Relative Bactericidal Effectiveness of Hypochlorous Acid and ...

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Regression analysis (Table 5) shows that the killtime correlates excellently with the free HOCl concentration(r 2 = 0.98). By contrast, correlations againstthe concentrations of the various chloroisocyanurateswere poor. The lone exception was dichloroisocyanurateion (Cl 2Cy – ) which showed a high correlation (r 2 =0.95). However, this is to be expected since the concentrationof Cl 2Cy – correlates with the concentration ofHOCl (see Figure 3). Indeed, any significant bactericidalactivity for Cl 2Cy 2– is very unlikely given the poorstatistical correlations found for the four otherchloroisosyanurates. The following equation was obtainedwhich shows that kill time at 20°C variesinversely with the calculated HOCl concentration at25°C.t 0.99= (0.0268q)[HOCl] –1.014Where: q is the relative disinfection time at 25°Cversus that at 20°C. This equation conforms to theChick–Watson Law: C n t = k, where C is the disinfectantconcentration, t the kill time, and n and k areconstants. The fact that n = 1 means that the concentrationof HOCl and time are equaly important indetermining the inactivation rate of bacteria.Effect of Temperature on Kill Time – Therate of disinfection increases with increasing temperature.Disinfection data can be extrapolated toother temperatures using the following equation:(~7.3 kcal/mol) for disinfection by hypochlorous acid(Fair et al 1948). This value represents the combinedtemperature dependance of HOCl ionization and inactivationof bacteria. Although the concentration ofHOCl decreases with increasing temperature at constantpH due to increased ionization, the disinfectionrate in cyanuric acid–free media actually increasesdue to greater disinfection efficiency. By contrast, in acyanuric acid–available chlorine system, increasedtemperature can lead to an increase in the HOClconcentration due to increased hydrolysis ofchloroisocyanurates, resulting in an additional increasein the disinfection rate.Although the activation energy for disinfectionin a cyanurate system has not been determined, it canbe estimated. The value of 30.5 kJ/mol (7.3 kcal/mol)for a cyanurate–free system can be adjusted to that ina cyanurate system by combining it with the appropriateenergy term applicable to a cyanurate system. Theprinciple chloroisocyanurate in pool water ismonochloroisocyanurate ion. It hydrolyzes to a smallextent at pool pH, providing increased HOCl concentrationsas temperature increases. A value of 99.2 kJ/mol/mol (23.7 kcal/mol) has been reported for theenergy (i.e., heat of hydrolysis) that determines thetemperature dependence of its hydrolysis (Wojtowiczto be published). Therefore, the overall estimatedactivation energy for disinfection in a cyanurate systemis: 30.5 + 99.2 = ~129.7 kJ/mol (31.0 kcal/mol). At25°C, this will result in a decrease in kill time by afactor (q) of 2.44 compared to that at 20°C.t 2/t 1= q = exp[(–DE/R)(1/T 2–1/T 1)]Where: t 1and t 2are the 99% kill times at temperaturesT 1and T 2(in kelvins), R is the gas constant (8.314 J/deg/mol; 1.987 cal/deg/mol), and DE is the disinfectionactivation energy. For enteric bacteria (such as E. coli)in a cyanuric acid–free system, DE = ~30.5 kJ/molSpeciesHOClClO –H 2ClCyHClCy –ClCy 2–HCl 2CyCl 2Cy –Coefficient ofDetermination r 2 *0.980.070.370.590.080.600.95*r 2 is the fraction of the variability in the data explained bythe regression equation.Table 5. Regression Analysisof Disinfection DataExtrapolation of Andersen’s Data to SwimmingPools – Since hypochlorous acid andchloroisocyanurates cannot be distinguished by testkit analysis (which is typically based on diethyl–p–phenylenediamine), the total free av. Cl concentrationis the only indicator of adequate disinfection. However,because disinfection is affected by the presenceof cyanuric acid, a better indicator is the ratio of totalcyanurate to av. Cl since it correlates perfectly withthe HOCl concentration at pH 7 and 20°C in Andersen’sstudy:HOCl (pH 7 & 20°C) = 0.576(Cy T/Cl T) –1.008 , r 2 = 1.00This equation shows that the HOCl concentrationvaries inversely with the ratio of total cyanurate tototal free av. Cl.Regression analysis shows that Andersen’s datafor 99% kill time (t 0.99) at pH 7 and 20°C varies linearlywith the ratio of total cyanurate (Cy T) to total freeavailable chlorine (Cl T) as shown by the followingequation:t 0.99(pH 7 and 20°C) = 0.119 + 0.0516Cy T/Cl T, r 2 = 0.98110 The Chemistry and Treatment of Swimming Pool and Spa Water
In order to apply Andersen’s data to swimmingpool conditions, the data must be adjusted for theeffect not only of temperature but pH as well. Since killtime varies inversely with pH (which influences theHOCl concentration), the 99% kill time at pH 7 and20°C was first adjusted to pH 7.5 (typical in swimmingpools) by multiplying by the factor 1.40, which representsthe ratio of HOCl at pH 7 to that at pH 7.5 instabilized water. Cyanuric acid moderates the effect ofpH on the HOCl concentration since the factor inunstabilized water is 1.48.t 0.99(pH 7.5 and 20°C) = [0.167 + 0.0722Cy T/Cl T]/qThe factor q allows extrapolation of kill timesfrom 20°C to other temperatures. The higher thetemperature, the higher the value of q and the lowerthe kill time. At a swimming pool temperature of 80°F(26.7°C), q = 3.29 which corresponds to 99% kill timesof 0.6 – 0.2 minutes over the recommended free av. Clrange of 1 – 3 ppm at 25 ppm cyanuric for entericbacteria such as S. faecalis. Higher cyanuric acidconcentrations will cause the 99% kill times to increase.By contrast, higher temperatures, especiallyin spas, will decrease kill times.Effect of Swimming Pool Contaminants onDisinfection Rate – Longer kill times were observedin pool water, compared to distilled water, whethercyanuric acid was present or not (Swatek et al 1967).This was attributed to unknown variables, which inreality are swimming pool contaminants. In swimmingpools, nitrogenous compounds such as ammonia,amino acids, and creatinine, introduced by bathers,can react with free av. Cl. This forms bactericidallyineffective combined chlorine, resulting in increasedkill times. Combined chlorine compounds do not hydrolyzeto a significant extent to HOCl. Thus, it isimportant to maintain a low combined available chlorinelevel by frequent superchlorination or shock treatmentin order to oxidize chlorinated compounds suchas monochloramine, N–chloroamino acids, and N–chlorocreatinine, which are much poorer disinfectantsthan HOCl.Addition of ammonia to stabilized aqueous av. Clresults in increased kill times proportional to theamount added (Fitzgerald and DerVartanian 1967).By contrast to ammonia, urea, a major swimming poolcontaminant, does not affect disinfection in short termexperiments (Fitzgerald and DerVartanian 1967).Although urea is a potential source of ammonia, itshydrolysis is very slow under swimming pool conditions.However, since urea serves as a nutrient forbacteria and algae, it is necessary to oxidize it byperiodic shock treatment.ReferencesAndersen, J. R. “A Study of the Influence of CyanuricAcid on the Bactericidal Effectiveness of Chlorine.”American Journal of Public Health 1965, 55, 1629–37.Brady, A. P., K. M. Sancier, G. J. Sirine. “Equilibria inSolutions of Cyanuric Acid and its ChlorinatedDerivitives.” Journal of the American ChemicalSociety 1963, 85, 3101–4.Fair, J. M., J. C. Morris, S. L. Chang, I. Weil, R. P.Burden. “The Behavior of Chlorine as a WaterDisinfectant.” Journal of the American WaterWorks Association 1948, 40, 1051–1061.Fitzgerald, G. P., M. E. DerVartanian. “FactorsInfluencing the Effectiveness of Swimming PoolBactericides.” Applied Microbiology 1967, 15, 504–509.Gardner, J., “Chloroisocyanurates in the Treatmentof Swimming Pool Water.” Water Research 1973,7, 823–33.Green, D. E., P. K. Stumpf. “The Mode of Action ofChlorine.” Journal of the American Water WorksAssociation 1946, 38, 1301–5.Morris, J. C. “The Acid Ionization Constant of HOClfrom 5 to 35°.” Journal of the American ChemicalSociety 1966, 70, 3798–3805.Nelson, G. D. “Special Report No. 6862 – SwimmingPool Disinfection with Chlorinated–s–triazinetrione Products.”, Monsanto Chemical Co.,St. Louis, Missouri, March 1967.O’Brien, J. E., J. C. Morris and J. N. Butler in Chemistryof Water Supply, Treatment, and Distribution.(Ann Arbor, Michigan: Ann Arbor SciencePublishers, Inc. 1972) pp. 333–358.Pinsky, M. L.; H–C Hu. “Evaluation of theChloroisocyanurate Hydrolysis Constants.”Environmental Science and Technology 1981, 15,423–30.Stewart, L. S.; L. F. Ortenzio. “Swimming Pool ChlorineStabilizers.” Soap and Chemical Specialties 1964,40, 79–82, 112–13.Swatek, F. E.; H. Raj; and G. E. Kalbus. “A LaboratoryEvaluation of the Effects of Cyanuric Acid on theBactericidal Activity of Chlorine in Distilled Waterand Swimming Pool Water.”, Paper presented atNSPI Convention, Las Vegas, NV, 1967.Wojtowicz, J. A. “Re–evaluation of ChloroisocyanurateHydrolysis Constants.” Submitted for review andpublication to the Journal of the Swimming Pooland Spa Industry 1996.John A. Wojtowicz – Chapter 6.1 111
Page 2 and 3: SpeciesHOClClO -HClCy -Cl 2Cy -l ma
Page 4 and 5: Figure 3 - SpeciesDisribution in th

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