the chlorine / cyanuric acid relationship and implications

Chlorine/CYAandNitrogenTrichloride 1

TheChlorine/CyanuricAcidRelationshipandImplicationsforNitrogenTrichloride

byRichardA.Falk

AbstractTheamountofhypochlorousacid(HOCl)inwaterwithCyanuricAcid(CYA)attypicalpoolpHisproportionaltotheFC/CYAratioandisordersofmagnitudelowerthantheFClevelitself.Theprimaryoxidizingandsanitizingcompoundishypochlorousacidwhilehypochloriteionandthechlorinatedisocyanuratecompounds(chlorineattachedtoCYA)haveordersofmagnitudeloweroxidizingorsanitizingcapability.TherateofproductionandtotalamountofnitrogentrichlorideproducedfromtheoxidationofammoniabychlorineisrelatedtothehypochlorousacidconcentrationandnottotheFCleveldirectly.Thoughtheprecisemechanismofoxidationofureabychlorineisunknown,proposedmechanismsleadtothesameconclusionofnitrogentrichloridequantitiesbeingproportionaltothehypochlorousacidlevel,allelseequal.Therefore,theuseofasmallamountofCYAinindoorpoolsshouldsignificantlylowertheamountofnitrogentrichlorideproducedinsuchpoolsandresultinlowerratesofoutgassingofchlorine,corrosionofmetaldirectlyexposedtowaterandoxidationofskin,swimsuitsandhair.

TheHypochlorousAcid/CyanuricAcidEquilibriumCyanuricAcid(CYA,akastabilizerorconditioner)isusedinpoolstoprotectchlorinebreakdownfromsunlight.ThoughCYAabsorbsultraviolet(UV)radiationdirectlythusshieldingthelowerdepthsofwaterandprotectingchlorineinthosedepthsfrombreakdown,theprimaryresultofhavingCYAinthewaterwithchlorine(hypochlorousacid)isthatitcombineswithchlorinetoformasetofchemicalspeciescollectivelycalledchlorinatedisocyanurates.ThesecompoundsalsoabsorbUVwithoutbreakingdownasquicklyaschlorine.Thefullchemistryiscomplicated(well,tedious)becausethereare6differentspeciesofchlorinatedisocyanurates(thatis,chlorineattachedtoCYA)and4differentspeciesofCyanuricAcidanditsdissociatedions.Thereare13simultaneouschemicalequilibriumequationsoftheCYA,chlorinatedisocyanurates,hypochlorousacidandtheircombinationsthoughonly10oftheseareindependentfromeachother.LookatthechemicalstructureforCYAhere(diagramsfromWikipedia)


andthatofTrichlorandDichlorhere

Trichlor Dichlor andnoticethatessentiallytheNitrogencanhaveeitherhydrogenorchlorineattachedtoitandthattherearethreesuchsites.Qualitatively,chlorinecombineswithCYAtoformnewchemicalsthatareessentiallynotdisinfectantsnoroxidizers(tobedemonstratedlaterinthispaper).CYAhasamoderatelystrongaffinityforchlorinesuchthatwhenCYA>>FC(whenbotharemeasuredintheirrespectiveppm),thenmostofthechlorineisattachedtoCYA.Forexample,whenthepHis7.5andtheFCis3.5ppmandtheCYAis30ppm,then97%ofthechlorineisattachedtoCYA.Nevertheless,thechlorineattachedtoCYAgetsmeasuredintheFCtestbecausethechlorinegetsreleasedfromtheCYAquicklyenoughtoreplenishthechlorinethatisconsumedbythetest(byreactingwithdye).Inaveryrealsense,CYAactsasahypochlorousacidbufferholdingchlorineinreserve,butsignificantlylowersitsconcentrationwhichdeterminestherateofanyreactioninwhichchlorineparticipates.YoucanseefromthestructureofHypochlorousAcid(ontheleft)thatitlookssimilartowater(ontheright)withachlorineatomsubstitutingforahydrogenatom.


HypochlorousAcid Water WhenchlorinecombineswithCYA,thisisachlorinesubstitutionforahydrogenatomoressentiallyanexchangeofthechlorineatomtotheCYAandthehydrogenatomfromtheCYAtomakewater.WhenchlorineisreleasedfromCYA,thentheoppositeexchangeoccurs.Thedefinitivescientificpaperthatdeterminedtheequilibriumconstantsbetweenhypochlorousacid,cyanuricacid,andthechlorinatedisocyanurateswaspresentedataconferencein1973andpublishedin1974.Thereferenceisthefollowing:J. O'Brien, J. Morris and J. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate”, 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. Thisbookisout‐of‐print,butIreceivedpermissiontoreprint/postitandyoucanretrieveaPDFofthispaperatthefollowingwebpageandIstronglyencourageyoutoreadatleasttheINTRODUCTIONandSUMMARYsectionsofthisdocument(notethattheterm“FreeChlorine”isnotasitisusedtodayandthattheFCtestreallymeasures“reservoirchlorine”asdescribedinthepaper:http://richardfalk.home.comcast.net/~richardfalk/pool/OBrien.htmIhavecreatedaspreadsheetthatcalculatesthevariouschemicalspeciesincludinghypochlorousacidconcentrationgivenstandardwaterchemistryparameters.Thisspreadsheetmaybefoundatthefollowinglink:http://richardfalk.home.comcast.net/~richardfalk/pool/PoolEquations.xlsOnecan,ofcourse,useanyofanumberofstandardchemicalequilibriumprogramstocomputetheconcentrationsofchemicalspeciesusingtheequilibriumconstantsfromO’Brienandconcentrationsconvertedfromstandardppmunits.SuchprogramsincludeCHEMEQL,EQS4WIN,MINEQL+,amongothers.


TheFC/CYARatioasaProxyforEquivalentFCwithnoCYA

SimplifiedChemicalEquationsTosimplifythedescription,IwillonlywriteaboutthemostdominantchemicalspeciesfoundatthepHofpoolwater.ForCyanuricAcid(whichIdesignateasH3CY),thespeciesathighestconcentrationistheonethathasdissociatedonehydrogenionwhichIwilldesignateasH2CY‐.Forthechlorinatedisocyanuratespecies,itisCYAwithonehydrogen,onechlorine,andoneopenslotsoisnegativelychargedwhichIwilldesignateasHClCY‐.Thefollowingistheprimaryrelevantchemicalequationtofocuson:HClCY‐+H2OH2CY‐+HOCl"ChlorineboundtoCYA"+Water"CYAion"+HypochlorousAcidHypochlorousAcidisthestronglydisinfectingandoxidizingformofchlorinesoisallIwillwriteabout(asopposedtohypochloriteion).Thechlorinatedisocyanuratesshowlittleifanydisinfectingcapabilityandminimaloxidationpower.Theaboveequationisdescribedbyachemicalequilibriumconstantasshownbythefollowing:[H2CY‐]*[HOCl]/[HClCY‐]=10‐5.62=2.4x10‐6At3.5ppmFreeChlorine(FC),thisisequivalentto4.9x10‐5moles/literconcentrationwhile30ppmCYAis2.3x10‐4concentration.SincetheCYAconcentrationismuchhigherthantheFCconcentration,evenifallthechlorinecouldattachtoCYAviatheaboveequation,theneteffectisthatthetotalamountof"chlorineboundtoCYA"can'tbemorethantheamountofFCandtheH2CY‐doesnotdropverymuch.Rearranging,wehave:[HOCl]=2.4x10‐6*[HClCY‐]/[H2CY‐]Hypochlorousacid(HOCl)isalsoinequilibriumwithhypochloriteion(OCl‐)whereatapHof7.5thisisroughlysplit50/50betweenthesetwospecies.SowecanrewritetheaboveintermsofmeasuredconcentrationsasfollowswhereCYAandFCaretotalconcentrations:FC=[HOCl]+[OCl‐]+[HClCY‐]CYA=[H2CY‐]+[HClCY‐][HOCl]=2.4x10‐6*([FC]‐[HOCl]‐[OCl‐])/([CYA]‐[HClCY‐])andatapHnear7.5,[HOCl]=2.4x10‐6*([FC]‐2*[HOCl])/([CYA]‐[FC]+2*[HOCl])Forpracticalpurposes,becauseCYAismuchlargerthanFC,theHClCY‐canbeinitiallyignoredintheabove.TheaboveequationimpliesthattheHOClconcentrationmustbeverysmallandthatmostofthechlorineisboundtoCYA.Thefollowingisanapproximationwecantest:[HOCl]isapproximately2.4x10‐6*[FC]/[CYA]


Thechlorinevaluesof[HOCl]and[FC]canbemeasuredinthesameunits(astheyareonbothsidesoftheequationsoanyfactorscancel),butwecanconvertthe[CYA]concentrationintoppmbymultiplyingtherighthandside(numerator)bythemolecularweightofCYA,129.075g/mole,and1000mg/g(multiplyingthedenominatorbythisnumberconvertsCYAintoppm)resultingin:HOClisapproximately0.3*FC/CYATheaboveapproximationisn'tterriblyfarofffromtheaccuratecalculation.AtanFCof3.5ppmandaCYAof30ppm,theactualHOClis0.051ppmwhiletheaboveapproximationgives0.035ppm.YoucanseewheretheFC/CYAratiocomesfrom‐‐itisadirectresultofthechemicalequilibriumbetweenchlorineattachedtoCYAvs.separatechlorineandCYA.Amoreaccurateapproximationisgivenbymodificationoftheformulanotremovingthe[FC]terminthedenominator(whichresultsinafactorthatistheratioofCYAandCl2molecularweights):HOClisapproximately0.31*FC/(CYA‐(1.8*FC))or0.31*(FC/CYA)/(1–(1.8*(FC/CYA)))whichwiththeFCof3.5ppmandCYAof30ppmresultsin0.046whichiswithin10%ofthecorrectresult.However,theaboveapproximationfallsapartratherquicklywhentheCYA/FCratioislessthan5anditisstillpHdependent(theassumptionswereatapHof7.5forthedominantspecieswhichdeterminestheequilibriumconstant).AtapHof7.5,thereisroughlyanequalamountofhypochlorousacidandhypochloriteionsothe3.5/30ratioof0.117isclosetotheFCthatproducesthesameamountofhypochlorousacidiftherewasnoCYA,namely0.106.ThefollowingtableshowshowtheFC/CYAratiocanbeusedasareasonableproxyforFCwithnoCYAwhenthepHisnear7.5. FCas%ofCYA

FC/CYA EquivalentFCwithnoCYA(basedon30ppmCYA)

EquivalentFCwithnoCYA(basedon50ppmCYA)


2.06*0.31*(FC/CYA)/(1–(1.8*(FC/CYA)))

1% 0.01 0.0075 0.0075 0.0076 0.00652% 0.02 0.0152 0.0153 0.0154 0.0135% 0.05 0.0396 0.0400 0.0404 0.03410% 0.10 0.0854 0.0866 0.0874 0.07820% 0.20 0.202 0.206 0.208 0.2030% 0.30 0.36 0.37 0.38 0.4240% 0.40 0.60 0.62 0.64 0.9150% 0.50 0.92 0.98 1.02 3.260% 0.60 1.39 1.50 1.61 N/A


FCas%ofCYA

FC/CYA EquivalentFCwithnoCYA(basedon30ppmCYA)



2.06*0.31*(FC/CYA)/(1–(1.8*(FC/CYA)))

70% 0.70 2.03 2.27 2.50 N/A80% 0.80 2.89 3.35 3.88 N/A85% 0.85 3.41 4.05 4.84 N/A90% 0.90 4.00 4.87 6.02 N/A95% 0.95 4.66 5.81 7.49 N/A100% 1.00 5.38 6.90 9.30 N/AYoucanseefromtheabovethataconstantFC/CYAratioresultsinthesamehypochlorousacidconcentration(orequivalentFCwithnoCYAwhichisaboutdoubleatpH7.5)independentofCYAlevel(from30to100ppm)evenwhentheFCasa%ofCYAisuptoaround50%atwhichpointtheerrorisaround10%ofthehypochlorousacidconcentration(orequivalentFCwithnoCYA).Thisiswhy3ppmFCwith30ppmCYAisthesameas6ppmFCwith60ppmCYAisthesameas10ppmFCwith100ppmCYA.AstheCYAlevelclimbs,theFCmustalsobeincreasedproportionatelyinorderforthehypochlorousacidconcentrationtoremainconstant.TheFC/CYAratioitselfasaproxyfortheequivalentFCisaroughguidewhentheratiois20%orless,butyoucanseethatit’sonlyroughsince1ppmFCat100ppmCYAisnotequivalentto0.0100butratherto0.0076ppm.Nevertheless,itdoesgiveanorder‐of‐magnitudesensefortheeffectofCYA.Themoreaccurateformulashowninthelastcolumn,stillbasedontheFC/CYAratio(the2.06factorconvertsHOCltoequivalentFCatapHof7.5),isreasonablyaccurateuptoanFC/CYAratioof30%.

ComplexChemicalEquationsSohowcanoneconcludewhatthedominantspeciesaresincethatistheassumptionIstartedwithabove?Let'slookatthedetailedequationsandgothroughaprocessofeliminationbasedonthepH.We'llstartwiththeeasiercasetoanalyze,namelyCYAanditsdissociatedspecies.Someofthefollowingequationsuseanadjustedequilibriumconstantfortheionicstrengthintypicalpoolwaterat300ppmCH,100ppmTA,30ppmCYAand525ppmTDS.Alloftheequilibriumconstantscomefromthe1974O'Brienpaper,butyoucanalsoseetheseconstants(withsomeminorerrorsduetousingslightlydifferentsources)inthefollowinglinkhttp://www.epa.gov/hpv/pubs/summaries/tricltrz/c14659rr.pdfondocumentpage12(PDFpage18).H3CYH2CY‐+H+ pK=‐log10(K)=6.83H2CY‐HCY2‐+H+ pK=11.26HCY2‐CY3‐+H+ pK=13.32


Let'stakealookatthefirstreaction'sequilibriumexpression:[H+]*[H2CY‐]/[H3CY]=10‐6.83Takingthenegativelog10ofbothsidesgives:pH‐log10([H2CY‐]/[H3CY])=pKlog10([H2CY‐]/[H3CY])=pH‐pKSofromtheabove,andgeneralizing,onecanseethatwhenpH<pKthentheratiointhelog10islessthan1whilewhenpH>pKtheratiointhelog10isgreaterthan1.SothismeansthatatapHof7.5,thefollowingaretrue:[CY3‐]<<[HCY2‐][HCY2‐]<<[H2CY‐][H2CY‐]>[H3CY]SothisiswherewegetourinitialassumptionofH2CY‐beingthedominantcyanuratespecieswherewecanseethatthenextmostdominantcyanuratespeciesisH3CY.Forthechlorinatedisocyanurates,wehavethefollowing(thepKareadjustedforionicstrength):H2ClCYH++HClCY‐ pK=5.28HClCY‐H++ClCY2‐ pK=9.98HCl2CYH++Cl2CY‐ pK=3.70wherewecanconcludethefollowingatapHnear7.5:HClCY‐>>H2ClCYClCY2‐<<HClCY‐Cl2CY‐>>HCl2CYSooftheabovespecies,HClCY‐andCl2CY‐aredominant,butwecannotyettellwhichismoredominantbetweenthesetwo.Thereareadditionalchemicalequationsrelatingtotheinteractionofchlorinewiththechlorinatedisocyanuratesasfollows:Cl2CY‐+H2OHClCY‐+HOCl pK=4.51HCl2CY+H2OH2ClCY+HOCl pK=2.93Cl3CY+H2OHCl2CY+HOCl pK=1.80BecausetheHOClconcentrationisrelativelysmall(pHOCl>4.6;actually>6),thisimpliesthefollowing:HClCY‐>Cl2CY‐


H2ClCY>>HCl2CYHCl2CY>>Cl3CYSotheassumptionthatHClCY‐isthedominantchlorinatedisocyanruatespeciesisreasonableandthenextmostdominantchlorinatedisocyanuratespeciesisCl2CY‐.Inspiteoftheaboveequilibrium,therateofreleaseofchlorinefromCYAisratherfast(seethehalf‐lifeconstantsintheaforementionedEPAlink)soallofthechlorineattachedtoCYAmeasuresasFCintheFCtestbecausetheHOClgetsusedupreactingwiththedyeinthetestandmoreHOClisreleasedfromthatattachedtoCYA(orisconvertedfromhypochloriteion)inthetimeofthetest.Essentially,theFCtestmostlymeasurestheamountofchlorinein“reserve”anddoesnotmeasuretheamountofactivedisinfectingandoxidizingchlorinealone.

GraphsofChlorinevs.pHThetraditionalindustrygraphshowinghypochlorousacidandhypochloriteionasafunctionofpHisincorrectinthepresenceofCyanuricAcid(CYA).ThetraditionalgraphandthecorrectgraphwhenCYAispresentareshownbelow:

SinceitishardtoseetheproportionaleffectsofpH,thefollowingaretheequivalentgraphsusingalogscaleforFCconcentration:


Ifyoulookcarefully,youwillseethattheloggraphontherightisthesameasshownintheO’BrienpaperinFigure14.5onpage352exceptthatmypHrangeisnarrowerandIhavecombinedmultiplechlorinatedcyanuratespeciesintoone“Cl‐CYA”curve.Inparticular,theshapeoftheHOCl(red)andOCl‐(green)curvesisthesameasintheO’Brienpaper.This,ofcourse,isnotasurprisesinceIusedtheO’Brienequilibriumconstantstogeneratethegraphfromaspreadsheetthatcomputeschemicalspeciesconcentration.TheabovegraphsshowthatCYAbuffershypochlorousacidsuchthattheeffectofpHonhypochlorousacidlevelisnotasstrongaswithoutCYA(i.e.theredcurveismore“flat”,especiallyabovepH7.5).

EffectsofCyanuricAcidonDisinfectionandOxidationThoughtheabovechemistrydemonstratestheverylowlevelofhypochlorousacidinthepresenceofCyanuricAcid(CYA),howdoweknowthatchlorineattachedtoCYAisnotaseffective?Therehavebeennumerousscientificstudiestoanswerthisquestionsolet’slookatonesreadilyavailableonlineandmostbeingfreeofcharge.Ifonecalculatesthehypochlorousacidconcentrationwhenthereisharddataforkilltimes(notjust“<30seconds”),oneisabletopredictthekilltimeswithinafactorof2inmostcasesthusdemonstratingthathypochlorousacidistheprimarydisinfectingagentandthatthechlorinatedisocyanurateshaveamuchdiminishedcapacityexceptasareservoirforhypochlorousacid.


BacteriaThefollowingfourpaperspublishedfrom1965to1969showthesignificantreductioninbacterialkilltimeswhenchlorineisinthepresenceofCyanuricAcid(CYA).NotethatthefourthreferencedemonstrateshowammoniainwaterreducessomeoftheapparenteffectofCYAonchlorinekillrates,mostlikelyduetotheformationofmonochloraminewhichisnotmoderatedinstrengthbyCYA(thoughisalesspowerfulbactericidethanhypochlorousacid).ItshouldalsobenotedthatthepresenceoforganicmatterinthewatercanconsumechlorinesuchthatlowFClevelscanbecomeineffective.ThisiswhereCYAcanbecomehelpful,wherehigherFClevelscanbeusedtonot“runout”ofchlorinewhilehavingmoderatedstrengthinspiteofthehigherFClevel.http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1256554&blobtype=pdfhttp://www.pubmedcentral.nih.gov/picrender.fcgi?artid=377704&blobtype=pdfhttp://www.pubmedcentral.nih.gov/picrender.fcgi?artid=546955&blobtype=pdfhttp://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1227093&blobtype=pdf

VirusesThefollowing1988paperdemonstratesthesignificantreductioninvirusactivationtimewhenchlorineisinthepresenceofCyanuricAcid(CYA).http://www.jstor.org/pss/3863297

ProtozoanOocystsThefollowingtwopapersin1983and2009demonstratethesignificantreductionininactivationofprotozoanoocystswhenchlorineisinthepresenceofCyanuricAcid(CYA).http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=239534&blobtype=pdfhttp://www.iwaponline.com/jwh/007/jwh0070109.htm

AlgaeThefollowing1981paperdoesnotdemonstrateanysignificantaffectofCyanuricAcid(CYA)onchlorineeffectivenessagainstkillingalgae.Thisisnotonlyinconsistentwithallpreviousstudiesasshownabove,butitisinconsistentwiththeexperienceofover20,000(mostlyresidential)poolownersreportingatThePoolForumandover10,000poolownersreportingatTroubleFreePoolwherethechlorine/CYArelationshiphassuccessfullykeptthesepoolsfreeofalgaeusingchlorinealone.Icanonlyspeculatethatthegrowthmediausedtoculturethealgaemayhavecontainedammonia‐likecompoundsthatcreated


compounds(suchasmonochloramine)thatinhibitedalgaegrowthandwerenotaffectedbyCYAlevel.http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=241858&blobtype=pdf

OrganicsThefollowing2001paperdemonstratesthatfortheoxidationofmonochlorodimedone(MCD),therateswithchlorinatedcyanurateswereestimatedtobe1/150ththatofthefreechlorinesolution(hypochlorousacidplushypochloriteion;mostlytheformerasthepHwas7.0).http://dx.doi.org/10.1016/S0043‐1354(01)00482‐1U.S.Patent5,591,692issuedin1997andassignedtoBio‐Lab(nowpartofChemtura)demonstratesasignificantreductionintherateorproductionofchloroform(trihalomethane,THM)fromhumicacidreactingwithchlorinewheneitherCyanuricAcidorglycolurilwaspresent.GlycolurilisverysimilartoCYAinthatitbindstochlorine,butdoessosomewhatmorestronglythanCYA.http://www.google.com/patents?vid=USPAT5591692

Real‐PoolStudiesonDisinfectionandORPThoughsomeofthescientificstudiesnotedaboveusedrealpoolwaterinadditiontochlorinedemand‐freewater,therehasbeenacontroversyintheindustryastowhetherthechemistryofchlorineandCYAappliesto“realpools”.Thefollowingtwostudiesusingrealpoolsattempttoanswerthisquestion.

PinellasCounty,FloridaPoolStudy1992(1994)ThefollowingisalinktothemainpageforthePinellasstudywithlinksundertheheading“FloridaStudyDetailsMicrobicidalProperitesofChlorine”.http://www.nspf.com/Research_Archives.htmlOneoftheconclusionsofthisstudywasthatFreeChlorine(FC)hadbyfarmoreinfluenceonthebacteriapopulations(disinfectionconditions)thananyothervariable.However,thestudydidnotlookspecificallyatcalculatedhypochlorousacidconcentrationsoIdecidedtodothatmyself.First,IputalloftherawdataintoaspreadsheetwhichcanbeviewedinHTMLatthefollowinglink:http://richardfalk.home.comcast.net/pool/Pinellas.htm


IextractedrelevantcolumnsofdataandsortedbyFreeChlorine(FC)andbycalculatedhypochlorousacid(HOCl)concentrationsasshowninthefollowinglinksandhighlightedinredthebacteriaandalgaecountsthatwereconsideredtobeaproblemaccordingtostudycriteria.http://richardfalk.home.comcast.net/pool/PinellasFC.htmhttp://richardfalk.home.comcast.net/pool/PinellasHOCl.htmToseetheeffectsofFCandcalculatedHOCl,Ireorganizedthedataintogroupsincreasinginalogarithmicfashion.Let’sfirstlookatthedatausingFC.Duetospacelimitations,Ionlyshowbacteriaandnotalgae,butthealgaedatadidn’tmakeanysenseanywaywithonly4poolswithgreenalgaeandthatpoolswithnoFCandhighbacterialcountshadnogreenalgae. Count FC HPC TCOLI FCOLI NCOLI PSEUD TSTAPH FSTREP 49 < 0.1 48.98% 36.73% 20.41% 57.14% 6.12% 6.12% 2.04% 4 0.1-0.19 25.00% 0.00% 0.00% 75.00% 0.00% 0.00% 0.00% 14 0.2-0.39 14.29% 7.14% 7.14% 14.29% 7.14% 28.57% 0.00% 35 0.4-0.79 8.57% 11.43% 5.71% 20.00% 2.86% 11.43% 0.00% 44 0.8-1.59 9.09% 6.82% 4.55% 9.09% 2.27% 2.27% 0.00% 111 1.6-3.19 4.50% 6.31% 4.50% 9.01% 0.00% 7.21% 0.00% 189 3.2-6.39 4.76% 5.82% 3.70% 12.17% 0.00% 3.17% 0.53% 29 6.4-12.79 0.00% 6.90% 3.45% 0.00% 0.00% 3.45% 0.00% 11 > 12.8 0.00% 9.09% 0.00% 0.00% 0.00% 0.00% 0.00% 486 Overall 9.88% 9.67% 5.76% 15.84% 1.23% 5.56% 0.41%YoucanseeanS‐curvelikeeffectwhereathresholdFCroughlyat0.2resultsinasignificantdropinthepercentageofpoolswithbacteriacounts,thoughthereisnolevelofchlorinethathasnocountswhatsoeverthoughpool#318hadanHPCof31000andanNCOLIof320(and2TSTAPH)andyethad5ppmFreeChlorine(FC)withapHof7.2andnoCyanuricAcid.Sothereareclearlynoabsolutes.Nowlet’slookatsomethingthereportdidn’texamine–calculatedhypochlorousacidconcentration.Count HOCl HPC TCOLI FCOLI NCOLI PSEUD TSTAPH FSTREP 58 < 0.001 43.10% 31.03% 17.24% 51.72% 5.17% 5.17% 1.72% 10 0.001-0.0019 30.00% 20.00% 20.00% 50.00% 0.00% 10.00% 0.00% 22 0.002-0.0039 9.09% 4.55% 4.55% 9.09% 0.00% 13.64% 0.00% 44 0.004-0.0079 2.27% 13.64% 6.82% 13.64% 4.55% 9.09% 0.00% 92 0.008-0.0159 7.61% 6.52% 3.26% 9.78% 0.00% 2.17% 0.00% 94 0.016-0.0319 4.26% 5.32% 2.13% 12.77% 0.00% 6.38% 1.06% 55 0.032-0.0639 3.64% 3.64% 3.64% 3.64% 0.00% 0.00% 0.00% 28 0.064-0.1279 0.00% 0.00% 0.00% 7.14% 0.00% 0.00% 0.00% 83 >0.128 4.82% 8.43% 6.02% 10.84% 1.20% 9.64% 0.00% 486 Overall 9.88% 9.67% 5.76% 15.84% 1.23% 5.56% 0.41%


HereyoucanseeanevensteeperS‐curveeffectataverylowchlorinelevelofaround0.002ppmHOClwhichatapHnear7.5isanFCof0.004ppmwithnoCYA.Basically,whatappearstobegoingonisthatittakesanincrediblylowchlorineleveltokillmostbacteriafasterthantheycangrowsoonecannotreallyconcludethatFCisthemostinfluentialvariablesinceonecouldjustasreadilyconcludethatitisHOClinstead.MostheterotrophicbacteriahaveaCTvalue(chlorineconcentrationinppmtimestimeinminutes)fora99%(2‐log)killofaround0.08andthisisequivalenttoa50%killCTvalueoflog10(1/0.5)*0.08/2=0.012.Bacterialgenerationtime(thetimeittakestodoubleinpopulation)isfrom15to60minutessousing15minutesthismeansitwouldtakeonly0.012/15=0.0008ppmFCtokillbacteriafasterthantheycanreproduce.Sincerealpoolwaterhasslowerkilltimesthanlabconditions,thestudyresultsarenotsurprisingandthemainpointisstillthesame–mostofthecommonbacteriaareincrediblyeasytokill(ingeneral)sothestudyresultsinrealitywereinconclusivewithrespecttoFCvs.calculatedHOCl(orevenFC/CYAwhichtheyalsodidn’tlookat).

Oxidation‐ReductionPotential(ORP)Thegraphsbelowweretakenfrom620samplesfrom194commercial/publicpoolsandspaswiththedatacollectedbyJeffLuedemaninBloomingtonandRichfield,Minnesota.There'salotofscatterandoutliers(asmallamountofvariationisfromthedifferentpH),butthebasictrendisprettyobvious.Inthelowergraph,thediamondstotheupperrightareunstabilizedpools,thediamondstothelowerleftarestabilizedpools,andthesquareswiththeredbordersarestabilizedpoolswithCYA<30so15ppmwasusedinthecalculations.(Thefirstgraphsays"DPD"fortheFreeChlorineaxis,butinfactaFAS‐DPDtestfromtheTaylorK‐2006Ckitwasused).


ItshouldbeprettyobviousthatORPismorehighlycorrelatedwithhypochlorousacidconcentrationthanwithFreeChlorine(FC)alone.

SideEffectsofChlorineSourcesCyanuricAcid(CYA)isadouble‐edgedsword.Ontheonehand,itprotectschlorinefromrapidbreakdowninsunlightanditalsomoderateschlorine’sstrength,butontheotherhandtoomuchCYAcanmoderatethatstrengthtolowerkillratesandoxidationtoanunacceptablelevel,especiallyforpreventingalgaegrowth.TheuseofstabilizedchlorinecanbuildupCYAfairlyrapidly.ThefollowingarechemicalfactsthatareindisputableandbasedonthecorechemistrywherechlorinemeasurementsareinunitsofppmCl2.Forevery10ppmFreeChlorine(FC)addedbyTrichlor,italsoincreasesCyanuricAcid(CYA)by6.1ppm.Forevery10ppmFCaddedbyDichlor,italsoincreasesCYAby9.1ppm.Forevery10ppmFCaddedbyCal‐Hypo,italsoincreasesCalciumHardness(CH)byatleast7.1ppm.Forevery10ppmFC,allsourcesofchlorineadd8.2ppmsaltasthechlorineconvertstochloridewhenitgetsusedup(oxidizingorganics,killingpathogens,breakingdownfromsunlight).Chlorinatingliquid,bleachandlithiumhypochloriteaddanadditional8.2ppmsaltwhileCal‐Hypoaddsaround2ppmadditionalsalt.Asimplecalculationshowsthatevenatalow1ppmFCperdaychlorineusage,continueduseofTrichlorwillincreaseCYAbyover100ppmin6monthsifthereisnowaterdilution.UsingCal‐HypowillincreaseCHbynearly130ppmin6months.Allsourcesofchlorinewillincreasesaltlevelsbyatleast150ppmin6monthswithchlorinatingliquid,bleachandlithiumhypochloriteincreasingsaltatdoublethisrate.Mostoutdoorresidentialpools


withoutpoolcovershavechlorineusagecloserto2ppmFCperdayandevenweeklybackwashingofsandfiltersisnotenoughtodilutethewaterexceptinthesmallestofpools.ThereisnoproductlabelingnordocumentationoftheabovechemicalfactsandsuchinformationisnottaughtinNSPFCPOnorAPSPTECHcoursesotherthanavaguequalitativedescriptionofhowstabilizedchlorinecanleadtoabuildupofCYAandthatCal‐HypoincreasesCH.Theissueisnotsomuchhavingtodowithsafety(except,perhaps,forperson‐to‐persontransmissionrisk)sinceithasalreadybeenshownhoweasyitistokillmostbacteriawithverylowlevelsofchlorine(thatis,lowFC/CYAratios),butalgaeismuchhardertokillsocontinueduseofstabilizedchlorinewithoutsupplementalalgaecidesorphosphateremoverscanleadinitiallytonascentnon‐visiblealgaegrowththatappearsasamysteriouschlorinedemandbeforeeventuallyshowingasanalgaebloom.TheswimmingpoolindustryonlytalksabouthowCYAprotectschlorinefromsunlightsodoesnotconsideritspositiveandnegativeeffectsasachlorinebuffermoderatingchlorine’sstrength.ThegeneralrecommendationofnotusinganyCYAatallinindoorpoolsorspasleadstoinconsistentsanitationwithactivechlorine(hypochlorousacid)concentrationsinindoorpoolsandspasbeing5‐20timesormorehigherthaninmostpoolsusingCYA.Thishasimplicationsforoxidationofswimsuits,skinandhair,corrosionrates,andforproductionofnitrogentrichlorideasdescribedbelow.

BreakpointChlorinationModelsandCyanuricAcidEffectsonNitrogenTrichlorideThebasicbreakpointreactionwasdescribedbyGriffin(1939),butthefirstreasonabledetailedmodelwasproposedbyWei&Morris(1974‐‐inChapter13inthesame"ChemistryofWaterSupply,TreatmentandDistribution"bookthathastheO'Brienpaperonthechlorine/CYAequilibriumconstants).SubsequentimprovementsweremadetothemodelbySaunier&Selleck(1976)andmostrecentlybyJafvert&Valentine(1992)whichshouldbeconsideredtobethebestmodelto‐date.ThefollowingisthefollowingpaperChad T. Jafvert and Richard L. Valentine, “Reaction Scheme for the Chlorination of Ammoniacal Water”, Environ. Sci. Technol., Vol. 26, No. 3, 1992, pp. 577-585. Alinktobeabletopurchasethispaperisthefollowing:http://pubs.acs.org/doi/abs/10.1021/es00027a022Thoughthemodellists14reactions,includingbothforwardandreversereactionrates,thedominantreactionsarethefollowing:(1)HOCl+NH3NH2Cl+H2OHypochlorousAcid+AmmoniaMonochloramine+Water


(2)HOCl+NH2ClNHCl2+H2OHypochlorousAcid+MonochloramineDichloramine+Water(3)HOCl+NHCl2NCl3+H2OHypochlorousAcid+DichloramineNitrogenTrichloride+Water(4)NHCl2+NCl3+2H2O2HOCl+N2(g)+3H++3Cl‐Dichloramine+NitrogenTrichloride+WaterHypochlorousAcid+NitrogenGas+HydrogenIon+ChlorideIonThefirstreactionproducingmonochloramineisbyfarthefastest.Itisover95%completeinoneminutewhentheFCisaround10%oftheCYAandtheammoniaismuchlessthanthechlorinesothatthechlorinelevelremainsfairlyconstant.WithnoCYA,thereactionismostlycompleteinacoupleofseconds.Thesubsequentreactionsarefarslower.Youcanthenseethathypochlorousacidparticipatesintworeactions(afterinitiallyproducingmonochloramine),oneproducingdichloramineandanotherproducingnitrogentrichloride.Sothenetreactiongoingfrommonochloraminetonitrogentrichloridetakestwohypochlorousacidandthisnetreactionratevariesasthesquareofthehypochlorousacidconcentration.Youcanseethatnitrogentrichlorideisbrokendownbydichloramineandthelatterisproducedwithareactionratethatvarieslinearlywithhypochlorousacidconcentration.Sointhesteadystate,theamountofnitrogentrichlorideislinearlydependentonthehypochlorousacidconcentration.Thiscanalsobeseenbythefollowingratereactionbalanceatsteadystate.k3*[HOCl]*[NHCl2]=k4*[NHCl2]*[NCl3]RateofformationofNitrogenTrichloride=RateofdestructionofNitrogenTrichlorideso,k3*[HOCl]=k4*[NCl3]Thenitrogentrichlorideconcentrationinthesteadystateislinearlyproportionaltothehypochlorousacidconcentration.Sincenitrogentrichlorideisveryvolatile,thisimpliesthattherateofoutgassingofnitrogentrichloridemaybeproportionaltothehypochlorousacidconcentrationsincetheoutgassingrateislikelytobeproportionaltoitsconcentrationinthewater.Asimilarratereactionbalancefordichloraminegivesthefollowing.k2*[HOCl]*[NH2Cl]=k3*[HOCl]*[NHCl2]+k4*[NHCl2]*[NCl3]RateofformationofDichloramine=RateofdestructionofDichloramineandsubstitutingtheearliersteady‐stateequationwehavek2*[HOCl]*[NH2Cl]=k3*[HOCl]*[NHCl2]+k3*[HOCl]*[NHCl2]whichreducesto


k2*[NH2Cl]=2*k3*[NHCl2]Sotheratioofmonochloraminetodichloramineisconstantandindependentofhypochlorousacidconcentration.Wecanlookatthesteady‐stateformonochloramineassumingaconstantintroductionofammoniaintothewater.k1*[HOCl]*[NH3]=k2*[HOCl]*[NH2Cl]RateofformationofMonochloramine=RateofdestructionofMonochloraminesotheratioofammoniatomonochloramineisconstantandindependentofhypochlorousacidconcentration.Finally,wecanlookatthesteady‐stateforammonia.k=k1*[HOCl]*[NH3]RateofformationofAmmonia=RateofdestructionofAmmoniawhichsaysthatforaconstantrateofintroductionofammonia,theamountofammonia,andthereforemonochloramineanddichloramine(fromabove),areinverselyproportionaltothehypochlorousacidconcentration.Earliermodelshadreactionsforminganintermediate,andtheJafvert&Valentinemodelhasthisaswell,butitisnotthedominantreactioninthatmodel.ThefollowingshowstheintermediatereactionssuchasfoundwithWei&Morris.(5)NHCl2+H2ONOH+2H++2Cl‐Dichloramine+WaterIntermediate+HydrogenIon+ChlorideIon(6)NOH+NH2ClN2(g)+H2O+H++Cl‐Intermediate+MonochloramineNitrogenGas+Water+HydrogenIon+ChlorideIon(7)NOH+NHCl2N2(g)+HOCl+H++Cl‐Intermediate+DichloramineNitrogenGas+HypochlorousAcid+HydrogenIon+ChlorideIonIntheWei&Morrismodel,thereisnodestructionofnitrogentrichloride,soit'srateofproductionistheproductofthehypochlorousacidconcentrationandthedichloramineconcentration.Intheabove,reaction(7)ismoredominantthanreaction(6).TheformationoftheintermediateNOHisaratelimitingstepsodichloramineisbuiltupandthereforetherateofproductionofnitrogentrichlorideislinearlydependentonthehypochlorousacidconcentration.Forarealisticexample,considerapoolwith3ppmFCandnoCYAvs.apoolwith3ppmFCand30ppmCYA.BothareatapHof7.5(thereisfarmorenitrogentrichlorideproducedatlowerpH)andthetemperatureis77F.Ifitisassumedthatthechlorinelevelismaintained


ataconstantlevelandthatthereisaconstantintroductionofammoniainthewateratarateof0.1ppmNperhour,thenwehavethefollowingsteadystateamountusingJafvert&ValentineinaspreadsheetImadeinthefollowinglinkhttp://richardfalk.home.comcast.net/~richardfalk/pool/Breakpoint.xlsOXIDATIONOFAMMONIASPECIES NOCYA 30ppmCYAMonochloramine 0.02ppm 0.70ppmDichloramine 2.97ppb 85.42ppbNitrogenTrichloride 70.96ppb 2.35ppbYoucanseefromtheabovethatwithnoCYAinthewater,thereislessmonochloramineanddichloraminebutmorenitrogentrichloridecomparedtohavingCYAinthewater.ThedifferencesareroughlyafactoroftheCYAlevelbecausethatisroughlythedifferenceinthehypochlorousacidconcentration(thebreakpointchlorinationspreadsheetassumes3ppmFCwith30ppmCYAresultsinabout0.05ppmhypochlorousacidatpH7.5‐‐theactualamountiscloserto0.042ppm).Nitrogentrichlorideisthemostvolatileandirritating.Themonochloramineodorthresholdis0.65ppm(650ppb);fordichloramineitis100ppb;fornitrogentrichlorideitis20ppb.Theequilibriumconcentrationsinairformonochloramineanddichloraminearesomewhatlowerthanthatinwater,butnitrogentrichlorideisextremelyvolatilesowillnotsaturatetheairbeforebecomingextremelynoticeableandirritating.Theaboveisjustforbreakpointchlorinationofammonia.AsseeninTable4.1ondocumentpage62(PDFpage85)ofthefollowinglinkhttp://www.who.int/entity/water_sanitation_health/bathing/srwe2full.pdfureahas68%ofthenitrogeninsweatcomparedto18%forammoniawhileinurineit's84%vs.5%.Thereisnodefinitivemodelforoxidationofureabychlorine,thoughsomemechanismshavebeenproposed(byWojtowicz)includingtheslowformationofaquad‐chloroureafollowedbyrapidbreakdowntodichloramineandnitrogentrichloride.IfIrepeattheaboveanalysisusingan80%/20%splitofureatoammoniaandassumeasteadystatebuildup,thenIgetthefollowingresults.OXIDATIONOFUREA&AMMONIASPECIES NOCYA 30ppmCYAMonochloramine 0.01ppm 0.28ppmDichloramine 1.19ppb 34.17ppbNitrogenTrichloride 70.84ppb 2.35ppbYoucanseethattheresultingnitrogentrichlorideisthesameasbefore,butthatthereislowermonochloramineanddichloraminebyafactorof2.5.


Therateofammonia/ureaintroductionof0.1ppmNperhourisforheavybatherloadssinceitrepresentsachlorineusageofnearly1ppmFCperhour.Oneswimmermayproducearound0.1ppmNperhourin1000gallonssoonlyapoolwithmanypeoplebeingactivewouldhavethissortofusage.Ofcourse,havingchildreninthewaterthaturinatewouldprovideaveryhighload.Ifachildurinates100ml(3.4fluidounces),thenin1000gallonsthisisabout0.3ppmN.SincetheUVinsunlightbreaksdownnitrogentrichloridefairlyquicklyandsinceaircirculationisalsogoodoutdoors,thecurrentrecommendationsforFCasa%ofCYAarereasonableforoutdoorpools.Theslowerbreakpointisnotgenerallyaproblemunlessthebatherloadishigh.Forcommercial/publicpoolswithhigherbatherloads,anFCthatis20%oftheCYAlevelmaybemoreappropriate.Forindoorpools,theslowerbreakpointmightbemoreofanissuesoperhapsanFCthatis20%oftheCYAlevelmaybebetterevenwhenthereisnothighbatherloadinsuchpools.Fromthemodels,notusinganyCYAatallinanypool(indoororoutdoor)canresultinfarhigherirritatingnitrogentrichlorideconcentrationsandalsohasthechlorinelevelbetoostrongforoutgassingofchlorine(mostlyhypochlorousacid),corrosionofimmersedmetalandoxidizingofswimsuits,skinandhair.Sinceitisnotpracticaltomaintain0.2ppmFCeverywhereinanindoorpoolduetolocalusageandimperfectcirculation,usingasmallamount(20ppm)ofCYAasahypochlorousacidbuffermakessense,butshouldnotbeoverdone.Toensuresufficientoxidationrates,anFCof4ppmwith20ppmCYA,whichisequivalentto0.2ppmFCwithnoCYA,couldbeareasonablebalancebetweenoxidationratesandabalancebetweenmonochloramine,dichloramineandnitrogentrichloridewhilestillprovidingplentyofchlorinetonotrunoutlocallyunderbatherload.

NextStepsClearly,investigationsonvolatilechlorinedisinfectionby‐products,includingmonochloramine,dichloramineandnitrogentrichloride,shouldincludeanalysisofwater(bothinthelabandinrealpools)usingCyanuricAcid(CYA)aswellasthosewithout.Unfortunately,allstudiesIhaveseenthathavebeendoneandthatarecurrentlyinprogressarenotlookingatCYAatall.TheEPASwimmingPoolWaterDisinfectantsstandard,DIS/TSS‐12,needstoberevisedtoreflecttheeffectofCYA.ThecurrentlaboratorystandardmeasureskillrateswithoutanyinitialCYAinthewater.Also,theEPAmaximumlimitof4ppmFCforswimmingpools,whichisbasedondrinkingwaterstandards,needstobere‐evaluatedinlightofCYA’smoderationofchlorine’sstrengthandtheminimalskinabsorptionofCYAand,byextension,thechlorinatedcyanuratesasdescribedinthefollowinglink:http://www.informapharmascience.com/doi/ref/10.3109/15569529309036260Foringestion,itisFCthatmattersindependentofCYA,butthe4ppmFCstandardwasbasedondrinkingquartsofwatereverydayandtheamountofswimmingpoolwaterthatisnormallyswallowedisafarsmalleramount.

the chlorine / cyanuric acid relationship and implications

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