Relative Bactericidal Effectiveness of Hypochlorous Acid and ...

SpeciesHOClClO –HClCy –Cl 2Cy –l maxnm235290215220Table 1. Absorption Maxima forMain Chlorine Specieswell below 290 nm, they are relatively stable to sunlight.Studies with chloroisocyanurate solutions (CA =26 ppm and av. Cl = 14 ppm) showed maximumstability at pH 6.8 which corresponds to minimalhydrolysis (O’Brien et al 1975). Stability decreases aspH is increased due to increased ionization of HOClto ClO – .Shortly after the commercialization ofchloroisocyanurates, it was shown that cyanuric aciddecreases the kill rate of bacteria such as S. faecalisand S. aureus by available chlorine in distilled water(Andersen 1965). Later studies confirmed these findings(Stewart and Ortenzio 1964; Fitzgerald andDerVartanian 1967; Swatek et al 1967). However, noattempts were made to correlate the observed killtimes with the concentration of biocidal species.In the absence of cyanuric acid, the rate of disinfectionby free av. Cl decreases with increased pH inaccordance with the ionization pattern of HOCl, demonstratingthat HOCl is the active biocidal agent andthat hypochlorite ion is a poor disinfectant (Fair et al1948). Disinfection by HOCl is believed to be due todiffusion through the cell wall of bacteria followed byoxidative attack resulting in disruption of metabolicprocesses (Green and Stumpf 1946). The poor bactericidalproperties of hypochlorite ion are attributed toslower diffusion through the cell membrane comparedto the neutral molecule HOCl.DISCUSSIONChloroisocyanurate Equilibria – The variouscyanurate equilibria in the cyanuric acid – availablechlorine system are shown in Figure 2. Thecomplex equilibrium system consists of six ionizationand six hydrolysis reactions, shown horizontally andvertically, respectively. Table 2 lists the reportedvalues of the equilibrium constants. The equilibriumconstants for the first 12 reactions were measured at24–25°C (O’Brien et al 1975). Other values of some ofthe chloroisocyanurate equilibrium constants havealso been reported (Brady et al 1963; Gardner 1973;Pinsky and Hu 1981). The equilibrium constant forionization of hypochlorous acid was determined overthe 4 to 34 °C temperature range (Morris 1966).Mathematical Model and Computer Program– A computer program (similar to O'Brien et al1975) was employed for calculation of the concentrationof all species in the cyanuric acid – availablechlorine system. Material balance yields the followingequations for total free av. Cl (Cl T) and total cyanurate(Cy T):ReactionpK NumberpKCl 3Cy + H 2O HCl 2Cy + HOCl 1 1.8 ± 0.2HCl 2Cy H + + Cl 2Cy – 2 3.75 ± 0.03HCl 2Cy + H 2O H 2ClCy + HOCl 3 2.93 ± 0.07H 2ClCy H + + HClCy – 4 5.33 ± 0.05H 2ClCy + H 2O H 3Cy + HOCl 5 4.07 ± 0.08H 3Cy H + + H 2Cy – 6 6.88 ± 0.04Cl 2Cy – + H 2O HClCy – + HOCl 7 4.51 ± 0.09HClCy – H + + ClCy – 8 10.12 ± 0.02HClCy – + H 2O H 2Cy – + HOCl 9 5.62 ± 0.05H 2Cy – H + + HCy – 10 11.40 ± 0.10ClCy 2– + H 2O HCy 2– + HOCl 11 6.90 ± 0.11HCy 2– H + + Cy 3– 12 13.5HOCl H + + ClO – 13 7.54Table 2. Equilibrium Constants for Reactions in theCyanuric Acid – Available Chlorine System @ ˜25ºC106 The Chemistry and Treatment of Swimming Pool and Spa Water