Environ. Sci. Technol. 1988, 20, 822-826
Alteration of Chemical and Disinfectant Properties of Hypochlorite by Sodium, Potassium, and Lithium Charles N. Haas," Medardas G. Kerallus, Dolores M. Brncich, and Michael A. Zapkln Prltzker Department of Envlronrnental Englneerlng, Illlnols Instltute of Technology, Chicago, Illlnols 60616
Previous studies have suggested the possible existence of alkali cation-hypochlorite ion pairs during disinfection, particularly at high pH. In this paper, we report quantitative measurements for the dissociation constants of the LiOC1, NaOC1, and KOCl ion pairs using obtained from titration. Furthermore, we report the effect of the Li+, Na+, and K+ cations on inactivation of Escherichia coli by free chlorine. The microbiological effects are consistent, and the bactericidal efficiency of the ion pairs is estimated as intermediate between hypochlorite and hypochlorous acid.
Introduction In 1972, it was reported that viruses were more rapidly inactivated by free chlorine at high pH than at low pH (I) in distinct contradiction to previously accepted behavior (2). Subsequent studies indicated that the addition of alkali cations to hypochlorite solutions enhanced the virucidal efficiency of such solutions (3-5). The basis for this effect has been postulated to be due to the formation of neutral alkali cation-hypochlorite ion pairs possessing biocidal potency greater than OCl- (5,6).Prior work has also established that addition of sodium to alkali hypochlorite solutions results in an enhancement of biocidal efficiency (7). This paper reports the first verified systemic measurements of the quantitative effect of formation of ion pairs of lithium, sodium, and potassium with hypochlorite on the inactivation of Escherichia coli by alkaline free chlorine solutions. Materials and Methods General. Chlorine demand free water was prepared by adding sufficient free chlorine to distilled water to produce a residual of 3-4 mg/L after 1week of exposure in the dark at 25 "C. After at least 1week, chlorine demand free buffer (CDFB) was prepared by adding 6.25 mL/L of 0.1 M boric acid and 12.5 mL/1 of NaOH. The buffer was then boiled for 30 min and dechlorinated under UV light until no detectable chlorine residual remained. The final buffer pH was 10.0, Chlorine demand free water (CDFW) was prepared in a likewise manner, omitting the NaOH and boric acid. pH was determined by using glass electrodes. Residual chlorine was determined by forward amperometric titration (8). All chemicals used were reagent grade. Determination of Dissociation Constants The effect of added ions on the hypochlorite dissociation equilibrium was determined by acid titration. Nitric acid titration was standardized against primary standard Na2C03,previously dried, and desiccated. This nitric acid was used a secondary standard for the standardization of NaOH solutions. To a known volume of solutions containing, prepared in CDFW, free chlorine and a known amount of nitrate salt of the test cation (Li, Na, or K), a known volume of standardized NaOH was added to bring the pH to ap-
* Author to whom correspondence should be addressed. 822
Environ. Sci. Technol., Vol. 20, No. 8, 1986
proximately 10.0. Titration by addition of 0.01-mL increments then proceeded, recording the equilibrium pH following each addition. In all cases, when the final computed ion pair dissociation constants were used, the contribution of cations present in the initial NaOH change was shown to have a negligible effect on the equilibrium. Titration was conducted in duplicate or triplicate at cation concentrations of 0.33, 0.67, 1.0, and 2.0 M. pH values were corrected for the effect of the background electrolyte on the response of the pH electrode to a series of standard buffer solutions (9).
Bactericidal Effectiveness In experiments using sodium, E . coli ATCC 112299was the test organism, and organisms were prepared from a 24-h nutrient agar slant obtained after incubation at 36 f 1.0 "C. Cells were washed from the slant in sterile pH 7.2 dilution water (8). In experiments with potassium and lithium a wild-type strain of E. coli was inoculated into a nutrient broth test tube, and following 24 h of incubation at 37 "C, cells were harvested by centrifugation and resuspended in sterile dilution water. In both cases, the cell suspensions were washed twice in dilution water, harvested by centrifugation, and resuspended. In all cases, the procedure resulted in all suspensions possessing negligible chlorine demand at the cell concentrations employed. Initial cell concentrations were approximately 106/mL. For each experiment, four identical flasks were prepared. The first and second flasks contained CDFB plus cells alone. The third and fourth flasks contained CDFB plus cells plus 0.1 M alkali salt (either as the nitrate or as the chloride). A t time zero, a known amount of chlorine was added to the second and fourth flask. At the conclusion of the experiment, chlorine residual was measured in the second and fourth flasks, and the experiment was discarded if a detectable change in chlorine residual was noticed. This design enabled control and correction for any inactivation due to the cation salt in the absence of free chlorine. At predetermined sample times, 5-mL aliquots were withdrawn from each of the four flasks and immediately added to sterile test tubes containing 5 mL of 0.2% peptone and 0.1 mL of 50 mM sodium thiosulfate. The sample was placed into a 4 "C cold room prior to analysis and held for less than 2 h. Viability was assessed by plating known volumes if necessary after prior decimal dilution using dilution water. Spread plates were made with nutrient agar, and colonies were counted after 24 h of incubation at 37 OC. Duplicate or triplicate plates at each chlorination were taken, and the counts were averaged. Analysis of Data Chemical Equilibrium. Let B equal the buffer index (= -dCA/dpH), where CA represents the volume of acid which is added, and pH represents the pH. If x is defined as the proton activity at which B is a maximum, and if y is defined as the difference between the two proton con-
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0.0031 free chlorine Z.0N LiNO 0 . 1 ~H N O ~ ~ 0.0049E1 NaOH
centrations at which B is half its maximum value he., width a t half-height), then it may be shown that the following are true (9, 10):
26’C
+ ((1/16x4) + ( . ~ ~ / 6 4 ~ ~ ) ) ~ / ~(1)] ’ / ~ = [(1/4) + (1/16 + y2/~2)1/2]1/2 - 1 (2)
Log KA/A Log %/B
A/KA = [(1/4x2) A’C,/KD
In eq 1 and 2, KA is the dissociation constant of HOC1, KD is the putative dissociation constant of the cation hypochlorite ion pair (MOCl),A is the ratio of the hypochlorite ion activity coefficients to that of hypochlorous acid, A’ is the product of the cation and hypochlorite ion activity coefficients divided by the ion pair activity coefficient, and C, is the total concentration of cation under investigation. These equations permit the simultaneous determination of K Aand KD without the determination of the final free chlorine concentration, which is relatively imprecise. The computed value of KA provides an initial check on the procedure and may be compared to published values for this constant (e.g., see ref 11). Since the determination of numerical values for derivatives increases noise, a data-smoothing procedure was employed prior to the substitution into eq 1 and 2. First, the pH vs. CA data were smoothed by using a five-term method (12).Then, the values of 1 / B (=-dpH/dCA) were obtained at each value of CAby using a five-term central difference relation (13). The resulting data set was then fit to a series of polynomials in CAby using least squares, and the highest degree polynomial providing a statistically significant improvement in fit was used to determine x and y for use in eq 1and 2.
Bactericidal Effectiveness Fair et al. (2), in estimating the relative efficiency of HOC1 to OC1-, assumed that the rate of inactivation by a mixture of the two species was a linear, additive function of their concentration. In this work, their model is extended as follows: kbt = kl[HOCl]
+ k,[OCl-] + ks[MOCl]
(3)
where kt, is the overall observed pseudo-first-order inactivation rate constant, and kl, k2, and k3 are the intrinsic inactivation rate constants due to each species. The present work was conducted at pH 10. At this pH, given the ratio of kl to k2,the contribution of the first term to kbt may be neglected. This simplification, combined with the definitions of KA and KD and the appropriate mass balances, may then be shown to yield (14) kbt
Cbt(k2KD + ~&,)/(KD + cd
(4)
In eq 4, CWtis the overall free chlorine concentration. If C, = 0 (no cation present) or, alternatively, C,
