, 01:45 PM

Cliff,

I understand your points, but I don't think we're really in thatmuch disagreement. Basically, though we start off by tellingpeople to measure and adjust water parameters, that's mostlyso people can get a "sense" of their pools and what they need.Then, over time, people don't have to measure as often (at leastfor some parameters) if they "know" their pools better. That is,one starts off with measurements and adjustments to keepthings in line and then transitions more towards what you aredescribing as practical ways of essentially maintaining the waterchemistry the same as would be done with measurements. In myown pool, I don't measure chlorine every day, but rather twice aweek before I normally add chlorine since I know its usage rateand 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 thesecond stage as you have without first doing the first stage of measurements. The reason is that the factors in every pool arecomplex and not easily calculated. Yes, one knows that if sunhits the pool then there will be loss due to sunlight, but theexact 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 sweatdifferent amounts and this depends a lot on water temperatureand level of activity. Chlorine demand from other organicsdepends on how much junk (leaves, pollen, etc.) gets into thepool and that can change over time depending on what getscaught 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

#9

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

23/34

found a timing approach that works well for you. I'm not so surethat the 100% SWG ontime only when the pump runs isnecessarily more efficient. My understanding of SWG percentageis 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 thinkthe SWG cell necessarily lasts longer trying to have it run 100%of the time that the pump is running. The pump run time shouldbe based on circulation needs while the SWG percentage time(and when this occurs) should be based on chlorine demand andthose are separate factors that may not coincide. I do agreewith your "when to run the SWG" analysis such that it runs more

frequently during expected higher demand.Richard

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

03-18-2008, 10:44 PM

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

#10

Guest

Reply With Quote

03-19-2008, 10:25 AM

I accidently wandered in here myself and I think I got a Briananeurysm.

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

#11

Join Date:Location:Posts:

May 2007Miami266

JCJR

Senior Member

20x40 24,000 gal IG plaster/CircuPool RJ45 SWG/2 skimmer/2sp 2.5hpJandy Stealth/340sq ft cart filter/600sqftHeliocol Solar Panels/6ft semi circlrtanning ledge/25ft deep end bench/5ft sheer descent/2 lion head waterfeatures/1300sqft travertine paver/2 Colorlogic lights/HaywardNavigator/3step 2ft raised bond beam

Reply With Quote

03-19-2008, 11:04 AM #12

chem geek Originally Posted by waterbear

8/10/2019 Pool Water Chemistry

24/34

Evan,

Just FYI -- this is not a new thread. Only the post from cliff_swas new and triggered this thread to show up in "View PostsSince Last Visit". The information on pool water chemistry ispretty old, much of it copied from the Pool Forum China Shopwhere I originally posted it here .

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

Richard

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

Senior Memberstill digesting all this. quite a bit here and some interestingimplications but no big surprises. We've always been prettymuch on the same page, Richard!

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

03-19-2008, 12:05 PM

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!

#13

Guest Originally Posted by chem geek

Just FYI -- this is not a new thread.Richard

Reply With Quote

03-19-2008, 12:21 PM

oooohhhh.... colors! I especially like the green!! 8)

(sorry to have hijacked this thread... just couldn't help myself!!!)

#14

The Mermaid Queen

Senior Member Originally Posted by chem geek

JCJR and Grace, aren't the graphs pretty? I thought mychoice of color scheme was particularly well planned.

8/10/2019 Pool Water Chemistry

25/34

Join Date:Location:Posts:

Mar 2007Northern KY 2,532

18x32 grecian IG vinyl; ~23000 gallons; 250# sand filter, Haywardsuperpump with AO Smith motor 1HP SF1.0; booster pump forpolaris. Handy Links: Jason's Calculator , TF-100 Test Kit , PoolSchool , CYA-Chlorine Chart

"Shock" is a process, not a product!

Reply With Quote

03-19-2008, 12:43 PM

I am sure glad that others are able to understand this stuff anddebate/add to the findings because it benefits us all.

#15

Join Date:Location:Posts:

May 2007Miami266

JCJR

Senior Member

20x40 24,000 gal IG plaster/CircuPool RJ45 SWG/2 skimmer/2sp 2.5hpJandy Stealth/340sq ft cart filter/600sqftHeliocol Solar Panels/6ft semi circlrtanning ledge/25ft deep end bench/5ft sheer descent/2 lion head waterfeatures/1300sqft travertine paver/2 Colorlogic lights/HaywardNavigator/3step 2ft raised bond beam

Reply With Quote

06-01-2008, 05:12 PM

I am adding a post that explains the chlorine/CYA relationship inchemical terms at varying levels of detail as some people ask fora more detailed explanation or derivation and the original 1974O'Brien paper is hard to find [EDIT] (you can see a copy of thepaper here ). [END-EDIT]

QUALITATIVE DESCRIPTION

Though Cyanuric Acid (CYA) absorbs ultraviolet (UV) radiationdirectly thus shielding the lower depths of water and protectingchlorine in those depths from breakdown, the primary result of having CYA in the water with chlorine (hypochlorous acid) is thatit combines with chlorine to form a set of chemical speciescollectively called chlorinated isocyanurates (and thesecompounds also absorb UV without breaking down significantly).The full chemistry is complicated (well, tedious) because thereare 6 different species of chlorinated isocyanurates (that is,chlorine attached to CYA) and 4 different species of CyanuricAcid and its dissociated ions. There are 13 simultaneouschemical equilibrium equations of the CYA, chorinatedisocyanurates, hypochlorous acid and their combinations thoughonly 10 of these are independent from each other.

#16

Join Date:Location:

Posts:

Mar 2007San Rafael, CA USA

8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

26/34

Look at the chemical structure for CYA here and that of Trichlorhere and of Dichlor here and notice that essentially the Nitrogencan have either hydrogen or chlorine attached to it and thatthere are three such sites. Qualitatively, chlorine combines withCYA to form new chemicals that are essentially not disinfect antsnor oxidizers (at least not even close to hypochlorous acid; morelike hypochlorite ion at best). CYA has a moderately strongaffinity for chlorine such that when CYA >> FC (when both aremeasured in their respective ppm), then most of the chlorine isattached to CYA. For example, when the pH is 7.5 and the FC is3.5 ppm and the CYA is 30 ppm, then 97% of the chlorine is

attached to CYA. Nevertheless, the chlorine attached to CYAgets measured in the FC test because the chlorine gets releasedfrom the CYA quickly enough to replenish the chlorine that isconsumed by the test (by reacting with dye). [EDIT] FreeChlorine (FC) does not measure active chlorine, but ratherthe chlorine reserve or reservoir that is mostly inactive.[END-EDIT]

In a very real sense, CYA acts as a hypochlorous acid bufferholding chlorine in reserve, but significantly lowers itsconcentration which determines the rate of any reaction inwhich chlorine participates. You can see from the structure of

Hypochlorous Acidhere that it looks similar to water with achlorine atom substituting for a hydrogen atom. When chlorinecombines with CYA, this is a chlorine substitution for a hydrogenatom or essentially an exchange of the chlorine atom to the CYAand the hydrogen atom from the CYA to make water. Whenchlorine is released from CYA, then the opposite exchangeoccurs.

SIMPLIFIED CHEMICAL EQUATIONS

To simplify the description, I will only talk about the mostdominant chemical species found at the pH of pool water. For

Cyanuric Acid (which I designate as H 3CY), the species athighest concentration is the one that has dissociated onehydrogen ion which I will designate as H 2CY-. For the chlorinatedisocyanurate species, it is CYA with one hydrogen, one chlorine,and one open slot so is negatively charged which I will designateas HClCY-. The following is the primary relevant chemicalequation to focus on:

HClCY- + H2O H 2CY- + HOCl"Chlorine bound to CYA" + Water "CYA ion" + HypochlorousAcid

Hypochlorous Acid is the strongly disinfecting and oxidizing formof chlorine so is all I will talk about (as opposed to hypochloriteion). The chlorinated isocyanurates show little if any disinfectingcapability and minimal oxidation power. The above equation isdescribed by a chemical equilibrium constant as shown by thefollowing:

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

At 3.5 ppm Free Chlorine (FC), this is equivalent to 4.9x10 -5moles/liter concentration while 30 ppm CYA is 2.3x10 -4concentration. Since the CYA concentration is much higher thanthe FC concentration, even if all the chlorine could attach to

8/10/2019 Pool Water Chemistry

27/34

CYA via the above equation, the net effect is that the totalamount of "chlorine bound to CYA" can't be more than theamount of FC and the H 2CY- does not drop very much.Rearranging, we have:

[HOCl] = 2.4x10 -6 * [HClCY-] / [H 2CY-]

Hypochlorous acid (HOCl) is also in equilibrium with hypochloriteion (OCl-) where at a pH of 7.5 this is roughly split 50/50between these two species. So we can rewrite the above interms of measured concentrations as follows where CYA and FCare 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, theHClCY- can be initially ignored in the above. The above equationimplies that the HOCl concentration must be very small and thatmost of the chlorine is bound to CYA. The following is anapproximation we can test:

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

The chlorine values of HOCl and FC can be measured in the sameunits (as they are on both sides of the equation so any factorscancel), but we can convert the [CYA] concentration into ppmby multiplying the right hand side (numerator) by the molecular

weight of CYA, 129.075 g/mole, and 1000 mg/g (multiplying thedenominator 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 accuratecalculation. At an FC of 3.5 ppm and a CYA of 30 ppm, theactual HOCl is 0.051 ppm while the above approximation gives0.035 ppm. You can see where the FC/CYA ratio comes from-- it is a direct result of the chemical equilibrium betweenchlorine attached to CYA vs. separate chlorine and CYA. Amore accurate approximation is given by modificat ion of the

formula not removing the [FC] term in the denominator (whichresults in a factor that is the ratio of CYA and Cl 2 molecularweights):

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

which with the FC of 3.5 ppm and CYA of 30 ppm results in0.046 which is within 10% of the correct result. However, theabove approximation falls apart rather quickly when the CYA/FCratio is less than 5 and it is still pH dependent (the assumptionswere at a pH of 7.5 for the dominant species which determinesthe equilibrium constant).

COMPLEX CHEMICAL EQUATIONS

8/10/2019 Pool Water Chemistry

28/34

So how can one conclude what the dominant species are sincethat is the assumption I started with above? Let's look at thedetailed equations and go through a process of elimination basedon the pH. We'll start with the easier case to analyze, namelyCYA and its dissociated species. Some of the following equationsuse an adjusted equilibrium constant for the ionic strength intypical pool water at 300 ppm CH, 100 ppm TA, 30 ppm CYA and525 ppm TDS. All of the equilibrium constants come from theoriginal 1974 O'Brien paper I refer to in the first post in thisthread, but you can also see these constants (with some minor

errors due to using slightly different sources) in this link ondocument page 12 ( PDF page 18 ).

H3CY H2CY- + H+ ..... pK = -log 10(K) = 6.83H2CY- HCY2- + H+ ..... pK = 11.26HCY2- CY 3- + H+ ..... pK = 13.32

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

[H+] * [H2CY-] / [H 3CY] = 10-6.83

Taking the negative log 10 of both sides gives:pH - log10([H2CY-] / [H 3CY]) = pKlog10([H2CY-] / [H 3CY]) = pH - pK

So from the above, and generalizing, one can see that when pH< pK then the ratio in the log 10 is less than 1 while when pH >pK the ratio in the log 10 is greater than 1. So this means that ata pH of 7.5, the following are true:

[CY3-] > H 2ClCYClCY2- > HCl2CY

So of the above species, HClCY - and Cl2CY- are dominant, butwe cannot yet tell which is more dominant between these two.There are additional chemical equations relating to the

8/10/2019 Pool Water Chemistry

29/34

interaction of chlorine with the chlorinated isocyanurates asfollows:

Cl2CY- + H2O HClCY- + HOCl ..... pK = 4.51HCl2CY + H2O H 2ClCY + HOCl ..... pK = 2.93Cl3CY + H2O HCl2CY + HOCl ..... pK = 1.80

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

HClCY- > Cl2CY-H2ClCY >> HCl2CYHCl2CY >> Cl3CY

So the assumption that HClCY - is the dominant chlorinatedisocyanruate species is reasonable and the next most dominantchlorinated isocyanurate species is Cl 2CY-.

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

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

Richard

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

06-06-2008, 03:26 AM

chem geek,

It sounds like you have the 1974 O'Brien paper. Are you able topost it to the forum or provide a link to the paper?

Thanks!

Titanium

#17

Join Date:Location:Posts:

Jun 2007SF Bay Area442

Titanium

Senior Member

the original 1974 O'Brien paper is hard to find.

8/10/2019 Pool Water Chemistry

30/34

24,000 gallon inground freeform pool/spa circa 1983 (113 ft perimeter, 625sq ft) with 350 gallon attached spill-over spa2007 2 HP, three-phase Hayward TriS tar pump which is powered by anIkeric VS-200 variable speed drive system1983 Laars XE Pool/Spa Heater Type ES 400,000 BTU, 1998 HaywardSuper Star-Clear C-4000 cartridge filter (400 sq ft, 4 separate cartridges)1998 Polaris 380 pressure-side cleaner w/ 3/4 HP booster pumpOne skimmer :( and one PoolSkim :), One Supervision Galaxy LED poollamp, Second story solar panelsHayward/GoldLine AquaLogic PS4 (replaced 1983 vintage dual circuitIntermatic timer)

Reply With Quote

06-06-2008, 12:33 PM

Titanium,

It's copyrighted material so I can't post it (at least not in itsentirety). I have a copy of the out-of-print book that it is in:Chemistry of Water Supply, Treatment and Distribution that Irefer to in my first post in this thread. I pretty much bought upthe world's supply of that book sending it to the most importantpeople in the world who should have that book -- the people onthe committee defining the APSP-11 standards as well as theCDC and others who should be aware of the chlorine/CYArelationship and that this isn't new. The only copy I can still seeavailable for purchase is here and it's much more expensivebecause, well, it's now more rare.

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

______________________________________________________ ______ ______________________________________________________ ______ _______________

Moreover, chlorinated cyanurates, in addition to acting asstabilizers, exhibit a limited degree of hydrolysis to yield arelatively constant level of germicidally potent, free chlorine. Inother words, chlorinated cyanurates may be considered asanalagous to a protected reservoir which liberates a small butrelatively constant level of free chlorine in accordance withclearly defined principles of chemical equilibrium.:Ordinarily, most of the reservoir chlorine consists of chlorinatedcyanurates. Although these provide a readily available source of active chlorine, Andersen 1 has submitted evidence thatchlorinated cyanurates, as such, are not particularly germicidal.As a result, the germicidal activity must be borne by therelatively small fraction of free chlorine present at any giventime. Since, in general, increase in cyanurate concentrationresults in decreased free chlorine, the use of large cyanurateconcentrations to achieve maximum stability will tend to giveinadequate germicidal activity. For the same reason thecontinual addition of chlorinated cyanurates as a source of

#18

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

chem geek

Senior Member

8/10/2019 Pool Water Chemistry

31/34

chlorine is not recommended since this will lead to build up of cyanurate concentration and consequent repression of theconcentration of free chlorine below that necessary for effectivegermicidal activity.:Although chlorinated cyanurates serve as a reservoir of freechorine, bactericidal efficacy is more closely related to therelatively small fraction of free chlorine present at equilibrium.Therefore, the use of excessive cyanurate in an overly zealousattempt to reduce photolysis may repress free chlorine to thepoint 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.:REFERENCES1. Anderson, J.B. "The Influence of Cyanuric Acid on theBactericidal Effectiveness of Chlorine," Ph.D. Thesis, Universityof 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 ourmodern terminology based on what is measured in the FreeChlorine (FC) test, FC is actually reservoir chlorine. So when thepaper says that higher CYA levels lower Free Chlorine, they arereferring to hypochlorous acid and hypochlorite ion, not to whatwe 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 carefulof such studies ( this PDF file) shows that the germicidal effec tof chlorine (against a species of protozoan cyst) is based on thehypochlorous acid concentration. Other studies show the effectsagainst bacteria, viruses, algae and oxidative power (againstamino acids).

Richard

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

06-06-2008, 04:07 PM

Richard,

Thanks for your excellent reply.

#19

Titanium

Senior Member

8/10/2019 Pool Water Chemistry

32/34

8/10/2019 Pool Water Chemistry

33/34

Forum Swimming Pool Care The Deep End... Pool Water Chemistry

-- Default Style

Previous Thread | Next Thread

Contact Us Archive Web Hosting Top

so you could certainly help me out.

As for the APSP-11 developing standard, I gave rather extensivecomments on it. I was very encouraged that it contained someinformation on the chlorine/CYA relationship though not to thelevel of detail I would have preferred so I sent some graphs and"rules of thumb" info in the comments. I have no idea what willcome of it since there is no feedback process back to those whomake comments. I proposed using an "FC as % of CYA" with aminimum FC as a standard and said that this wasn't thatdifferent than the LSI that is already a multi-parameter index

and is referred to in the standard. I also made the radicalproposal of allowing (at least not recommending against) a smallamount of CYA for indoor pools.

As for the CDC, they are very busy and generally underfundedand besides, they don't set pool standards. They are mostly justfocussed on disease prevention. Nevertheless, I have written tothem about possible ways of handling Crypto, but I'm probably

just a PITA to them -- I don't have any relevant credentials anddon'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.

Thanks,Richard

Join Date:Location:Posts:

Mar 2007San Rafael, CA USA 8,657

16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VFpump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square footcartridge filter12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater(200,000 BTU/hr output); automatic electric pool safety cover; 4-wheelpressure-side "The Pool Cleaner"

Reply With Quote

Page 1 of 4 1 2 3 4 Last

Quick Navigation The Deep End... Top

You may not post newthreads

You may not post replies You may not postattachments

You may not edit your posts

Posting Permissions

BB code is OnSmilies are On[IMG] code is On[VIDEO] code is Off HTML code is Off

Forum Rules

8/10/2019 Pool Water Chemistry

34/34

All times are GMT -5. The time now is 11:00 AM.

Powered by vBulletin Version 4.2.2Copyright 2014 vBulletin Solutions, Inc. All rights reserved.