pH

There are three reasons to maintain pH: * Efficacy of the CHLORINE, * BATHER comfort, and * Maintenance of balanced water. Each of these reasons are discussed briefly below: Efficacy of Chlorine The efficacy of CHLORINE/hypochlorous acid is dramatically impacted by pH and therefore pathogen inactivation can be severely affected by higher pH levels where only a small percentage of FREE CHLORINE is active. Lower pH levels below this range allow a greater percentage of FREE CHLORINE to be “active”. Further data are needed to ensure that lower levels (e.g., 6.8 to 7.2) do not adversely impact membranes, particularly eyes. The present practice of maintaining the pH between 7.2 and 7.8 has been developed by coupling physical chemistry with empirical observations. There is no definitive peer-reviewed study that extensively covers the subject of pH in POOL and SPA water except those showing the titration of hypochlorous acid and the importance of pH for assuring maximal efficiency. The best general authority is the 1972 edition of the Handbook of Chlorination by Geo. Clifford White. The 1972 edition of this widely recognized authority on CHLORINE chemistry is the only edition that has a chapter especially on POOLS. Much, but not all, of the POOL chemistry chapter can be found in subsequent editions. Copies of the 1972 edition are difficult to locate in libraries but are available for sale on the internet as of July 2009. The discussion on efficacy and BATHER comfort is a summary of the 1972 edition discussion on pH. CHLORINE used in POOLS refers to hypochlorous acid. Hypochlorous acid (HOCl) is a weak acid that readily dissociates to form hypochlorite (OCl-) and hydrogen ion (H+). The mid-point of the dissociation (the pKa) is at pH 7.5. Functionally, this means that at pH of 7.5, 50% of the FREE CHLORINE present will be in the form of hypochlorous acid and 50% will be in the form of hypochlorite. As the pH decreases below 7.5, the proportion of hypochlorous acid increases and proportion of hypochlorite ion decreases. The opposite occurs as the pH increases above 7.5. Numerous investigators have reported that hypochlorous acid is approximately 100 times more effective at killing microorganisms than the hypochlorite ion. Thus from a public health perspective, it is desirable to maintain the pH so as to maximize the portion of hypochlorous acid portion of the FREE CHLORINE present in the water. Bather Comfort As BATHERS enter the water, their skin and eyes come into direct contact with the water and its constituent components. In general, the eyes of BATHERS are more sensitive to irritation than the skin. Studies on the sensitivity of BATHERS’ eyes to pH changes of the water show wide variations in tolerance limits. The tolerance of the eye to shifts in pH is also impacted by the concentration of FREE CHLORINE, combined CHLORINE, and alkalinity. Under normal POOL conditions, the optimum limits for BATHER comfort appears to be from pH 7.5 to 8.0. Potential for Lowering pH in the Future During the review of the data, the MAHC had a broad interest in lowering the minimum pH. This would increase the efficacy of the CHLORINE by increasing the proportion of hypochlorous acid (at the expense of hypochlorite) and thus increase DISINFECTION efficacy. This was not recommended because of the lack of data on the impact on BATHERS, particularly the eyes. If additional information on the impact of lower pH on BATHERS’ skin and eyes is developed, the MAHC suggests that the acceptable range for pH be reexamined. As part of the reexamination, consideration should also be made concerning how this change will impact the water balance and any possible negative impact on the facility.